FORAGING BEHAVIOR, REPRODUCTIVE SUCCESS, AND COLONIAL NESTING IN OSPREYS JOHN M. HAGAN lip AND JEFFREYR. WALTERS Departmentof Zoology,Box 7617, North CarolinaState University, Raleigh,North Carolina27695-7617 USA ABSTRACT.--Wetested several hypotheses about the evolution of colonial nesting in Os- preys.We examined foraging behavior and reproductive success of membersof a densecolony in coastalNorth Carolinato test(I) if the colonyfunctioned as a centralplace for exchanging information about the location of food patches(Information Center Hypothesis), (2) if the colonywas locatedcentrally relative to foragingsites, which would minimize foragingtravel costs,(3) if the colony functioned to promote synchronizedbreeding, which would swamp predatorswith more young than they could consume,(4) if nestingin the centerof a group decreasedpredation pressure,and (5) if safe nest-sitedistribution controlsactual breeding distribution.The hypothesisthat the colony functionedas an information center was sup- portedby synchronyin departure,but contradictedby evidenceof fidelity to foragingareas. The colony did not function as the geometric center of an individual's foraging locations. Colony memberstraveled 1.5-7 timesas far as they would have traveled had they nestedat their geometric centers.There was a negative correlation between the proportion of eggs lost in the colony and the number of eggsavailable to predators,suggesting some benefits to synchronousnesting in a dose spatialgroup. However, there was no numericalresponse in predation relative to the total number of eggsavailable in the colony.Birds nesting toward the centerof the colonyproduced significantly more fledglingsthan peripheral nesters,but strong nest-sitefidelity prevented shifting to more central locationsby peripheral nesters. Spatialdistribution of nestsreflected the distribution of potential nest trees,both within the lake and within the region. This and the plasticity of nesting density shown in Ospreys supportLack's (1968) Nest-siteHypothesis. We concludethat the spatialdistribution of safe nest sitesand advantagesrelated to predation maintain coloniality in Ospreys.Received 22 May 1989,accepted 15 January1990. COLONIALnesting in birds is a common yet 1968,Lack 1968). The firsttwo explanationspos- poorly understoodphenomenon. A colonymay tulate that individualsnesting near othersde- be defined as a group of animals that nest at a rive a higher fitness as a result of interaction. centralized location, from which they recur- The third explanation postulatesno such ad- rently depart in search of food (Wittenberger vantage. For most speciesit is not clear whether and Hunt 1985). Members of a nesting colony nestingnear othersis beneficialor is a by-prod- of birds are constrainedby the immobility of uct of a habitat constraint. their clutch or brood. The costsof traveling to The Information Center Hypothesis (Ward and from such a central location are obvious, andZahavi 1973) relates colonial nesting to for- but the benefitsare not. Explanationsof colonial aging success.Individuals may learn about the breeding fall into three categories(Alexander locationof patchilydistributed, ephemeral foods 1974).Colonial breeding may enhanceforaging from other colonymembers. Information need (Crook 1965, Emlen 1971, Fisher 1954, Lack 1968, not be exchangedactively. Simply following or Ward and Zahavi 1973), reduce the probability cuing in on the return direction of an individ- of predation (Burtonand Thurston 1959,Kruuk ual known to have been a successfulforager 1964, Horn 1968, Burger 1974, Hoogland and qualifiesas "exchange."Ward and Zahavi's In- Sherman 1976), or result from resource con- formationCenter Hypothesis has become widely straints(e.g. nestsites, food distribution) (Horn cited (ISI 1983), but clear evidence that infor- mationtransfer induces colonial nesting is lack- ing. It is a difficult hypothesisto test and in- ' Presentaddress: Mahomet Bird Observatory, P.O. volves distinguishing between successfuland Box 936, Mahomet, Massachusetts 02345 USA. unsuccessfulforagers, and then demonstrating 506 The Auk 107: 506-521. July 1990 July1990] ColonialNesting inOspreys 507 that unsuccessfulforagers improve their suc- similarlyenhanced by groupingamong nesting cessby obtaininginformation on food location birds. Nests in the center of colonies suffer less from successfulbirds (Mock et al. 1988). predationthan thoseon the peripheryin many Nevertheless, evidence that information ex- species,which is consistentwith the proposed changeseems to occurin colonies(or roosts) effectsof groupmobbing or predatordetection has recently been obtained for three avian (e.g.Patterson 1965, Tenaza 1971, Siegfried 1972, species(Brown 1986, Rabenold 1987, Greene Feare1976, Siegel-Causey and Hunt 1981). 1987). The role of the information exchangein Synchronousbreeding of an avian assem- the evolution of colonies remains debatable, blagecan also produce benefits to members.By however. Information centers could be wide- swampingpotential predators with a synchro- spreadphenomena, critical to the evolutionof nized productionof young, colonymembers coloniality,or they couldsimply be a secondary can reducethe probability that their offspring adaptationthat sometimesoccurs as a result of will be preyed upon (Patterson1965, Nisbet group living (colonialityhaving evolved for 1975, Feare 1976, Veen 1977). Emlen and De- other reasons). mong(1975) suggested that synchronous breed- To addressthis question,we examinedanother ing in BankSwallow (Riparia riparia) colonies colonyof a speciesthat hasalready been shown increased opportunities for social foraging, to exchangeinformation. Demonstration of in- therebyincreasing foraging efficiency. We tested formation exchange at an independent site this hypothesisby relating breedingchronol- would imply that it can be a widespreadphe- ogyto reproductivesuccess in the colony. nomenon, and therefore may be an important Finally,we testedtwo hypothesesrelated to factor leading to, or at leastmaintaining, group resource constraints'. Horn's Geometric Center living. Its relative importanceto colonymain- Hypothesisholds that colonyformation could tenance should be correlated to its common- be due to food distribution (Horn 1968). By ness. nestingin the centerof the distributionof a We testedthe Information Center Hypothesis patchy,ephemeral food supply,birds can min- for an Osprey(Pandion haliaetus) colony in coast- imize overall foraging distancetraveled. No al North Carolina. This site is distant from the benefit of socialityis implied; rather, foraging colonyin NovaScotia where Greene (1987) doc- considerationsthat act on many individuals in umentedinformation exchange, and the strong an identicalway leadto colonyformation. The natal-sitefidelity of Ospreysmakes it extremely evidenceto supportthis hypothesiscomes from unlikelythat individuals of thesecolonies would Horn's(1968) study of Brewer'sBlackbird (Eu- mix (Poole 1989: 137). Male Ospreys supply phaguscyanocephalus). nearly all of the food for the female and nest- The Nest-siteHypothesis (Lack 1968)holds lings.Males in the North Carolinacolony must that colonialityis relatedto the distributionof travel long distancesto any of severalforaging safe nest sites. If safe nest sites are clumped, locations, and exploit schoolsof marine fish, nestswill also be clumped. For those colonial which fluctuatespatially and temporally. The speciesthat feed solitarily, Lack (1968) main- colony is large (50-60 pairs) and dense,and tainedthat safenesting sites--rather than feed- returning foragersare readily visible. If infor- ing habitats--arethe criticaldeterminants of mationexchange is a prominentfeature of group nestingdistribution. It has never been dem- living in Ospreys,it shouldbe evident at this onstrated that nest-site limitation alone can in- site, where conditionstheoretically conducive duce coloniality. to informationexchange are nearly ideal. We used a comprehensiveapproach to the We testedother hypothesesrelated to pre- studyof colonyformation and functionin Os- dation reduction as a result of group living in preys,recognizing that many factorsmay op- Ospreysby examinationof spatialand temporal erate to maintain colonial nesting. Although patternsof reproductivesuccess in the colony. not commonlyconsidered a coloniallynesting Detectionof predatorsis improved by flocking species,Ospreys historically nested in colonies. (Powell 1974, Siegfried and Underhill 1975, Coloniesbecame rare along the U.S. Atlantic Kenward 1978, Lazarus 1979, Caraco et al. 1980, seaboard in the 1950s and 1960s as a result of Barnard 1980, Bertram 1980, Jennings and Ev- extensiveuse of DDT and its effectson eggshells ans 1980,Thompson and Barnard1983), and it (Amesand Mersereau1964, Ames 1966, Hickey seemslikely that detectingpredators would be and Anderson 1968). Before this time, colonies 508 HAGANAND WArTEES [Auk, Vol. 107 noon sampleswere followed by subsequentmorning samplesto obtain this equivalent of a full day of sam- pling of a pair. We sampled three pairs twice (two daysfor eachpair), but at differenttimes during the breeding cycle. We observedpairs for a total of 170h, and assigned behaviorsonly to broad categoriesbecause we were interestedin qualitativepatterns in foraging by the males,which provide most of the food for their mates and young until fledging.The categoriesincluded (1) perched over nest, (2) perched on nest (includes in- cubation),(3) perchedin territory (not nest tree), (4) away from territory, (5) flying within territory,