Ecological, Morphological, and Bioenergetic Correlates of Hunting Mode in Hawks and Owls

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Abstract
We examined four correlates of hunting mode, dichotomized as active search (AS) and sit-and-wait (SW), in five assemblages of sympatric hawks and owls. Examining twenty-four hawk and ten owl species we found that: (1) Linearized wing loadings are light for kites, harriers, and eagles (which are primarily AS), and heavy for buteonines, falcons, and accipiters (which are mixed-mode hunters using both AS and SW techniques). Primarily AS-owls (mainly strigines and tytonines) have light linearized wing loadings, whereas SW-owls (mainly bubonines) are more heavily loaded. AS-hawks do not differ significantly from AS-owls, nor do SW-hawks from SW-owls; (2) Among hawks, diet breadth increases from accipiters through harriers, to falcons, eagles, buteonines, and to kites; and among owls, from strigines through bubonines to tytonines. No significant differences in diet breadth are detected between AS and SW hawks (or owls), nor between single-mode and mixed-mode hawks (or owls); (3) Among hawks, the ratio prey mass/body mass (PM/BM) increases from harriers through falcons to buteonines, accipiters, eagles, and to kites, and from tytonines through strigines to bubonines, among owls. No significant differences in the ratios PM/BM are found between AS and SW hawks (or owls); (4) Energetic costs incurred per prey capture are greater with the AS than with the SW mode inthree out of four cases analyzed, but information on energetic gains are available for only one. Here, AS rendered a larger differencebetween benefit and cost than did SW; in the other three cases that differential was not known, but several advantages associated to AS were claimed to offset its high energetic cost. Our main conclusions are: (1) Calculation of linearized wing loading of a hawk (or owl) enables prediction of its main hunting mode; (2) Although use of AS or SW techniques (singly or in combination) seems not to constrain access of raptors to prey resources (in terms of diversity and size), further studies focusing on single subfamilies are likely to detect differences in prey use by raptors using different techniques; (3) Little can be predicted about the extent of hunting modes used by different raptors, until the associated energetic gains are evaluated.