SEED HANDLING ABILITY, BILL STRUCTURE, AND THE COST OF SPECIALIZATION FOR CROSSBILLS CRAIG W. BENKMAN • Department of Biological Sciences, State University of New York at Albany, 1400 Washington Avenue, Albany, NewYork 12222 USA AssT•cT.--Crossbills (Loxia) arespecialized to extract andhandle seeds fromconifer cones. I evaluated the abilityof crossbills to handle nonconifer seeds by comparing seed handling efficiencies with other cardueline finches. For all seed sizes, crossbills requiredseed encounter rates or seedabundances 2-3 times greater than other species to meet their daily energy requirements. Consequently, crossbills may suffer highmortality during conifer cone failures. Crossbills are inefficient at meetingtheir energydemands on nonconifer seeds because of their narrowmandibles, loweredhorny palate, and largebodysize.The narrowmandibles enable crossbills to efficiently extract seeds fromconifer cones and thelowered palate enables them to handle small seeds rapidly. Crossbills have evolvedlarger bills, and associated musculature and bodymass, to provide the powernecessary to separate cone scales. Some of the increase in body mass, however, maycounterbalance thelarge billsto improve predator evasion.Received 27 October 1987, accepted 17 June 1988. THEcardueline finches(subfamilyCardueli- nae) are highly specialized seedeating birds (Newton 1967, 1972). Most consume seeds, mainly fromdicotyledonous plants, throughout the year(Martin et al. 1951; Newton 1967,1972; Austin 1968). The crossbills (Loxia) eat conifer seeds to the near exclusion of other seeds(New- ton 1967, 1972; Austin 1968; Nethersole- Thompson 1975; Benkman1987a). Crossbills are clearly more efficientat utiliz- ing seeds in closed conifer conesthan other carduelinefinches(Newton 1967, 1972). Many characteristics of the crossbill's feeding appa- ratusare adapted for exploitingconifer seeds (Benkman 1987b). Experimental ablationof the crossed portion of the crossbill's mandibles demonstrated that this portion wasessential for crossbills to extract seeds from closed conifer cones (Benkman 1988). Further, no carduelines other than crossbills have been found to extract seed from tightly closed conifercones (Smith and Balda 1979; pers. obs.). Crossbills may be limited to foraging on co- nifer seed because they arerelativelyinefficient at handling other seed types (e.g. Roberts 1936). The reduced efficiency of crossbills is of partic- ular interest because about every 3-4 years co- nifer seed cropsfail over much of the boreal forests of North America (Bock and Lepthien • Present address: Departmentof Biology,Prince- ton University,Princeton, New Jersey 08544-1003 USA. 715 1976)and Eurasia (Newton 1970, 1972),and at this time many crossbills feed on nonconifer seed (see Newton 1970, 1972). These periods of conifer seed shortage oftencoincide with fail- ures of other boreal tree seedcrops(Bockand Lepthien ! 976). Thus, competition for available seed among cardueline finches wouldbe inten- sified. I compared the seed-handling rates, on grad- ed seed sizes, of 3 noncrossbill species of car- duelines with the2 species of crossbills in North America. These data are used to determine the relative seedhandling abilities of crossbills in comparison to othercarduelines and those fea- tures that make crossbills inefficient when feed- ing on nonconifer seeds. The dataon noncross- bill carduelines are compared to those on emberizine sparrows, elsewhere (Benkman and PullJam 1988). METHODS Red Crossbills(L. curvirostra), American Goldfinches (Carduelis tristis), House Finches (Carpodacus mexicanus), and Evening Grosbeaks (Coccothraustes vespertinus) were capturedin Albany Co., New York. White-winged Crossbills (L. leucoptera), were caught in Laurentides Reserve, Quebec. Several days prior to and during the experiments, all species were fed commercial sun- flower (Helianthus spp.) and niger thistle (Guizotia abyssinica) seeds,and vitamin-enriched water. For- aging experiments were conducted on the crossbills after a minimum of 5 months in captivity.The other species werecompleted within 6 days of capture. Even The Auk 105: 715-719. October 1988