Prey partitioning between sympatric grassland raptors
AuthorGiovanni, Matthew D.
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I collected diet data at ferruginous hawk and Swainson's hawk nests in the northwest Texas panhandle, the southwest Oklahoma panhandle, and northeast New Mexico during the 2003 and 2004 breeding seasons. I used video recording systems to document 959 ferruginous hawk prey deliveries and 1,058 Swainson's hawk prey deliveries during approximately 5,618 hours of video monitoring at 26 nests. The primary prey (n = 4 species ≥ 4.5% total biomass) delivered by ferruginous hawks were black-tailed prairie dogs (Cynomys ludovicianus; 29.9%), black-tailed jackrabbits (Lepus californicus; 15.8%), cottontails (Sylvilagus spp.; 14.4%), and pocket gophers (Geomys bursarius and Cratogeomys castanops; 10.9%). The primary prey (n = 8 species ≥ 3.3% total biomass) delivered by Swainson's hawks were cottontails (Sylvilagus spp.; 32.4%), pocket gophers (Geomys bursarius and Cratogeomys castanops; 9.8%), black-tailed jackrabbits (Lepus californicus; 8.5%) bull snakes (Pituophis melanoleucus sayi; 8.2%), southern plains woodrats (Neotoma micropus; 4.9%), thirteen-lined ground squirrels (Spermophilus tridecemlineatus; 4.8%), hispid cotton rats (Sigmodon hispidus; 3.8%), and western coach whips (Masticophis flagellum testaceus; 3.6%). Swainson's hawks delivered significantly more primary prey items than ferruginous hawks (P = 0.003). However, Smith's Measure of Niche Breadth did not indicate different dietary breadth measures (P = 0.39) for the ferruginous hawk (FT = 0.86) and the Swainson's hawk (FT = 0.88). The Simplified Morisita Index (CH) for Niche Overlap indicated an inter-specific dietary overlap of 0.31 in terms of prey species used, and 0.56 in terms of prey species biomass contribution. Ferruginous hawks delivered 4.6 deliveries/day at 480 grams/delivery for a nestling provisioning rate of 670 grams/nestling/day. In contrast, Swainson's hawks delivered 7.0 deliveries/day at 147 grams/delivery for a nestling provisioning rate of 401 grams/nestling/day, and this was significantly less than that of ferruginous hawks (P < 0.001).