©        1979





             Gregory Scott Mills

A Dissertation Submitted to the Faculty of the


  In Partial Fulfillment of the Requirements
              For the Degree of


           In the Graduate College


                   19 7 9

     Copyright 1979 Gregory Scott Mills

                                             GRADUATE COLLEGE

     I hereby recommend that this dissertation prepared under my

direction by ___________ Gregory Scott Mills______________________


             RELATION TO AN OPTIMAL FORAGING MODEL__________________

be accepted as fulfilling the dissertation requirement for the

degree of ______________ Doctor of Philosophy_____________________

             Dissertation Director                                      Date

     As members of the Final Examination Committee, we certify

that we have read this dissertation and agree that it may be

presented for final defense.

                 C     -I
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           ...       u^u_   .   '   v.   . 'Lz-v ________                Jijzc* 7 g____

                        l j. l           U     L .                   r ^        ??

Final approval and acceptance of this dissertation is contingent
on the candidate's adequate performance and defense thereof at the
final oral examination.

        This   dissertation has been submitted in partial fulfillment of
requirements   for an advanced degree at The University of Arizona and
is deposited   in the University Library to be made available to bor­
rowers under   rules of the Library*

        Brief quotations from this dissertation are allowable without
special permission, provided that accurate acknowledgment of source
is made* Requests for permission for extended quotation from or
reproduction of this manuscript in whole or in part may be granted by
the copyright holder*

                           SIGNED;            5

        I would like to thank Ho R=, Pulliam, Co R= Tracy, Jc Re Silli-

man, W 0 Ao Calder, Stephen Mo Russell, and, especially, Jo Ho Brown

for their comments and suggestions concerning the ideas presented in

this papero   Steve Sutherland made valuable contributions to the con­

cept of optimal perch height and Tom Caraco kindly shared his ideas on

risk aversiono   Jo Ho Brown and Ao Co Gibson made valuable contribu­

tions to the preparation of the manuscripto    I thank the staff and

officers of The Research Ranch for their consent and aid in some

aspects of this study0


   LIST OF ILLUSTRATIONS    o o o o o o o o o o o    o o o o o o o         v

   LIST OF TABLES  © © © © © © © © © © © © © © © ©©          © © © ©    vx

   ABSTRACT   © © o o © © © © © © © © © © © © ©     © ©   ©©   © © © ©   vxx

1© INTRODUCTION © © © © © © © © © © © © © © © © ©         ©©   © © © ©     I

2© PATTERNS OF HUNTING FROM PERCHES © © © ............. ©            ©     4

       An Equation for Net Energy Gain © © © © © © © © © © ©               4
       Metiiods © o © © © © © © © © © © © © © © © © © © © © ©              ^
       Patch Choxce © o © © © © © © © © © © © © © © © © © © ©              6
            Geometry of Hunting from Perches © © © © © © © © ©             7
            Optxmal Perch Hexght © © © © © © © © © © © © © © ©            12
            Predictions and Tests © © © © © © © © © © © © © ©             19
       Movement Between Patches © © © ©© © © © © © © © © © ©              27
       Allocation of Time in Patches     ©© © © © © © © © © © ©           30
       Opt xmal Dxet © © © © © © © © © ©© © © © © © © © © © ©             3^*
       Comparison of Foraging Patterns of Kestrels and
         Shrxkes o o © © © © © © © © © © © © © © © © © © © ©              43
       Concurrent Goals © © © © © © © © © © © © © © © © © © o             44
       Conclusxons © o o © © © © © © © © © © © © © © © © © ©              46

3© PATTERNS OF HUNTING WHILE HOVERING     ©© © © © © © © © © © ©          48

       Methods © © © © © © © © © © © ©© © © © © © © © © ©•           ©    49
       Advantages of Hunting While Hovering © © © ©© © © ©           ©    49
       Costs of Hoverxng © © © © © © © © © © © © © © © © ©           ©    31
       Hoverxng Hexght o o © © © © © © © © © © © © © © © © ©              39
       Optimal Wind Speed for Hovering © © © © © © © © © © ©              61
       Hoverxng Txme © © © o © © © © © © © © © © © © ©   © ©         ©    66
       Hovering as an Alternate Hunting Technique ©© © © ©           ©    68

   LIST OF REFERENCES   © © © © © © o © © © © © © © © © © © © ©           71


Figure                                                                 Page

  lo     Geometric considerations of perches and vegetation „ = =        8

  2=     Relative areas visible to a perched bird showing effects
         of grass height and density 0 = 0 0 0 0 0 0 = o b o o =        10

  3=    Approximate increase in visible area with increasing
       hunting height 0 0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0       11

  4o    Approximate prey encounter rate with increasing perch
       height o 0 0 0 0 0 o 0 0 - 0 0 0 0 0 00 o o o o 00 0 0 0        15

  5o    Effects of increasing height on net energy gain per
       attack 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0         l6

 60      Rate of net energy gain as a function of hunting height.       17

 7o      Index of grasshopper abundance in months of August
         through December 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ,   00    2^

 80      Success rates and lengths of giving-up times for shrikes
         (A) and kestrels (B) as functions of season 0 0 0 0 0 0        33

 9= Two possible mechanisms for threshold renewal          00000        41

10o The effect of air speed on the power required to fly 0 o            92

11= Hovering effort as a function of wind speed         0=00=0          98

12= Hovering height as a function of wind speed at 2.m = « =           69


Table                                                                                                                  Page

  lo    Effects of perch height on the distances traveled to
        prey for kestrels and shrikes » o o . < , » . o o . . o o o                                                     13
  2=    Perch height related to time of year                                      o o =, 0 « 0 , o = o-                 26
  3= The relation between perch height and wind speed                                                                   26
 4.e. Effects of perch height on distances traveled between
        perches   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o   o    o   o   o   o   o   o   o   b   o    29
  5® Effects of wind speed on distance traveled between
        perches 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0                                                       29
 60     The effects of perch height on giving-up time for
        kestrels and shi*ikzes o o o o o o o o 0 0 0.0.00 0 0 o o                                                       32
 7o Distances to prey at different times of year                                                   0 0 o o 00           36
  80    Effects of distance to prey on success rates of kestrels
        and shrr k e S o o o o o o o o o o 0 0 0 0 0 0 0 0 0 0 0 0 0                                                   38
 9 o    Effects of time on perch on success rates of kestrels
        and shrikes 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0                                                      38
10o     Effects of time since last prey capture on success rate
        of kestrels 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0                                                       39
11o Comparison of prey types' and rates of prey capture from
        perches and hovers                0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0                                        55
1 2 o   Calculations of Vmp for the American Kestrel                                               o o o 0 0 o         63


        Although considerable literature on optimal foraging theory

exists, few field tests have been conducted*    To make such tests,

winter foraging patterns of American Kestrels (Falco sparverius) and

loggerhead Shrikes (Lanins ludovicianus) were observed in southeastern

Arizona to compare actual patterns with predictions of an optimal

foraging model developed for predatory ground-hunting birds*       The model

is developed from considerations of foraging theory, energetics, and

perch and vegetational characteristics that influence vision of the

predator*    Two hunting techniques are analyzed;     hunting from perches

by kestrels and shrikes, and hunting while hovering by kestrels*

        Analysis of hunting from perches includes patch selection,

movement between patches, allocation of time in patches, and prey

selection*   For kestrels and shrikes, patch selection primarily in­

volves selection of a perch*    Considerations of factors affecting

hunting from perches predict the existence of an optimal hunting height

which increases with decreasing prey abundance and increasing prey size*

When comparable prey decrease in abundance, kestrels and shrikes hunt

more often from higher perches*    Selection of perch height is also

affected by wind; birds perch lower at high wind velocities*       Kestrels

and shrikes appear to minimize time and energy spent traveling between

patches; they nearly always forage unidirectionally and travel greater

distances between high perches than low ones*       Givirig-up times, i*e*,

times spent in patches where no prey were attacked, appear to be de­

termined in part by previous hunting times; giving-up time correlated

better with the previous three hunting times than just the last one0

Prey selection appears to be strongly influenced by three factors:

distance from perch, evaluation of probability of success, and size

and type of preyo   Success rate decreases with hunting time„ The

interpretation is that a threshold of prey selectivity diminishes with

timeo   Such a diminishing threshold could account for partial prefer­

ences in diets0 Contrary to predictions of some optimal foraging

models, prey selectivity appeared to increase with decreasing prey

densityo   An explanation for this pattern may be that birds minimize

variance in food intake by avoiding riskso

        'Analysis of hunting while hovering primarily concerns the

energetics of hovering flight and their effects on the utilization of

this foraging method,.   Hovering allows kestrels to hunt in areas with­

out suitable perches, but the relatively high energetic costs restrict

its use to times of favorable wind speeds,.   The optimal wind speed at

which to hover is apparently equal to the air speed at which flight is

least costly= Most hovering occurs when optimal wind speed and

optimal hunting height coincide; when they do not, kestrels appear to

adopt a compromise between the two„   Because wind speed increases with

height, hovering height decreases as wind speed increases« Duration

of individual hovers from which prey was not attacked was affected by

time of year, duration of the previous hover from which prey was

attacked, and wind speed*    Rate of energy intake is greater when
hovering than when hunting from perches» Hovering appears to be an

important alternative foraging" strategy for some species of birds at

times of favorable environmental conditions^


         Optimal foraging theory shows great promise for providing a

 better understanding of animal behavior and community structure (Pyke,

 Pulliam and Charnov 1977), but relatively few studies have fully

 assessed its application in natural systems*     In most papers, Optimal

 foraging has been treated only theoretically on a strategic level,

 e *go, Schoener 1971 and Charnov 1973= The scarcity of field tests may

 be in part due to difficulties in translating theory on a strategic

 level to testable predictions on a tactical level*    On a strategic

 level, terms are often vaguely defined and it is possible to focus on

 only one variable while others are ignored*     On a tactical level, terms

 must be defined more precisely and many variables that potentially

 affect an animal's behavior must be considered simultaneously* Another

 problem that may contribute to the scarcity of field tests of foraging

 theory is the difficulty in selecting a system where an animal can be

 observed for extended periods*

         A crucial part of optimal foraging models is identification of

 an animal's goal (Schoener 1971, Charnov 1973, Pyke et al* 1977)=      Al­

 though the choice of goal may affect the overall time budget of an

 animal, many goals ultimately reduce to the prediction that an animal

' should attempt to maximize net energy intake while foraging*   To do

this, an animal must make a number of choices,     Charnov (1973)

identified a hierarchy of such choices:   a habitat in which to hunt, a

patch within that habitat, a foraging method to use in the patch, and

prey types to be pursued.   Although I believe that such a hierarchy, is

a useful tool for analyzing foraging behavior, I do not believe that

the four choices must occur in the order listed.    In particular, forag­

ing method may be determined before habitat or patch selection occurs

because particular kinds of animals may be constrained by evolutionary

adaptations which restrict their range of foraging methods.

        In this paper I construct a tactical model for some aspects of

foraging of ground-hunting predatory birds from considerations of perch

and vegetation characteristics, energetic costs, and ideas from optimal

foraging literature.   Foraging of these birds provides a good system to

test foraging theory because complicating variables are minimized,

terms can be operationally defined, and foraging activities are easily

observed.   The model is developed assuming that these birds are

attempting to maximize net energy intake while foraging.     This goal

appears to be appropriate for predatory birds, and all foraging be­

haviors in this study were predicted from this assumption.    However,

some data collected during this study suggest that prey selection may

also be influenced by minimizing variance in energy intake.     In most

cases predictions of foraging behavior generated from both goals are

the same, and, therefore, discrimination between the two is not usually

critical.   Concurrent goals, such as avoiding predation, or maintaining

territories do not appear to significantly affect foraging behavior of
these birds,,   A more thorough discussion of these factors is presented

later in this paper»

          Qualitative predictions of the model developed are tested and,

in many cases, verified in the field with foraging patterns of American

Kestrels (Falco sparverius) and Loggerhead Shrikes (Lanins ludovicianusX

Predictions are based primarily on foraging theory and bonsiderations

of flight energetics and geometric properties of hunting from perches,

but were also biased by known information of kestrel biology0 Some

predictions were changed during the course of the study in light of new

considerations, but all predictions were a priori in the sense that

they were made before the extensive data were analysed«,    These predic­

tions can also be treated as hypotheses and tested independently by other

investigators working with other organises or in different habitatso

          Pyke et al= (1977) have divided foraging theory into four cate­

gories:    diet, patch choice, allocation of time in patches, and pat­

terns of movement between patcheso    In Chapter 2 of this paper,

foraging method, hunting from perches, is treated as a constant while

behaviors associated with patch choice, allocation of time in patches,

and patterns of movement between patches are examinedo     I also analyze

some aspects of diet, specifically quality evaluation of prey by dis­

tance and capture success rate*    In Chapter 3 , I analyze factors in­

fluencing the choice between two foraging methods for ground-hunting

predatory birds, hunting while hovering and hunting from perches^

Patch choice and allocation of time in patches for birds hunting while

hovering are also examined®


                        An Equation for Net Energy Gain

        Rate of net energy gain of a bird hunting from perches can be

represented by the equations

                  Eg = E/A ° A/t - RMR - C/t                         (1 )
where Eg is net rate of energy gain; E/A is the net energy gained per

attack; A/t is the attack rate; RMR is resting metabolic rate, here

defined as all the energy required to hunt from a perch including

thermoregulation; and C/t is the rate of energy expended changing

perches when no prey are attacked,,

        Net energy gained per attack (E/A) is a function of other

variables such that:

                  E/A = fs(e) - a                                    (2 )
where fs is the frequency of success (success rate), e is mean energy

content of prey attacked, and a is the mean energy expended in making

an attack including costs to fly to the ground and return to a perch0

        Similarly, attack rate (A/t) is a function of other variables'

such that:

                  A/t = Pp (N/t)                                     (3)

where Pp is the proportion of prey encountered that are attacked and

N/t is the encounter rate with prey over the entire foraging bouto
• 5
          To increase net energy intake, a bird can increase E/A or A/t

or decrease KMR or C/t, E/A can be increased by increasingfs, or e, or

by decreasing ao    Attack rate can be increased by increasing N/t or Ppo

Because these variables are interrelated and tradeoffs occur between

some, the exact combination of values that results in a maximum net

energy gain depends on the relative values of eacho    Because many of

the terms cannot be measured, these equations will not be evaluated

numerically but used to provide an understanding of the factors that

affect hunting from percheso    Qualitative predictions and analyses of

foraging behaviors can then be madeo


          Observations of foraging kestrels and shrikes were made in the

grasslands of southeastern Arizona from September 1975 to March 1977°

Although some data were collected throughout the year, most observa­

tions were made in fall and winter monthso     Most data were collected

between 0900 and 1500 h0 Birds were watched with lOx binoculars or a

15-60x telescope from a parked vehicle0

          Data taken on foraging birds included perch height, distance

traveled between perches, distance to prey, success rate of attacks,

and time spent hunting on percheso     A bird was considered to be forag­

ing when it showed active signs of searching the groundo     Except for a

few times in early fall, birds appeared to forage almost constantly„

Time spent in nonforaging activities (such as preening) was subtracted

from the time on perches*   Most birds were followed as long as pos­

sible *
Times were measured with a stopwatch and data were recorded on

a portable tape recorder and transcribed later.    Heights and distances

were estimated visually but were calibrated periodically by taking

precise measurements.    Wind speeds were measured with a Dwyer hand­

held wind meter.

          In one area perches consisting of poles (agave stalks) 3 to

5 m high were erected on three successive fenceposts spaced 3 m apart

such that perch height increased from 2 (fenceposts) to 5 m at approxi­

mately one meter intervals.    Only 2 such units were erected, but 13

others of perches 2, 3 , and 4 m high and 4 units of 2 and 3 m poles

were also constructed serially in the same area.

          No quantitative study of prey populations was conducted but

grasshoppers were censused along a 1750 m route in grassland habitat.

Kestrel diets were monitored by analysis of pellets found at roosts.

                              Patch Choice

          Although the term "patch" has been widely used in the litera­

ture, it is often ambiguous and poorly defined.    For perch hunting

birds, a patch can be operationally defined as the area that can be

hunted from a perch; thus, time in a patch and movement between patches

are easily measured.

          Net caloric intake can be increased by foraging in patches

where encounter rate with prey (N/t) is high.   For birds hunting from

perches, encounter rate is a function of prey availability and area

hunted.   Prey availability is some function of prey density, prey type,

vegetational structure, and weather.    One way encounter rate can be
increased is by hunting in areas where prey availability is highero      In

a fine-grained situation, patches must be visited for prey availability

to be assessed^   Thus, variations in prey availability would have

little effect on patch selection, although it would contribute sig­

nificantly to habitat selection*   Much of the area in which kestrels

and shrikes foraged appeared to be homogeneous so that birds probably

could not assess prey availability before visiting patches*

         Encounter rate can also be increased by hunting a larger area*

Area hunted can be increased by hunting in habitats with little vege­

tation so visibility is increased, and by using higher perches.   But

increasing perch height also increases foraging costs and handling

time of prey.   The following analysis of the geometry of perches and

vegetative structure on the terms of equations (1) and (2) suggests

that there exists an optimal height from which to hunt arid that patches

should be chosen on the basis of perch height and vegetative structure.

Geometry of Hunting from Perches

         Figure 1 is a model that provides a basis for estimating the

relative area of ground that is visible from a perch, where h equals

perch height, g is the. average height of grass clumps or other vege­

tation, d is the average distance between these clumps, and y is the

distance from a given clump to the base of the perch (y is a multiple

of d).   There is a distance, x, behind each clump where the ground is

not visible from the top of the perch.   This distance increases with

increasing distance of the clump from the perch until at some point

it equals the average distance between grass clumps and no ground is



Figure 1. Geometric considerations of perches and vegetation,

visible« Thus, ground area visible to a perched bird can be visualized

as concentric rings of decreasing width around a percho    Though the

width of each ring decreases with distance, the size increases so that

the area of each ring does not necessarily decrease„ Ring area as a

function of distance from the perch, depends on the average clump dis­

tance and height but, in general, increases to a point and then de­

creases = Some examples are shown in Figure 2o When grass clumps are

tall and closely spaced, very little ground is visible0

        By increasing perch height, a bird can hunt more area because

the distance behind each grass clump that is not visible decreases»

But the geometric properties are such that increasing increments of

perch heights result in successively small decreases in x For .the

area within a given radius around a perch, area that.can be hunted

increases in the manner shown in Figure J> and becomes asymptotic at

the maximum area within the specified radiuso

        This development of effects of perch height is based on simpli­

fied but robust assumptions«, Grass clumps or other vegetation obvious­

ly are not opaque and of even height, and do not occur in continuous

concentric rings at regular distances around perches=   The following

analysis also assumes that prey are flato    However, considerations of

the real properties of vegetation and prey have little effect oh the

qualitative aspects of the model which realistically indicates the

unavailability of some prey in vegetation*

        This analysis of perch geometry and area of the ground visible

from perches leads to the following prediction*

                           DISTANCE FROM PERCH

 Figure 2. Relative areas visible to a perched bird showing effects of grass
           height and density. — For curves A and B, g = 50 (tall grass)
           and d = 8 and 12* respectively. For curves C and D, g = 5 (short
           grass) and d = 8 and 12* respectively. Areas were calculated
           on the basis of a perch height (h) equal to 900. All numbers
           in cm.




  Figure 3* Approximate increase in visible area with increasing
            hunting height. — Dotted line represents maximum area
            visible within a specified radius around a perch (see
    Prediction 1: A greater proportion of attacks should occur at

greater distances from tall perches than from short ones because more

area is visible at greater distances,.

    Test of Prediction Is    Kestrels and shrikes made greater propor­

tions of attacks at greater distances from higher perches (Table 1)„

Distances traveled for prey were significantly shorter for shrikes

than for kestrels from perches of equal heights (for perches 

If prey are taken primarily from the ground, which appears to be a

valid assumption for kestrels and shrikes, rate of prey encountered per

search time (N/tg) should increase with height in approximately the

same manner as area that can be hunted (Figo 3)° But handling time
Table   lo   Effects of perch height on the distances traveled to prey
             for kestrels and shrikes»

                         Kestrels                       Shrikes
  Perch                  % attacks at:                   attacks at:
Height (m)     n    0-20 m 21-40 m >40 m     n    0-10 m 11-20 m >20 m
also increases with height because the time to attack and return to a

perch increases*     As handling time increases, search time decreases;

thus, encounter rate for the total time hunting (N/t) increases with

perch height to a maximum and then decreases, as shown in Figure 4*

The exact shape of the curve depends on the relative values of N/tg and

th/tSo Increasing prey density increases encounter rate per search

time (N/tg) but does not affect handling time; therefore, the perch

height where encounter rate is maximized decreases as prey density in­


           Increases in perch height also increase foraging costs*    Cost

to attack prey (a) increases with height because the cost to return to

the perch increases, though the cost of the drop from the perch to the

ground is probably negligible because it is gravity assisted*        It seems

reasonable that cost of an attack is directly proportional to height*

The increased height causes the energy gained per attack (E/A) in

Equation (1) to decrease as shown in Figure 5° If mean energy content

of prey were increased, the line in Figure 5 would shift'Upwards*

           If attack rate were proportional to encounter rate arid net

energy gain per attack decreased with height as outlined above, an

optimal hunting height, where the net rate on energy gained is maxi­

mized, could be found by multiplying the equations of the curves in

Figures 4 and 5* The result of such a multiplication is shown in

Figure 6*. The preceding analysis suggests that optimal hunting height

increases as mean prey size increases or as density decreases*

           In some cases, resting metabolic rate might have a significant

effect on optimal hunting height*     RMR varies with environmental


  Figure 4. Approximate prey encounter rate with increasing perch


Figure 5* Effects of increasing height on net energy gain per
          attack. — Line B is for larger prey.


Figure 6.   Rate of net energy gain as a function of hunting height.
            — This curve is obtained by multiplying net energy
            gain per attack (Fig. 5) and attack rate, which is
            assumed to be proportional to encounter rate (Fig. 4).
            See text for further explanation.
conditions, especially temperature and windo   At times of high winds,

RMR could increase due to heat loss or an increase in the effort re­

quired to remain on a percho   Because wind speed increases with height,

BMB should be greater on higher percheso   Birds could reduce this cost

by perching lower or in a more protected place, otherwise RMR is a

fixed cost for any given time or place
increase in cost is probably small in comparison to costs of making

attacks because only a horizontal flight is required*

Predictions and Tests

           The previous discussion of the factors influencing costs and

benefits of foraging from perches suggests that patch selection should

be based on perch height and vegetation density*    From this analysis I

make the following predictions*

   Prediction 2s     Areas of tall, dense vegetation should be avoided

because little ground is visible regardless of perch height and the

probability of prey escaping in the vegetation is high*

   Prediction 3s     Because larger birds generally take larger prey

than smaller ones, their optimal hunting height should be higher and

they should select higher perches*    Different-sized birds are not

strictly comparable, however, because energy to gain height is not the

sane * Nevertheless , female kestrels, which weigh about 110 g, would

be expected to perch the highest, male kestrels (about 100 g) slightly

lower, and shrikes (about 50 g) considerably lower than either sex of


   Prediction 4:     Optimal hunting height should increase as mean prey

size increases or as prey density decreases*

   Prediction 5:     If wind velocity is sufficient to increase ener­

getic costs due to heat loss or effort to remain on perches, optimal

hunting height should decrease with increasing wind speed because wind

speed is lower near the ground*

        Testing these predictions in the field was complicated by

several factors,.     Perches in nature rarely present birds with con­

tinuous choices of height« In the study area, fenceposts (lo5 to 2 m)

and utility poles and wires (8 to 10 m) were the most common and often

the only perches available„ Some perches were apparently not suitable

for reasons other than height,.    Neither kestrels nor shrikes were ever

seen perched on electric wires of utility poles; telephone wires were

always usedo   Both species also showed a definite preference for

perches that provided greater stability; wooden fenceposts were pre­

ferred to metal ones, utility poles or wires near poles were preferred

to wires midway between poles,.    Because these respective perches were

usually close in proximity and of similar height, however, these

preferences had little influence on perch height selection,,

    Test of Prediction 2:     Kestrels and shrikes clearly avoided hunt­

ing in areas of tall, dense vegetation,.   During the months when grass­

hoppers were abundant and were the primary food, kestrels and shrikes

were observed hunting only in areas of short grass even though grass­

hoppers appeared to be more abundant in areas with tall grasso     Avoid­

ance of areas of tall, dense vegetation was best demonstrated by

several observations of kestrels foraging sequentially along utility

wires that crossed an area of tall, dense grass (Sporobolus wrightii)

bordered by areas of short, sparse grasso     Upon reaching the area of

tall grass after foraging in the area of short grass, kestrels made

flights much longer than the usual distance between hunting perches

across the tall grass and resumed foraging in the area of short grass

on the other side,.

        Hunting in areas of tall, dense grass might be profitable if

higher prey availability or greater prey size compensated for the low

visibility0 Observations of a male kestrel hunting in a small clearing

in tall, dense grass in February, when insect prey were scarce, sug­

gested that such compensation may sometimes occur» After making a

number of aborted attacks near the edges of the tall grass, a cotton

rat (Sigmodon sp0) was capturedo    Cotton rats are among the largest

prey items that I recorded in the diets of kestrels in southeastern

Arizona and were very abundant in the tall grass areas that winter«,

        This observation also provided a possible example of hunting

height being affected by a decreased probability of success with an

increased distance from prey0 It seems reasonable that cotton rats

were exposed to capture only at the edges of the tall grass for short

periods of time» In order for an attack to be successful, the kestrel

would have to perch a short distance away to reduce the time to reach

the preyo    Even though utility wires were available nearby, the kestrel

hunted only from perches barely higher than the surrounding grass

(t 105 m)o    Such a reduction in hunting height is profitable only if

encounter rate is high or prey size is large=

    Test of Prediction J>% Mean perch height was highest for female

kestrels (7°5 m), intermediate for male kestrels (6=3 m), and lowest

for shrikes (5=0 m)»    Because perches were normally either fenceposts

or utility lines, perch differences are perhaps best shown by the per­

centage of times the birds perched higher than ?06 m (25 ft0)0 Kes­

trels perched at heights of 7=6 m or higher significantly more often

(53^9 n = 602) than did shrikes (3^, n = 217;      = 23=9, p < o001),

but there was no significant difference between male ( ,        n = 11?) and
female kestrels (52^, n = 485; ^       = 069, p > o90)= These patterns are

as expected for reasons of optimal hunting height, but I have other

data which suggest that perch selection also was affected by aggressive


        The most striking characteristic of perched kestrels was that

they chose the top of the tallest available perches*      Ninety-three per­

cent (n = 688) of kestrels observed were on the tallest perches avail­

able within a distance of 25 m*     Shrikes also perched frequently on the

highest perches but they did so a smaller percentage of the time (85#,

n = 359)   Selection of the tallest perches was most clearly demon­

strated from observations of birds on manipulated perches.      Both

kestrels and shrikes always chose the highest pole in a set (n = 14 and

40, respectively) even though the highest poles were of different abso­

lute heights in different sets.

        In many areas differences between the highest and lowest

perches were substantial, and perches of intermediate height were not

available, but even in areas where intermediate perches were available,

the highest perches were chosen.       Where telephone wires ranged from

heights of 6 to 9 m, the highest were chosen.      Likewise, in leafless

trees where an almost continuous range of perch heights was available,

kestrels and shrikes nearly always perched within 1 m from the top on

the highest good-sized branch.     Large leafy trees presented an inter­

esting situation.   To maximize area hunted, a bird should perch on the

side of the tree because from the top the view of the ground below

would be blocked by the foliage.       Kestrels consistently perched on the

sides of these trees rather than at the top0 Shrikes were not observed

in these treeso

        These observations suggest that optimal hunting height for

kestrels was usually higher than available perches®   This is also sug­

gested by the heights of birds using an alternative hunting techniques

hoveringo   Hovering height was usually between 1101 and l4®3 m (see

Chapter 3), higher than virtually all perches on the study area®

    Test of Prediction 4:   It is difficult to assess whether changes in

prey size or density affected perch height becauses   (1) optimal hunt­

ing height often appeared to be higher than available perches; (2) it

was difficult to assess changes in prey sizes and densities; and (3)

wind was a confounding variable = It is also possible that optimal

hunting height is primarily determined by the largest prey if these

account for a large proportion of the prey biomass®   However, field

observations indicated that from August through December diets of kes­

trels and shrikes consisted primarily of grasshoppers®   Grasshopper

populations showed a marked decrease during this time (Fig® 7)® Al­

though the grasshopper population Consisted of individuals of many body

sizes much of the year, most seen after August were large ( >2®5 cm)

and from September through December no marked change in their size was

apparent®   Analysis of kestrel pellets showed that the diet contained

more rodents toward the end of this period®   This decrease in prey

density for both kestrels and shrikes and the inclusion of more rodents

in kestrel diets should cause an increase in optimal hunting height®

Kestrels and shrikes perched more often on perches >8 m on days of wind





                   5 00

                     15 AUG I SEP   15 SEP I OCT    15 OCT I NOV    15 NOV I DEC   15 DEC

Figure ?•                 Index of grasshopper abundance in months of August through
                          December. — Points indicate census dates.
(Table 2) => Differences are significant; for kestrels,     = 8o3? P <

=005; for shrikes, X     = 19=0, p < o005o

           Other evidence for changes in hunting height due to changes in

prey density and size comes from data on hovering kestrels (Chapter 3) o

On several occasions kestrels hovering lower than usual were observed

apparently capturing small abundant prey iterns0 Also, when no attacks

were made on prey, successive hovers tended to be at increased heights

Suggesting that the birds' estimates of prey densities decreased and

hunting height was adjusted accordingly„ This may also explain obser­

vations by Pinkowski (1977) that bluebirds (Sialia sialis) moved to a
higher perch (n = 65) significantly more often (X = 8o3? p

Table 2=   Perch height related to time of year=

                        Time Period                Times Perched

                                                8 m

Kestrels           1 Septo=17 Octo               39            45

                  30 Octo~31 Deco                20            60

Shrikes           1 Septo-17 Octo                23             5

                  30 Octo-31 Deco                23            46

Table 3o The relation between perch height and wind speedo

                   Wind Speed (mph)      Times Seen at Perch Height

                                         0=3 m         4-7 m          >8 m

Kestrels                10               116          33             96

Shrikes                 10               100          13              7
Movement Between Patches

        In addition to choosing patches, foraging animals must make

decisions about moving between patcheso    In many cases movement between

patches is very complex because of the multidimensional nature and the

effects of patch boundaries (e0go, Pyke 1978)0 Probably for these

reasons few predictions or tests concerning movement between patches

have appeared, though Gharnov (1973) has discussed some theoretical

aspects of this topic and suggested that prey distribution is an im­

portant factoro    For kestrels and shrikes, hunting from utility lines

or fences in fairly homogeneous grasslands, movements between patches

are limited to one dimension; thus, aspects of between-patch movement

are simplified» Choices concerning movements between patches are re­

stricted to whether to return to the same perch after an attack, which

direction to go to the next perch, and how far to move*     Here I con­

sider only the latter two choices; the decision whether to return or

not is apparently complicated and will be discussed elsewhere» However,

kestrels and shrikes usually did riot return to the same perch after an

attempt for preye

        Net energy intake can be increased by decreasing the time and

energy spent traveling between patches (C of Equation 1)®    For birds

hunting from a line of continuous perches, I make the following predic­


   Prediction 6s    Kestrels and shrikes should forage unidirectionally

and should move only far enough between patches so that overlap with

adjacent patches is minimal®   Due to difficulties in calculating the

area that can be seen from a perch and the difficulties measuring
appropriate parameters in the field, I cannot predict actual distances

between patches,.     However, a qualitative prediction that can be made is

that distance between perches should be greater from higher perches

than low ones because more area is visible from each percho

      Test of Prediction 6: Kestrels and shrikes nearly always foraged

unidirectionaily along a line of continuous percheso      Only occasionally

did a bird return to a perch after a visit to a different one„      Dis­

tances between perches were significantly greater from tall perches

than short ones when kestrels and shrikes left without attacking prey

(Table 4; t = 3o5» P < =005; t = 6,1, p
Table 40 Effects of perch height on distances traveled between
         percheso — Only distances between continuous equal-height
         perches after giving-up times are includedo

Perch Height (m)      x Distance (m)» Kestrels    x Distance Cm), Shrikes

     2-3                         18*2                       15=5

     >7                          53=9                       68 oO

Table %    Effects of wind speed on distance traveled between perches*

                          x Distance (m) Between Perches for:
  Perch              Kestrels                        Shrikes
Height (m)   Wind 10 mph      Wind 10 mph

   2-3             23°2            13o4           13o9          13=2

   >7              67=5            35=1           38=9
tendency was noted for shrikes on low perches to hunt into the wind at

times of high wind speeds, and there was no difference in distance be­

tween these perches for times of high and low wind (t = 37* p >o30)o

Sample size of shrikes for distance between perches on high perches was

too small for analysis0

                       Allocation of Time in Patches

        Most studies of optimal allocation of time in patches concern

"giving-up timeso”     Giving-up time is the period waited since the last

capture before an animal leaves a patch*    Although there is general

agreement that giving-up times are derived from information from pre­

vious experience, the kind and quality of information animals use has

not been determined*    Charnov (1973) has proposed the marginal-value

theorem, a deterministic model that relies only on the mean times

waited in previous patches*    This model has recently been criticized

by Oaten (1977), who suggested that a stochastic    model, where an

animal uses the variance as well as the means, is necessary for optimal

foraging*   It also seems likely that information gathered while forage

ing in a patch may affect giving-up time*    For birds hunting from

perches, one such source of information may be assessment of prey that

are seen but not attacked*

        -Qiving-up time could be measured for kestrels and shrikes when

they left a patch without attacking prey*    My limited data on kestrels

and shrikes does not allow a determination of exactly how these birds

use past experience to determine giving-up time*    However, it seems

that part of the information used should be the means of some number
of times waited in previous patches before prey were attacked,.     There­

fore, ’I make the following prediction*

    Prediction 7:   Giving-up time should correlate with some number of

previous times waited for prey*

    Test of Prediction 7°   The mean of the last three times waited in

a patch before prey was attacked was a better predictor of giving-up

time than just the last time*   For kestrels r = =59 (n =17, p < o005)

and r = .14 (n = 32, p >.25), respectively; for shrikes r = .78 (n =

32, p
Table 6=   The effects of perch height on giving-up times for kestrels
           and shrikeSo

                     Perch Height (m)    h*   x Giving-up Time (s)

Kestrels                 0-3            34           146 o6

                                        34           18606

Shrikes                  0-3            72            67=7

                          >3            13           l83o4

 Includes only giving-up times of less than 600 s0

                  75                                  95

                  65                                  85 i
            5     55                                  75

            i 45
                                                      65 z>
            if)                                            o
            8  35                                     55   5
            Z)                                             5
                                                      0 lx
                            SO   ND    JF      M
                  75                                  280 «
                  65                                           4
                  55                                  220
            cr 45                                      190 %
            if)                                                 i
            if)                                                o
            O     35                                  160 >
            z>                                                 o
                                                      o     |x
                            SO   ND    JF   M
                           TIME PERIOD (MONTHS)
Figure 8   Success rates and lengths of giving-up times for
           shrikes (A) and kestrels (B) as functions of season.

is not unexpected because diets of both species varied during the year.

If diets shifted to smaller more abundant prey items as a preferred

prey decreased, giving-up. time would likely decrease even though over­

all prey quality decreased.   This appeared to be the case for shrikes

in. January and February; smaller prey were taken and giving-up time


                              Optimal Diet

        Optimal diet theory assumes that animals evaluate prey and make

decisions whether or not to attack each item encountered.   For birds

hunting from perches, evaluation can be based on prey size (e), energy

and time required to attack (a), or the birds' estimates of chances of

success (fs).   Time and energy to attack increase with and are affected

primarily by distance to prey.   As previously discussed, success rate

might also decrease as distance to prey increases.    Evaluation for

chance of success seems especially likely because of the high cost of

an unsuccessful attack.   Any evaluation of prey and subsequent selec­

tivity lowers the proportion of prey attacked (Pp).    From my observa­

tions, I am able to examine aspects of prey selection based on distance

to prey and chance of success.

        I am certain that kestrels evaluate prey.    Hunting birds often

showed evidence of sighting prey with behaviors normally associated

with attacks such as head-bobs, tail jerks, and plumage depression, and

then did not attack.   Shrikes showed similar behavior but less

        If kestrels and shrikes do not evaluate prey on the basis of

distance, the proportion of attacks made at any distance from a perch

should be proportional to the visible area at that distance= I cannot

evaluate whether this occurs because it requires a quantitative measure

of ground area that is visible as a function of distance from the

perch*   Topographic irregularities and vegetation opacity affect areas

that can be hunted and are difficult to measure in the field*   Even if

these problems are neglected, quantitative evaluation of even the

simple model presented earlier is too complicated to be practical

Table 7=    Distances to prey at different times of yearo

                        From P e r c h e s m High   From PerchesJ>8 m Hi^h
            Time               x Distance                  x Distance
           Period         n     to Prey (m)           n    to Prey Cm)

            1 Septo-
                         32        1106              31       25=2
            17 0cto
            30 Octo—
                         10        I606              kl       50=0
            31 Deco

            1 Septo™
            17 Octo                 5=2              3       15=0
            30 Octo«-                                        23=0
                         19        11 o2             32
            31 Deco
        Success rate did not decrease significantly with increasing
distance- of attack for kestrels or shrikes (Table 8? X = 6=2, p > o10;
X = o20, p o95» respectively)c The apparently lower success rate for.

kestrels at very great distances is due almost entirely to attacks on

birds (11 of l6)»   Either the probability of success did not decrease

with distance for the majority of prey or kestrels and shrikes evalu­

ated their chance of success and attacked only more vulnerable prey at

greater distances= Laboratory work by Sparrowe (1972) showed that

attack responses by kestrels were affected by prey exposure time0 This

suggests that evaluation of probable success is at least partly re­

sponsible for the constant success rate with distance0

        Evidence that prey are evaluated on the chances of success is

that success rates of kestrels decreased significantly with hunting

time on a perch (Table 9| X^ = 15=59 p p >=10)o The last two time categories for shrikes were

combined for analysis0 Distance to prey did not increase significantly

with hunting time for kestrels or shrikes (t = =62, p > 025? t = =29,

p > o40, respectively) o This pattern suggests that prey items are

evaluated on the basis of chance of success, and the threshold for an

attack diminishes with time=   This threshold apparently is renewed when

birds change patches= The exact manner by which the threshold is re­

newed cannot be determined from my data because success rate also

decreased significantly (X^ = 12=59 p < =005) with time since the last

capture for kestrels (Table 10; small samples precluded analysis for

Table 80 Effects of distance to prey on success rates of kestrels and

                                             Distance to Prey (m)
                                     0-20       21—40     41-60         >6o
             attempts successful       66         24        10            8
Kestrels     attempts unsuccessful     44         16         6           16
            % success                  60         60        65           33

                                     0-10      11-20       >20 '
             attempts successful       28         12         6
Shrikes      attempts unsuccessful     21          8         3
            % success                  57         60        67

Table 9°   Effects of time on perch on success rates of kestrels and
             attempts successful        50         21        15     .     7
Kestrels     attempts unsuccessful      25         16        13          22
            % success                   67         57        54          24

                                      0-4o     41-120      > 120
             attempts successful        19         18         8
Shrikes      attempts unsuccessful      12         13        13
            % success                   61         58        38
Table 10=   Effects of time since last prey capture on success rate of

                                        Time Since Last Capture (s)
                                       0-300       301-600     >600

attempts successful                       40            8         18
attempts unsuccessful                      8            7         19
% success                                 83           53         49
shrikeso     Renewal may be complete (Fig* 9a) or only partial (Figo 9b)

with each change of perch0

           The diminishing threshold model of evaluation of capture suc­

cess provides a simple mechanism for partial preferences in diets if

assessment of prey types changes on a short time scale in a manner

similar to the chance of successo    Most theories of optimal diet pre­

dict that animals should not show partial preferences? i0e0? a prey

type should either be included in the diet every time it is encountered

or not at alio    Pulliam (1974) has suggested that partial preferences

would be expected if dietary constraints were important or if the

predator’s assessment of prey densities changed during the time it

searched for prey« In a later review (Pyke et alo 1977), dietary con­

straints are discussed at some length but short-term assessment is not

mentioned*    The diminishing threshold shown by kestrels and shrikes

support the latter theory and suggest that dietary constraints may not

be necessary to explain partial preferenceso

         Kestrels also appear to evaluate escape strategies of prey0

Roest (1957) and Collapy (1973) have mentioned differences in attack

behavior for different prey types*    These were also noted in this study*

For insects, kestrels usually glided down from a perch with few wing-

beats; for attacks on rodents and lizards, flights from perches were

usually direct with many wingbeats as if to minimize time to reach the

prey; for birds, attack flights were fast and powered but kestrels

dropped quickly from the perch and completed the attack from grasstop

level*   The latter method suggests that surprise is important when

birds are attacked*                                 .'

                 a   perch change
                 ▲ perch c h ange after prey capture

                                                    TIM E

Figure 9«             Two possible mechanisms for threshold renewal. — In ’’A*' threshold renewal
                      occurs with each perch change regardless of prey capture; in "B” the
                      threshold is only partially renewed with each perch change and completely
                      renewed only after a prey capture.
         The above discussion suggests that birds can control their

success rate by varying their threshold of selectivity«, One factor

that influences this threshold is prey size„ If prey are small rela­

tive to the size of the predator, success rate must be high to forage

profitably, especially if the cost to attack each prey is higho       If

prey are large, a lower success rstte may be toleratedo

         Some data suggest that success rate    may be affected by anaver­

sion to the risk of   starvation or fallingbelow a positive energy

balance rather than   simply maximizing net energy: gaino   Figure 8shows

how success rates of kestrels and shrikes covaried with the lengths of

time waited in patches where no prey were attacked (giving-up times)o

If lengths of giving-up times are inversely     proportional to preyden­

sities, as suggested by Charnov (1975)» these data indicate    that

selectivity based on estimates of chances of success increases as prey

density decreaseso    Craig (1978) presentsdata for shrikes, showing a

similar relationship between prey density and success rate®    This

pattern conflicts with optimal foraging theories predicting selectivity

should decrease as prey density decreaseso     An explanation for this

pattern may be that when prey are scarce, birds minimize variance in

food intake by attacking only prey that have a high probability of

capture, even if such behavior may also lower the mean net energetic

gaino   In this way risk of starvation decreaseso   As food becomes less

plentiful and the probability of starvation increases, risk aversion

increaseso   This seems especially likely for selectivity based on

chance of success because of the high cost of an unsuccessful attacke

If prey reach a critically low level, this conservative strategy may
not be sufficient to provide the food requirements of the animalP At

such times, birds may be forced to take more risks and attack prey with

a low probability of capture success but a high energetic reward
Concurrent Goals

        As outlined in the introduction, all predictions were made

assuming the goal of maximizing net .energy, reward while foragingo

This goal seems reasonable for many animals (see Schoener 1971; Charnov

1973; Pyke et alo 1977), and the agreement between predicted and ob­

served foraging behaviors suggests it is appropriate for kestrels and

shrikes in winter0 Some aspects of prey selection, however, appear to

be influenced by risk avoidance = In some systems, other goals such as

escaping predators, searching for mates, maximizing a specific com­

ponent of the diet, or territoriality, may operate concurrently and

influence foraging behavior0 I do not believe that any of these sig­

nificantly influenced the aspects of foraging behavior discussed in

this papero

        Of the concurrent goals that might influence foraging behavior

of kestrels and shrikes, territoriality appears to be most likely0

Both species are territorial in winter (Cade 1955, Mills 1975, Miller

cited in Bent 1950)o One might argue that kestrels choose the tallest

perches to "advertize" territories or to better survey territories for

intruderso     Unidirectional foraging may be a mechanism to patrol ter­

ritory boundaries,.    But some patterns are not consistent with goals of

territorial defense*    Shrikes do not always perch on the highest

perches; kestrels perch on the sides of leafy trees where the area

that can be hunted is maximized, not at the top where intruders are

more easily located*    Behaviors associated with boundary conflicts

suggested that kestrel foraging behavior was little influenced by ter­

ritoriality*    Birds at territorial boundaries appeared to forage no

differently than others, even when a neighbor was nearbyQ Very little

time was spent in territorial interactions and rarely did birds fly

long distances to pursue an intruder» In the boundary disputes I ob­

served, an intruding bird was attacked only when it flew off a perch

after prey=   Neither bird involved appeared to notice the other until

movement occurred,.   Cade   (1955)   and Welty   (1962)   also have noted that

movement of an intruder is often necessary to elicit an attack from a

kestrel= This seems a reasonable method to defend a feeding territory

at relatively low costo      An intruder is no detriment as long as it

takes no prey from the territoryo       If an intruder is prevented from

capturing prey, it will be advantageous for it to forage elsewhere0

        Although kestrels and shrikes are occasionally preyed upon by

other raptors, it is apparently rare0 During this study the only,

attack on a kestrel that I witnessed was an unsuccessful one by a

Cooper* s Hawk (Accipiter cooperi)o      This attack occurred in an area of

fairly dense oak woodland? no Cooper*s Hawks were seen in the open

grasslandso   Kestrels showed little concern for other raptors except

to occasionally mob a Red-tailed Hawk (Buteo jamaicensus) or Prairie

Falcon (Falco mexicanus)o     One shrike showed some alarm when a Marsh

Hawk (Circus cyaneus) passed near but, except for attacks by kestrels

which appeared to be motivated by competition rather than predation,

no attacks on shrikes were observed,.

        Most data were taken at a time of year when searching for mates

was evidently of little importance= Some kestrels remained paired in

winter; these birds appeared to forage no differently than unpaired


        Although some particular component of the diet may be an espe­

cially important requirement for some species, it seems unlikely that

carnivorous animals would have to take certain prey types in order to

obtain essential nutrients.   Even if this were the case, the searching

behaviors studied here would be little affected.    At times, however,

kestrels appeared to search for a specific prey type.    In addition to

a kestrel apparently hunting Sigmodon in tall grass, on at least two

other occasions it appeared that rodents were being hunted specifically.

In these cases, kestrels hunted small areas for long periods.    It

appeared that a rodent had been sighted there previously and the kes­

trel was waiting for it to reappear.


        Foraging patterns of kestrels and shrikes are consistent with

predictions of a tactical model for ground-hunting raptors developed

from considerations of perch geometry and optimal foraging theory.

These patterns show that kestrels and shrikes can measure distance and

time and respond appropriately to quantities such as means and, perhaps,

variances.   These are not unexpected results.   Perhaps more important

than demonstrating that animals appear to be selected to optimize

foraging behavior is the demonstration of the uses of optimal foraging

theory as a tool to better understand animal behavior.    Optimal forag­

ing theory is certainly useful in understanding and examining decision­

making processes that enable animals to solve problems posed by alter­

native prey types with variable temporal and spatial distributions.      It

also shows great promise in analyzing and understanding community
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