1979 GREGORY SCOTT MILLS ALL RIGHTS RESERVED
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© 1979 GREGORY SCOTT M I L L S A L L R I GH T S RESERVED
FORAGING PATTERNS OF KESTRELS AND SHRIKES AND
THEIR RELATION TO AN OPTIMAL FORAGING MODEL
by
Gregory Scott Mills
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF ECOLOGY AND EVOLUTIONARY BIOLOGY
In Partial Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
19 7 9
Copyright 1979 Gregory Scott MillsTHE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my
direction by ___________ Gregory Scott Mills______________________
entitled FORAGING PATTERNS OF KESTRELS AND SHRIKES AMD THEIR
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.
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... u^u_ . ' v. . 'Lz-v ________ Jijzc* 7 g____
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Final approval and acceptance of this dissertation is contingent
on the candidate's adequate performance and defense thereof at the
final oral examination.STATEMENT BY AUTHOR
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; 5ACKNOWLEDGMENTS
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 study0TABLE OF CONTENTS
Page
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
ivLIST OF ILLUSTRATIONS
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
vLIST OF TABLES
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
VIABSTRACT
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*,
viitimes 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 whenhovering 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^
CHAPTER 1
INTRODUCTION
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
1this, 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 ofthese 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®CHAPTER 2
PATTERNS OF HUNTING FROM PERCHES
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
Methods
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 beincreased 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 is8
\
\
\
h
\\
\\
Figure 1. Geometric considerations of perches and vegetation,9
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*AREA
VISIBLE
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.
H
O11
AREA
VISIBLE
HEIGHT
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
text).12
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 time13
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 malso 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
creases*
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 environmental15
RATE
ENCOUNTER
PREY
HEIGHT
Figure 4. Approximate prey encounter rate with increasing perch
height.16
PER ATTACK
GAIN
NET ENERGY
HEIGHT
Figure 5* Effects of increasing height on net energy gain per
attack. — Line B is for larger prey.17
GAIN
ENERGY
NET
OF
RATE
HEIGHT
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.18 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
19
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
kestrel*
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*20
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,.21
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),22
but there was no significant difference between male ( , n = 11?) and
p
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
interactionso
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 the23
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
speeds2500
2000
OF GRASSHOPPERS
1500
1000
NUMBER
5 00
15 AUG I SEP 15 SEP I OCT 15 OCT I NOV 15 NOV I DEC 15 DEC
DATE
Figure ?• Index of grasshopper abundance in months of August through
December. — Points indicate census dates.25
(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
2
higher perch (n = 65) significantly more often (X = 8o3? p26
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 7Movement 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
tion®
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 measuring28
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, p29
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=930
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 numberof 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, p32
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 s033
75 95
<
I
65 85 i
if)
LU
5 55 75
i 45
if)
65 z>
i
if) o
LU
8 35 55 5
Z) 5
if)
0 lx
SO ND JF M
B
75 280 «
I
I
65 4
250
if)
LU
55 220
LxJ
f-
cr 45 190 %
if) i
if) o
LU
O 35 160 >
U
z> o
if)
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.34
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
decreased.
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
obviously.
If kestrels and shrikes do not evaluate prey on the basis of
distance, the proportion of attacks made at any distance from a perch35 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
36
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
Kestrels
30 Octo—
10 I606 kl 50=0
31 Deco
1 Septo™
22
17 Octo 5=2 3 15=0
Shrikes
30 Octo«- 23=0
19 11 o2 32
31 Deco37
Success rate did not decrease significantly with increasing
p
distance- of attack for kestrels or shrikes (Table 8? X = 6=2, p > o10;
2
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 for38
Table 80 Effects of distance to prey on success rates of kestrels and
shrikeso
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
shrikes300
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 3839
Table 10= Effects of time since last prey capture on success rate of
kestrelso
Time Since Last Capture (s)
0-300 301-600 >600
attempts successful 40 8 18
attempts unsuccessful 8 7 19
% success 83 53 494o
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* .'OF SELECTIVITY
THRESHOLD
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.42
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 maynot 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 nok5
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
oneSo46
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.
Conclusions
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 communityYou can also read
