SOME PREY PREFERENCE FACTORS FOR A L. SNYDER

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1 SOME PREY PREFERENCE FACTORS FOR A RED-TAILED HAWK RN L. SNYDER SEW AL studies have reprted selectin against cnspicuus prey. Dice (1947) reprted differential selectin against cnspicuus phentypes f mice (Permyscus maniculatus) and Kaufman (1974a) shwed that Barn Owls (Tyt alba) and Screech Owls (Otus asi) selected against cnspicuus phentypes f ld field mice (Permyscus plintus). Metzgar (1967) reprted that Screech Owls preferred transient white4ted mice (Permyscus leucpus) ver resident nes in a labratry experiment. Metzgar suggested that transient mice may be mre active and thus mre cnspicuus t a predatr than residents. Prey activity was suggested as a cnspicuusness factr by Cushing (1939) and Ingles (1940). Kaufman (1974b) reprted supprt fr a prey activity explanatin. He allwed Barn Owls t select either an active (live) r inactive (dead) huse muse (Mus musculus) in a field enclsure. The wls preferred significantly mre f the live mice. The purpse f the present study was t examine further the rle f activity in prey selectin. The first f three experiments reprted here examined the rle f prey activity when a Red-tailed Hawk (Bute jamaicensis) was ffered a chice between tw live prey animals. The secnd experiment examined changes in prey activity preferences when the hawk was ffered tw cmparatively large prey animals. In the third experiment the hawk was ffered tw prey f different weights, t determine if this wuld affect the selectin against mre active prey. METIIODS One adult male Red-tailed Hawk was used, hused in an utdr 8.0 X 7.5 m enclsure 2.3 m high. The sides f the enclsure were cvered with plywd and the rf was cvered with wire mesh. The hawk was perched n a dwel 1 m high at the nrth end f the enclsure. Tw plywd prey pens were placed in the crners f the suth end f the enclsure. These measured 2.6 X 1.3 m with 0.6 m high sides. Plexiglass was used in the end facing the hawk in rder t give the hawk maximal visibility f the prey items. Tw metal tubes 2.5 m lng and 15 cm in diameter were psitined s that prey items culd be simultaneusly pushed int the prey pens frm utside the enclsure. The flrs f the prey pens were marked ff in 10-cm square grids. Huse mice (Mus musculus), dmestic chicks (Gallus dmesticus), and labratry rats (Rattus nrvegicus) were used as prey. During each prey presentatin trial, an hserver, statined in a blind abve the suth end f the enclsure, recrded the number f grids each animal crssed until ne was taken by the hawk. This terminated the trial. The prey item chsen, the latency t strike, and the ttal grid crssings fr bth prey items were recrded fr each trial. Grid 547 The Auk 92: July 1975

2 548 Rw L. S ryd [Auk, Vl ' I' Mre Active Less Active 15 Mice Chicks Rts Mice xchicks p=(o.01 p=(o.01 n,s. p=(o. 0I Fig. 1. Prey chices as a functin f prey activity fr fur cnditins. Open bars are mre active animals and clsed bars are less active animals. Significance determined by a sign test fr tw prprtins (Bruning and Kintz 1968). crssing was cnsidered an index f activity f the prey animals. The prey item nt chsen was remved while the hawk ate the chsen animal. EXPERIMENT 1 T determine if the hawk preferred the mre active prey, 84 trials were cnducted in 4 series. Prey were randmly assigned t prey pens and thse nt chsen n a given trial were used during later trials. N prey animal was used fr mre than fur trials. In the first series, 20 trials were run using adult huse mice as prey. Tw trials, 6 h apart, were cnducted every day fr 10 days. Mice were presented in same-sex pairs with n weight difference between tw mice n a given trial exceeding 4 g, The hawk maintained its weight at apprximately 850 g thrughut this and the fllwing prcedure. In the secnd series f trials 20 pairs f 5- t 15- day-ld chicks were presented. Chicks were matched by weight (apprximately 35 g) but nt by sex. In rder t magnify the differences in activity levels between chicks, a 0.5 mg/kg intraperitnea] injectin f chlrprmazine (CPZ, a majr tranquilizer) was given t ne f each pair f chicks 15 min befre the trial. The psitin f the injected chick was randmly alternated thrughuthis series. The third series f trials was cnducted with 24 pairs f rats. They were matched by sex and weight fr each trial. Rats weighing frm 125 t 385 g were used, but n any single trial the difference between the tw rats never exceeded 12 g. The hawk's weight during the rat trials was held t 864 g (-+ 5 g) by hlding its intake t apprximately 110 g f whle rat per day. T maintain this intake level, ne trial was given every 3 days fr the heaviest rats and ne trial per day with the lighter rats. This methd was cnsidered preferable t remving half-eaten carcasses because remval may have disrupted the hawk's natural feeding behavir. The furth series f trials tested activity preferences between mice and chicks. One chick and ne muse were pre-

3 July 1975] Red-tailed Hawk Prey Preference 549 TABLE 1 MEAN GRID CROSSINGS OR PREY ANIMALS D'URING 84 ACTIVITY PREFERENCE TRIALS Prey Mre active Less active Mice (N - 20) Chicks (N: 20) Rats (N = 24) Mice and chicks (N -- 20) sented n each trial. Twenty trials with randm psitining f these species were cnducted using the same prcedure as the first three series. Only 5- t 8~day-ld chicks were used t keep the weight differences between species within 5 g. A 0.5 mg/kg CPZ ip injectin was given t the mice n ten trials t reduce their activity, as the mice were nrmally much mre active than the chicks. Results.--On all but the third series f trials (rats) the hawk chse significantly mre f the higher activity animals (Fig. 1). The preference fr the mre active f tw prey items was cnsistent even when CPZ treated; less active animals were nearly immbile r behaving as if sick r dying. The mean number f grid crssings fr high and lw activity animals is shwn in Table 1. Mean latencies t strike in minutes were 3:25 fr mice, 3:54 fr chicks, 4:20 fr rats, and 3:45 fr mice vs. chicks. These differences were nt significant. There was n significant psitin preference fr either prey pen, with 39 chices n the left pen and 45 n the right. Further analysis f the rat chice data (series 3) revealed a lwer mean weight (141 g) f rats that were high activity chices. The mean weight fr rats in lw activity chices was 220 g. In this series the hawk apparently chse the less active rat when the chice was between heavier animals. In rder t explre this pssibility further, anther experiment was perfrmed. EXPERI1VIENT 2 Using the same delivery and recrding system as abve, the hawk was presented with 18 pairs f female rats. The rats were divided int six weight classes in 50-g increments, frm < 100 g t > 300 g. Three pairs were presented in each weight class n a semi-randm schedule. Chices were recrded and these data were cmbined with the rat data frm experiment 1 by including rats that fit the apprpriate weight classes. RES rt. Ts.--Fig. 2 shws the relatinship between rat weight, activity, and hawk preferences. Generally the hawk tk the mre active rat unless their weights exceeded 200 g. In chices invlving rats ver 200 g the hawk significantly preferred the less active animal (P: <0.05). The latencies fr sme heavy rats exceeded 10 min; n latency beynd 7 rain was recrded fr rats <200 g. Fig. 2 als shws greater variability f chice with heavier rats. This may be due t a reductin f the high weight-lw activity preference ver several trials. In experiment 1 n higher activity, >250 g rat was ever taken, but in the last experiment, tw such rats were chsen. EXPERI1VIEN T 3 Twenty pairs f chicks were used, each pair cnsisting f a cmparatively large CPZ treated (less active) chick and a cmparatively small, untreated chick. Weight

4 550 RaN L. SNYDER [Auk, Vl. 92 x x x x x x x x x x x x x x x x x x (100 I _00 2_50 > Weight Classes (grams) Fig. 2. Prey chices as a functin f the weight f rats. Circles are Experiment 1 rats and crsses are Experiment 2 rats. differences were never less than 55 g. The mean weights fr the chicks were 42 g fr the smaller and 117 g fr the larger. The chicks were intrduced int the prey pens as in the previus experiments and the side with the large chick was randmized. As the rle f size preference xvas t be tested independent f activity preference, all f the large chicks were injected with 0.5 mg/kg CPZ ip t hld their activity levels belw nrmal. RESXZLTs. The hawk chse the larger chick in 15 trials and the smaller chick in 3 trials. This difference was significant (P = <0.01). Tw trials were terminated befre a chice because the large chicks escaped frm the prey pen. Mean latencies t strike were nt significantly different frm the chick latencies in experiment 1. Activity levels were higher in the small chick grup with a mean number f grid crssings f 214 per trial while the mean crssings fr the large chicks was 85. The hawk tk ne large chick that was cmpletely immbile and lying n its back. This was the nly instance f a preference fr immbile prey in all three experiments. DISCUSSION In the first experiment the hawk preferred the mre active f tw prey animals when n ther differences were apparent between them. Relative activity may have perated by itself as a prey cnspicuusness factr in this experiment. This result extends the findings f Metzgar (1967) and Kaufman (1974b), wh suggested that activity played sme rle in the

5 July 1975] Red-tailed Hawk Prey Preference 551 selectin f prey by the wls in their studies. White and Weeden (1966) als reprted that the activity f Willw Ptarmigan (Lagpus lagpus) appeared t attract Gyrfalcn (Falc rusticlus) predatin. These authrs nted that ptarmigan freeze when a Gyrfalcn alights near them. Freezing behavir may be an example f a prey respnse t predatr selectin against cnspicuusly active prey. The mechanism respnsible fr the hawk's high activity preferences in the present study may be a respnse t a stimulus chain set up by the prey. Once a predatr has begun attending t a given prey animal, its intentin mvements tward the prey r sme physilgical respnse, such as arusal, may reinfrce further attentin and eventual attack. The hawk's attentin rarely appeared "lcked n" t the mre active prey during the trials. It appeared t cmpare bth animals, stepping frm ne side f the perch t anther, watching each ne clsely. The attacks were sudden and difficult t predict by the bserver; further research int predatr preferences culd investigate physilgical changes during these cmparisns. If chice is related t arusal, the heart rate, fr instance, culd be mnitred while the hawk cmpares the available prey items. The results f the secnd experiment may suggesthe existence f a mechanism in the Red-tailed Hawk that is related t the ptential frmidability f its ptential prey. A large rat is certainly capable f injuring a raptr the size f a Redtailed Hawk and there may be sme advantage t selecting the less active and therefre pssibly less frmidable ppnent. Over many trials this preference fr the less active animal may be replaced by a high-activity preference if the hawk is successful in subduing these larger animals. The variability in the heavier weight classes in experiment 2 may be explained n this basis. Further experiments are needed t examine this questin. Experiment 3 shwed a clear preference fr heavier, less active chicks. Cmparing the chick data in experiment 1, shwing a strng preference fr the mre active prey, with the third experiment where the larger chicks were less active and still preferred, may demnstrate a tendency in the hawk t selecthe apparently mre prfitable prey item in terms f relative bimass. Testing these preferences under different cnditins f fd deprivatin wuld determine the pint at which a prey animal becmes t large and preferences switch t the smaller prey. Financial supprt was prvided by the Envirnment and Man Prgram grant V at Utah State University, Carl Cheney f the Psychlgy Department, Principal Investigatr. I thank Dr. Cheney fr supplying prey animals and valuable assistance in preparing the manuscript.

6 552 Rw L. S YDER [Auk, Vl. 92 LITERATURE CITED BRIJNII 'G, J. L. AND B. L. KINTZ Cmputatinal handbk f statistics. Glenview, Illinis, Sctt, Fresman and C. CusI ING, J. E., JR The relatin f sme bservatins upn predatin t theries f prtective clratin. Cndr 41: DICE, L. R Effectiveness f selectin by wls f deer mice (Permyscus maniculaius) which cntrast in clr with their backgrund. Cntrib. Lab. Vert. Bil., Univ. f Michigan 34: INGI ES, L.C Sme bservatins and experiments bearing upn the predatin f the Sparrw Hawk. Cndr 42: KAUFMAN, D.W. 1974a. Differential predatin n white and aguti Mus musculus. Auk 91: KA rf AN, D. W. 1974b. Differential predatin n active and inactive prey by wls. Auk 91: METZ AR, L.M An experimental cmparisn f wl predatin n resident and transient white-fted mice (Permyscus levcpus). J. Mammal. 48: WtIITE, C. M., AND R. B. WEEDON Hunting methds f Gyrfalcns and behavir f their prey (Ptarmigan). Cndr 68: Institute f Animal Behavir, Department / Psychlgy, Utah State University, Lgan, Utah Accepted 11 July 1974.

The Effect of Various Types of Brooding on Growth and Feed Consumption of Chickens During the First 18 Days After Hatch

The Effect of Various Types of Brooding on Growth and Feed Consumption of Chickens During the First 18 Days After Hatch The Effect f Varius Types f Brding n Grwth and Feed Cnsumptin f Chickens During the First 18 Days After Hatch H. G. BAEOTT AND EMMA M. PRINGLE Animal Husbandry Divisin, Bureau f Animal Industry, Agricultural