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 Mills
THE 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. C -I j }v ______ __ f z /s ^ y 'T 'S ... 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.
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; 5
ACKNOWLEDGMENTS 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
TABLE 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 iv
LIST 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 v
LIST 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 VI
ABSTRACT 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*, vii
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^
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 1
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®
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 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
8 \ \ \ 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 O
11 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 time
13 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 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 environmental
15 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 the
23 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 speeds
2500 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? p
26 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 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 measuring
28 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
29 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
30 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
32 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
33 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 perch
35 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 Deco
37 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 for
38 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 38
39 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 49
4o 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 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
k5 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 oneSo
46 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 community
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