Diets of North American Falconiformes*
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DIETS OF NORTH AMERICAN FALCONIFORMES* Steve K. Sherrod Laboratoryof Ornithology CornellUniversity Ithaca, New York 14850 Introduction Most birds are predatorysometime during their lives whether we considera hum- mingbirdsuch as the Purple-MountainGem (Lamporniscalolaema) hawking insects in the forestsof CostaRica (Snow1977) or the giant Monkey-eatingEagle (Pithecophaga iefferyi)killing a primatein the Philippinejungles (Kennedy 1977). Generally, however, one thinksof membersof the ordersFalconiformes and Strigiformesas birds of prey, and occasionally,certain other species,such as membersof the Corvidae,are included within this classification in the functional sense. For the purposeof this discussion,only the Falconiformes,or the vultures,kites, hawks,eagles, and falcons of NorthAmerica, are considered.Evidence presented by Sib- ley and Ahlquist(1972) indicatesthat this order is polyphyleticin origin,and its mem- bers may representanalogous solutions by severalgroups to fill certain similar voids within the myriadof ecologicalniches. Nevertheless, the physicalcharacteristics com- monto thesebirds usually include a sharphooked beak for tearingflesh, strong grasping feet with largesharp talons for holdingor killing prey (vulturesexcluded), and particu- larly keen eyesight.Other adaptationsvary widely and may directly influencethe diet of a raptorby facilitatingthe captureof prey,or moreindirectly, may assista particular predatorybird in penetratingan environmentless crowded with competitors. Someraptors, such as the EvergladeKite (Rostrhamussociabilis) with its long and pointed,hooked beak adaptedfor manipulatingand removingthe soft bodiesof the Apple Snail (Pomacea)from its shell (Snyderand Snyder1969), are highly specialized for predationupon a singleanimal. Consequently the rangeof the predatoris limitedby the rangeof the prey, and any declinein the prey populationinfluences that of the predator.Other raptorssuch as the BaldEagle (Haliaeetusleucocephalus) are more ver- satileand feed on a wide varietyof prey,often utilizing for the mostpart differentclass- esof foodsin differentparts of their range(Herrick 1924;Sherrod, White, and William- son1977). Changes in diet accordingto the seasonand the availabilityof certainprey species,as duringmigration, are alsocommon, exemplified by the switchfrom verte-' brate to invertebrateprey in the springby AmericanKestrels (Falco sparverius) in cer- tain localities(Breckenridge and Errington1938). Both of the last two mentionedrap- torsare examplesof generalists.As a rule they are muchmore widely distributedthan specialists,and, althoughdeclines in one prey speciesmay affect the predatorpopu- lations,these raptors can rely more heavily on other prey speciesduring periodsof *Ed. Note:This paper was adapted from Rechcigl,M., CRC Handbookin Nutritionand Food,in press.Many membersof RRF may be unawareof CRC series,and thus it is reproducedhere for yourbenefit and by permissionof the publisher. 49 Raptor Research12(3/4):49 50 RAPTOR RESEARCH Vol. 12, No. 3/4 hardship.Most birds of prey are closerto beinggeneralists but canbe classifiedas inter- mediatebetween these two extremes.Overall, raptors are opportunistsand givencer- tain circumstances(prey abundance, vulnerability of prey,hunger of hawk)will attacka widevariety of preywithin their general capabilities. The Diet of Raptorsin the Wild There are roughlyfive methodsof determiningthe diet of wild raptors.Each method seemsto have advantagesand disadvantagesas discussed,in part, in the literature(Err- ington1930, 1932; Bond 1936; Hartley 1948). The first is simplyto collectthe raptorand examinethe contentsof its cropand stom- ach. This methodyields a higherpercentage of smalleror invertebrateitems in raptor dietsthan othermethods, but someprey in the stomachwill be unidentifiable.In addi- tion to the fact that somestomachs will be empty, this techniqueyields a minimum amountof dataper bird whencompared with mostof the otherprocedures used. More importantly,since the existenceof a numberof raptorsis threatened,this procedureis rarely acceptabletoday. It wasprincipally used by early governmentbiologists (Fisher 1893,McAtee 1935)to determinewhich species of raptorswere "good"and which were "harmful."If stomachcontents are deemedessential, emetic techniquds such as those discussedby Tomback(1975) might be adaptedto raptorswithout sacrificing the bird. A secondmethod is to examinenests or pluckingperches near the nestfor remainsof prey animalswhich adults have been eating or feedingto the young.This technique may favor largerprey speciessince smaller specimens may be swallowedwhole or in a few biteswith little plucking,while feathers,fur, and bonesare nearlyalways left from larger items. Examinationof regurgitatedpellets comprises a third methodof prey identification. The pelletscan be pickedup regularlyunder roosts or nestsand analyzedfor content. The valueof pellet examinationvaries, and sincebones can be digestedto differentde- greesand fur or featherslikewise destroyed, these castings can sometimesreveal qual- itativelybut not quantitativelythe specieseaten. Certain foods, such as fish or insects, may leavelittle castingmaterial that canlater be foundwhen regurgitated since a pellet formedof scalesor exoskeletonsquickly flakes apart and is lost. A fourth techniqueis to squeezethe food in the nestlings'crops out throughthe mouthand examinethe contents.As the fledglingsgrow older,they are tetheredat the baseof the nest tree to facilitate their examination.This givesan accurateaccount of the itemswhich the adultsare catchingduring the breedingseason, and hasbeen used successfullyby someexperienced raptor biologists. However, there are fourreasons why inexperiencedworkers should avoid this technique.Firstly, nestlingsare fed certain amountsof roughage,including bones, and to force this massback up the throatmay tear esophagealtissue. Secondly, it subjectsthe youngto an obviousdecrease in nutri- tion, unlessthe sampleis replacedor substitutedwith otherfood. Thirdly, young raptors in nestsare frequentlydepredated by mammalsfollowing human scent up the trees,and to placethese birds on the groundincreases depredation. Peterson and Keir (1976)pre- sentsuggestions for constructingplatforms which should reduce such losses. Lastly,to holdfledglings in thismanner "far pastthe time that the youngwould have left their nests,until the adultsno longerbrought food" as suggestedby Erringtonand Breckenridge(1936) prescribes an uncertainfuture for theseoffspring. Most raptorsun- dergo an extensivepost-nest departure dependency period during which the family Fall/Winter 1978 Sherrod--Diets of Falconiformes 51 grouptravels and may even hunt together as do the PeregrineFalcon (Falco peregrinus; Sherrod,in prep.).Preventing this family groupassociation up to the time when the adultdeserts the tetheredyoung may delaythe developmentof normalhunting behav- ior until after offspringcan no longerreceive additional food from the parents,thus in- creasingthe chanceof mortalityfor suchindividuals. The lastmethod is by observationof raptorswith prey.This may be accomplishedby constructinga blind near the nestand observingthe prey that the adultsbring in. In somecases, birds may be observedhunting in the field or frightenedoff kills. Prey broughtto the nestis often pluckedthoroughly and may be difficult to identify. This methodalso employs a considerableamount of time.To witnessa raptorkill in the field is rare for the proportionof time spent,and giveslittle quantitativelyconcerning over- all diet. Whatever the methodused, data collectedfrom only one bird or one nest can be highlymisleading as to diet of the speciessince certain individuals may tend to prey on a particularanimal which is plentifulin their huntingarea or which they havebecome very adeptat catching.The materialavailable for analysisof the diet of wild Falconi- formeshas been collected by all of the techniquesdiscussed above and is explainedfor eachaccount under "Methods Information" in the tables.It is presentedin the literature in a numberof differentmanners. For somespecies the materialis voluminous,and for otherslittle data exists.In the latter case,I have askedbiologists who are workingwith the speciesfor unpublisheddata which they havecollected. This collectionof diet data is presentedmerely as being representative, but by no meansis it inclusiveof all infor- mationavailable in the literature.! have generallyused the followingformat in treat- ment of eachspecies, but have foundit necessaryto divergein certaincases. To give someidea of the overall diet, ! have consultedcollective references (Bent 1937, Brown and Amadon1968). Accountsrelated by Bent (1937) are referencedas Bent (1937 or 1938)only. This and other collective references allow the possibilityof someduplication of materials.I havethen included the percentagesof broadclasses presented by Snyder andWiley (1976). Next I have taken the extensivestomach content records collected throughout the UnitedStates and published by Fisher(1893), and recalculated them as percentage diets by numbers.This accountis oftenvague and in somecases expressed in plurals(e.g., insects).In sucha caseI have calculatedthese entries to mean two specimensof the speciesin question.This liberty undoubtedlywill resultin error sincethere may have beentwenty insects instead of two. However,these samples are usuallytaken through- out the year and throughoutthe range of the bird in North America and, I feel, are extremelyvaluable even with errorsincluded. Considering the opportunisticnature