Seed Handling Ability, Bill Structure, and the Cost of Specialization for Crossbills

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Abstract
Crossbills (Loxia) are specialized to extract and handle seeds from conifer cones. I evaluated the ability of crossbills to handle nonconifer seeds by comparing seed handling efficiencies with other cardueline finches. For all seed sizes, crossbills required seed encounter rates or seed abundances 2-3 times greater than other species to meet their daily energy requirements. Consequently, crossbills may suffer high mortality during conifer cone failures. Crossbills are inefficient at meeting their energy demands on nonconifer seeds because of their narrow mandibles, lowered horny palate, and large body size. The narrow mandibles enable crossbills to efficiently extract seeds from conifer cones and the lowered palate enables them to handle small seeds rapidly. Crossbills have evolved larger bills, and associated musculature and body mass, to provide the power necessary to separate cone scales. Some of the increase in body mass, however, may counterbalance the large bills to improve predator evasion.