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Home , Accipitrinae, Greater spotted eagle, Hieraaetus

OBLIGATE AND FACULTATIVE BROOD REDUCTION IN : AN EXAMINATION OF FACTORS THAT INFLUENCE FRATRICIDE

THOMAS C. EDWARDS,JR. •'2 AND MICHAEL W. COLLOPY3 •Departmentof Biology,University of New Mexico,Albuquerque, New Mexico87131 USA, and 3Schoolof ForestResources and Conservation, 118 Newins-Ziegler Hall, Universityof Florida, Gainesville,Florida 32611 USA

ABSTRACT.--Fratricidein eagles (: ) is either obligate[second hatched nestling (C2) always dies] or facultative (C2 occasionallydies) and appearsto be a function of relative size differencesbetween siblings.Several factors,including time between hatch- ing, differencesin hatching weights, and possiblythe sex of the nestling in conjunction with its hatching sequence,influence size difference. Size differencesare modified further by feeding rates of young by adults. These factors determine the relative differencesin locomotordevelopment and coordinationand thus the ability of the first-hatchednestling to control accessto and monopolize parent-provided resources.Significantly greater (P < 0.0001) within-clutch volume differences(an indicator of disparity in hatching weight) and greater time between hatching (mode of 3 versus 2 days) are found in eagles in which fratricide is obligate (three )than in those in which fratricide is facultative (eight species).We suggestthat these differencesare proximate mechanismsthat accountfor ob- served differencesin the frequencyof fratricide in eagles.Although size differenceis deter- mined in part by the sex and hatching sequenceof the nestling, the role of offspring sex in fratricide is not clear. Received1 November1982, accepted 21 March 1983.

IN many speciesof the older, dominant factor leading to fratricide. Size difference de- nestling (hereafterreferred to as C1) is instru- termines the ability of a nestling to dominate mental in causing the death of its younger, and outcompete its smaller sibling(s) (Lack subordinate(C2) sibling [e.g. South Polar Skua 1954). As nestlings age and gain weight, size (Catharacta maccormicki),Proctor 1975; Brown difference typically decreases.We suggestthat Booby (Sula leucogaster),Dorward 1962; many fratricide becomesprobable when size differ- speciesof raptors,Meyburg 1974,Stinson 1979]. ence increasesand exceedsa hypothetical frat- The death of C2 may result from direct aggres- ricide threshold. Before this threshold is sion by C1, but more often indirect harassment reached, size difference is such that dominance and intimidationprevent C2 from obtainingthe is indeterminate. This does not imply a lack of resourcesnecessary for growth and survival dominance when only slight differencesexist (e.g. food, brood time). This form of brood re- (see Poole 1979) but rather that the outcome of duction, termed fratricide, is observedin many sibling aggressiveinteractions may vary. Nei- birds of prey. Within this group, it is mostcom- ther sibling can effectively dominate the other. mon in eagles(Accipitridae: Accipitrinae) with Fratricide is unlikely unless size difference is a clutch size of two. Published literature sug- great enough to ensure exclusion of C2 from geststhere are two categoriesof fratricide:an parent-provided resources.For theoretical con- obligateform where C2 invariably diesas a re- siderations of fratricide see O'Connor (1978) and suit of fratricide, and a facultative form where Stinson (1979). C2 occasionally dies (Table 1). Size difference is a function of several factors The relative size difference of siblings (cal- acting in concert. Here, using Golden Eagles culated as Dt/weight C2, where Dt = sibling (Aquilachrysaetos) as a test species,we examine weight difference) is an important proximate several factors that we feel are important in understanding fratricide in eagles. We have used data from the literature, museum speci- 2 Present address: School of Forest Resources and mens, field studies, and, where available, from Conservation, 118 Newins-Ziegler Hall, University other eagles. Comparisons also are made be- of Florida, Gainesville, Florida 32611 USA. tween the eaglesgrouped in the two categories

630 The Auk 100: 630-635. July 1983 July 1983] Fratricidein Eagles 631

TABLE1. Speciesof eaglesexhibiting either obligoteor facultative fratricide as defined in text.

Type of Species fratricidea Hatch intervalb Source Crowned O 3 Brown and Amadon (1968) (Stephanoaetuscoronatus) Verreaux's Eagle O 3 Meyburg (1974) ( verreauxii) Brown and Amadon (1968) Lesser-spottedEagle O 3-4 Meyburg (1974) (A. pomarina) Brown and Amadon (1968) Greater-spottedEagle F 2 Brown and Amadon (1968) (A. ) F "several days" Brown and Amadon (1968) (A. rapax) Imperial Eagle F 2 Brown and Amadon (1968) (A. heliaca) F 2 Brown and Amadon (1968) (A. chrysaetos) This study Wedge-tailed Eagle F 1-4 Brown and Amadon (1968) (A. audax) African Eagle F 3 Brown (1952) (Hieraaetusfasciatus) BootedEagle F "several days" Brown and Amadon (1968) (H. pennatus) Bald Eagle F 2 Brown and Amadon (1968) (Haliaeetusleucocephalus) O - obligate, F facultative. Represents mode in days. of fratricide to determine whether or not pos- establishesdominance with a minimum of ag- sible differencesin proximate mechanismscan gressiveinteractions. Both Meyburg (1974) and account for the observed differences in the fre- Newton (1977) consider hatch interval and its quency of fratricide. effect on size difference an important factor in fratricide.

METHODS AND RESULTS One result of hatching asynchronyin eagles is the establishment of a feeding hierarchy Hatchinterval.--Comparing the categoriesof among the young whereby C1 can control ac- fratricide reveals that, in general, hatching in- cessto the female and feed first (Meyburg 1974, terval is greater when fratricide is obligate Collopy 1980). The presentationof food to the rather than facultative (Table 1). The effect of young in small,easily controlled bits allows C1 time between hatching on size difference can to deny C2 accessto the female and to ensure be seen in a comparison of 1-, 2-, and 3-day adequatefood for itself. As C1 becomessatiat- hatch intervals in 12 broods of Golden Eagles ed, its response to a begging attempt by C2 studied in southwesternIdaho (Fig. 1). Nest- becomesless aggressive.An analysisof time- ling weight measurements,used to determine lapse photography and over 1,950 h of brood- size difference,were taken every 4-6 days dur- rearing observationsfrom blinds showed that ing brood rearing (Kochert 1972,Collopy 1980). parentsmade no effort to distributefood equal- The percentagesize difference was calculated ly between the nestlingsand did not interfere from sibling weight difference/weightC2. Note where C1 responded aggressivelytoward C2 that the peak value in size difference increases (Collopy 1980). Thus, if feeding ratesare low, as time between hatching increases.Although siblings do not receive equal sharesof food, not examined here, the intensity of sibling and C2, receiving less, will exhibit slower aggressionis probably greater when size dif- growth than C1. Relative size difference can ference is small and dominance less determi- rapidly increase. nate (D. Mock, pers. comm.).A large difference Hatching weight.--Data on differences in 632 EDWARDSAND COLLOPY [Auk,Vol. 100

FEMALE-FEMALE FEMALE-MALEC

20

MALE-FEMALE

AH=[

do •'o AGE OF C2 (DAYS)

Fig. 1. Effectof nestlingsex, hatching sequence, and time betweenhatching on siblingsize difference in 12 broodsof Golden Eaglesin southwesternIdaho. Arrows on horizontalaxes indicate point in a particular broodwhere siblingsapproach equality in size.Note verticalscale difference in Fig. lb.

hatching weights of sibling eaglesare limited. weight properly, one should know the laying The lower relative hatching weight of C2 is sequencein each museum egg set. Except for documented for Verreaux's Eagle (Aquila ver- Verreaux's Eagle, however, this information is reauxii;Gargett, unpubl. data) and the African unavailable. Thus, our results represent a mag- Hawk Eagle(Hieraaetus fasciatus; Meyburg 1974) nitude of volume difference within a clutch, and is suggestedfor other large eagles (New- and, although inappropriate for testing hy- ton 1977). One possible indicator of hatching pothesesregarding hatch-weight differencesin weight is egg volume (seeParsons 1970, Howe particular species, the results can be used to 1976). Egg-volume data were obtained from comparepotential hatch-weight differencesbe- museum specimensfor eight speciesof eagles tween eaglesin which fratricide is obligate and exhibiting facultative fratricide and three those in which it is facultative. This compari- speciesin which fratricide is obligate. Volume son shows that significant differencesdo exist: was estimated from the equation V = 0.51LB2, those speciesin which fratricide is obligate ex- where L = length and B = breadth of the egg hibit greater within-clutch volume difference (Hoyt 1979). The equation (volume difference than do those defined as facultaive (n = 11, P < between eggs/volume smaller egg) was used to 0.0001, Kruskal-Wallis single-factortest). calculate the percentageof volume difference Nestlingsex and hatching sequence.--In a brood within each museum clutch examined (Table in which both nestlings obtain sufficient food 2). to maintain maximum growth, size difference To test the effect of difference in hatching peaksslightly after C2 hatches,then decreases July 1983] Fratricidein Eagles 633

TABLE2. Analysis of within-clutch egg-volume difference for eagles exhibiting fratricide.

Mean percentage Type of of volume fratricidea Species nb difference SE 0 Aquila pomarina 34 13.04 0.27 0 A. verreauxii 49 c 12.08 1.00 0 Stephanoaetuscoronatus 10 11.93 0.26 F A. chrysaetos 36 5.37 0.66 F A. heliaca 36 4.91 0.55 F A. audax 20 4.84 0.74 F Hieraaetusfasciatus 30 4.83 0.60 F A. rapax 35 4.36 0.67 F Haliaeetusleucocephalus 35 3.57 0.46 F A. clanga 15 3.52 0.78 F Hieraaetuspennatus 14 3.09 0.50 O obligate,F = facultat•ve. Number of two egg clutchesexamined. 42 clutchesfrom Gargett(unpubl. data).

as the nestlings age and gain weight. The rate difference in casethree is based upon a weight at which size difference decreases, however, measurementtaken 1 day after C2 died (age at depends in part on the sexual compositionof death = 14 days, calculated from analysis of the brood and the sequencein which the nest- time-lapse photography). Examination of both lings hatch (Fig. 1). To determine nestling sex, the dead nestlingsrevealed numerousscars on we useda combinationof body weight and foot- and around the head, neck, and back, indicat- pad length (Kocherr 1972, Collopy 1980). A fe- ing fratricide had occurred. Necropsies per- male-male sequence is relatively shallow in formed on C2 in cases two and three revealed slope, and, due to size dimorphism, the differ- that the nestlingswere males.Based upon body ence does not approach zero (Fig. la). In con- weight and foot-pad length measurements,the trast, a male-female sequenceresults in a size two nestlings that subsequently fledged were difference of short duration and a relatively female. The third was a male that hatched 5 steep slope (Fig. lb). By 35 days of age females days before his sibling. typically had overtaken and passedmale sib- lings in weight. Single sex broods (Figs. lc, d) DISCUSSION were intermediate in value. Similar results are predicted for other dimorphic eagles. The differential mortality associatedwith The duration of the peak value in size dif- asynchronoushatching is consideredan adap- ference affectsthe probability of fratricide oc- tive mechanismthat increasesparental repro- curring. In Golden Eagles,peak size differences ductive successwhen food is limited (Lack 1954, occurred 6-15 days after C2 hatched, a time 1968, but see Clark and Wilson 1981). Benefits frame coinciding with our three observations associated with fratricide as a means of brood of fratricide in this species.Before the occur- reduction will vary from year to year. For this rence of fratricide, size differences at the last reason mechanisms capable of yearly adjust- nest visit before death equaled 98% [hatch in- ment are adaptive. Year-to-year adjustment of terval (H)= 5 days], 251% (H = 3), and 329% hatch interval, differences in sibling hatch (H = 3). The small size difference in case one weights, and possibly manipulation of off- may be a result of limited sampling. C2 was 3 spring sexand hatching sequencecan establish days of age when weights were taken, and, conditionseach breeding seasonwhereby frat- when the nest was visited 6 days later, C2 was ricide can be facilitated and parental reproduc- dead. Remainson the nest and previous obser- tive success maximized. vations of active aggressionby C1 toward C2 Manipulation of offspring sex and hatching suggested fratricide. In case two, fratricide oc- sequence(see Howe 1976, Ankney 1982) may curred when C2 was 9-13 days of age. The size be one adaptive mechanism that can maximize 634 EDWARDSAND COLLOPY [Auk,Vol. I00 yearly reproductive success.A comparisonof sure that fratricide will occur and minimize en- Figs. la and b shows that a female-male se- ergy waste. The greater reproductive success quence increasesnot only size difference but associatedwith a two-egg clutch may counter- also its duration. This suggeststhat fratricide balanceany selectivepressure toward reducing may be more likely in a female-male brood. clutch size from two to one. These factorssug- Breeding females,if able to "perceive" current gestthat, if parentscannot raise more than one environmental conditions,may manipulate off- offspring, it may be more adaptive to evolve spring sex and hatching sequencein a manner mechanismsthat virtually ensurethat the brood that facilitates fratricide if brood reduction be- will be reduced through fratricide rather than comes necessary.In years of poor prey avail- through reduction in clutch size. ability, a second-hatchedmale, whose growth On the basis of Hamilton's (1964) fitness ar- rate decreasesas a result of decreasedfeeding gument, sibling aggressionand parental be- opportunities,rapidly increasessibling-size dif- havior suppressingthese interactions should be ference. Requirements for fratricide can be apparent.This suggests,due to the active par- quickly met, and brood reduction can occurbe- ticipation of C1 in the death of C2 during frat- fore parents invest too much energy into the ricide, the possibility that a parent-offspring younger sibling. conflict (sensuTrivers 1974) might exist.Paren- In addition to the possiblemanipulation of tal suppressionof sibling aggressionis report- offspringsex to facilitatefratricide, females that ed for South Polar Skuas, Sandhill Cranes (Grus overwinter poorly and who are taxed energet- canadensis),and both White (Suladactylatra) and ically may increasethe time between hatching. Brown (S. leucogaster)boobies (cf. O'Connor The poorer a female's condition, the longer it 1978). This led O'Connor (1978) to conclude takes to produce an egg (King 1973). The in- that discriminationagainst smaller nestlingsis crease in hatch interval increasessibling-size practiced only by larger siblings and not by difference, establishing a situation whereby adults. It is important to note, however, that fratricideis easilyfacilitated if prey availability the parents'ability to suppresssiblings aggres- for the nestlingsis poor. sion in these speciesdepends upon adequate Greater within-clutch differences in egg vol- space for segregation of the young. Further- ume (indicating greater disparity in hatching more,suppression of sibling agonisticbehavior weight) and longer hatch intervals are major through brooding (Young 1963, Meyburg 1974) differencesbetween speciesin which faculta- is temporary and does not necessarilyprevent tative and obligate fratricide occur. The extra the occurrence of fratricide or imply that day of growth, coupled with a greater differ- brooding is part of a parental strategy to de- ence in sibling hatch weight, predisposethe creasesibling aggression. brood to fratricide. Why these differences In contrast,that adult Golden Eaglesignore evolved is unknown and will remain specula- aggressive interactions between siblings and tive. The result is a set of proximate mecha- make no attempt to distribute food equally sug- nisms that establish conditions whereby the gests,at least for this species,that no parent- fratricide threshold is invariably exceeded. offspring conflict exists.Similarly, Steyn (1973) The death of C2 before it has any possibility reportsthat adult Tawny Eagles(A. rapax)make of fiedging poses interesting evolutionary no attempt to distribute food equally among questionsas to the significance and continued the young, nor do adult Great Egrets (Casme- maintenanceof the secondegg. The prevalent rodius albus) or Great Blue Herons (Ardea hero- hypothesisis that C2 actsas insurancein case dias) actively suppressthe aggressiveinterac- C1 fails to hatch or suffers early mortality tions of siblings (D. Mock pers. comm.). In (Meyburg 1974, Stinson 1979). An additional addition, adequate space for segregation is factorhelping to maintaina two-eggclutch may lacking in cliff- and tree-nesting species.It is be the relatively low costof producing the sec- more likely that the aggressive interactions ond egg. Of six major groupsof birds examined leading up to fratricide serve the best interests by Ricklefs (1974), the energetic cost of egg of the parents. productionis lowest in raptors.Smaller second eggs may be a result of a conflict between the ACKNOWLEDGMENTS requirementsfor an egg large enough to pro- Golden Eagle data were collected as part of the duce viable offspring yet small enough to en- Bureauof Land Management'sSnake River Birds of July1983] FratricideinEagles 635

Prey ResearchProject. Other Golden Eagledata were HOYT,D. 1979. Practicalmethods of estimating vol- kindly providedby M. Kocherr.Special thanks go to ume and fresh weight of eggs.Auk 96: 73- Lloyd Kiff, curator, Western Foundation of Verte- 77. brate Zoology, and David Niles, associatecurator, KING, J. 1973. Energeticsof reproduction in birds. Delaware Museum of Natural History, for allowing Pp. 78-107 in Breedingbiology of birds (D. S. accessto their respectiveoologicai collections.Val- Farrier, Ed.). Washington, D.C., Natl. Acad. Sci. erie Gargett deservesspecial mention for allowing KOCHERT,M. 1972. Population statusand chemical use of unpublisheddata on measurementsof Ver- contamination in Golden Eagles in Southwest- reaux'sEagle eggs. We alsowish to thankM. Kochert, ern Idaho. Unpublished M.S. thesis. Moscow, J. D. Ligon,D. Mock,R. J. O'Connor,K. Steenhof,C. Idaho, Univ. Idaho. Stinson, and the students of a UNM social systems LACK,D. 1954. The natural regulation of seminarfor their help and review of earlier drafts. numbers. London, Oxford Univ. Press. This paper is contributionNo. 4474 of the Journal ß 1968. Ecologicaladaptations for breeding in Series, Florida Agricultural Experiment Station, birds. London, Methuen. Gainesville, Florida 32611. MEYBURG,B. 1974. Siblingaggression and mortality amongnestling eagles. Ibis 116:224-228. LITERATURE CITED NEWTON,I. 1977. Breedingstrategies in birds of prey. Living Bird 16: 51-82. ANKNEY,C. 1982. Sex ratio varies with egg sequence O'CONNOR,R. J. 1978. Brood reduction in birds: se- in Lesser Snow Geeseß Auk 99: 662-666ß lection for fratricide, infanticide and suicide? BROWN,L. 1952. On the biology of the large birds Anim. Behav. 26: 79-96. of prey of the EmbuDistrict, Kenya Colony. Ibis PARSONS,J. 1970. Relationshipbetween egg size and 94: 577-620. post-hatchingmortality in the HerringGull (Larus ß, & D. AMADON. 1968ß Eagles, and fal- argentatus).Nature 228: 1221-1222. cons of the world. New York, McGraw-Hill. POOLE,A. 1979. Sibling aggressionamong nestling CLARK,A., & D. WILSON. 1981. Avian breeding ad- Ospreysin Florida Bay.Auk 96: 415-416. aptations:hatching asynchrony,brood reduc- PROCTOR,D. 1975. The problem of chick lossin the tion, and nest failure. Quart. Rev. Biol. 56: 253- South Polar Skua Catharacta rnaccorrnicki.Ibis 117: 277. 517-520. COLLOPy,M. 1980. Food consumptionand growth RICKLEFS,R. 1974. Energetics of reproduction in energeticsof nestling Golden Eagles.Unpub- birds. Pp. 152-297 in Avian energetics(R. A. lished Ph.D. dissertation. Univ. Michigan, Ann Paynter, Jr., Ed.). Proc. Nuttall Ornithol. Club Arbor, Michigan. No. 15. DOWARD,D. 1962.Comparative biology of theWhite STEYN,P. 1973. Observations on the Tawny Eagle. Boobyand the Brown BoobySula spp. at Ascen- Ostrich 44: 1-22. sion. Ibis 103: 174-220. STINSON,C. 1979. On the selective advantage of HAMILTON,W. 1964. The genetic evolution of social fratricide in raptors. Evolution 33: 1219-1225. behavior: I, II. J. Theor. Biol. 7: 1-52. TRIVERS,R. 1974. Parent-offspringconflict. Amer. HowE, H. 1976. Eggsize, hatching asynchrony,sex, Zool. 14: 249-264. and brood reduction in the Common Grackle. YOUNG,E. 1963. The breedingbehavior of the South Ecology57: 1195-1207. Polar Skua. Ibis 105: 203-233.