We collected diet data at 12 Ferruginous Hawk (Buteo regalis) and 14 Swainson's Hawk (B. swainsoni) nests in a short-grass prairie and agricultural community in the panhandle area of northwest Texas and southwest Oklahoma, and in northeastern New Mexico during the 2003–2004 breeding seasons. We documented 959 Ferruginous Hawk and 1,058 Swainson's Hawk prey deliveries during ∼5,618 hrs of video monitoring. Ferruginous Hawks delivered 10.0 ± 0.7 (x̄ ± SE) prey species per nest and typically larger prey. Swainson's Hawks delivered 13.4 ± 1.1 prey species per nest and typically smaller prey. There was a dietary overlap (Simplified Morisita Index [CH]) of 0.31 in prey species delivery frequency and 0.56 in prey species' biomass. Ferruginous Hawks made 4.6 deliveries/day at 480 g/delivery whereas Swainson's Hawks delivered smaller prey items (147 g/delivery) but more frequently (7.0 deliveries/day). Deliveries/day and mass/day increased with increasing brood sizes of both species, but deliveries/day/nestling and mass/day/nestling decreased. Provisioning rates did not vary significantly over the nestling period. These data represent the most accurate diet quantification to date for Ferruginous and Swainson's hawks. Ferruginous Hawks used a larger array of prey types than shown in other studies based on indirect diet analysis methods. The low interspecific diet overlap suggests that prey is partitioned, which may facilitate the well-documented sympatric distribution of the two species.
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Vol. 119 • No. 4