Feeding ecology of 11 Sharp-skinned Hawk (Accipiter striatus) pairs ne
sting in aspen (Populus tremuloides), conifer (Abies, Picea spp.), and
mixed aspen-conifer habitats in southwest Colorado was investigated d
uring 1988-1989. Small birds (xBAR = 20.9 g, SE = 0.8 g) and mammals (
xBAR = 41.1 g, SE = 3.3 g) comprised 91 and 9% of 513 prey identified
at nests that fledged at least one young, respectively. Sixty percent
of the birds eaten during the hawks' nestling and fledgling stages wer
e nestlings or fledglings. Accordingly, median mass of birds eaten dec
reased from 17.4 g during incubation to 12.1 g during the nestling sta
ge. Although more birds were consumed than mammals during all nesting
stages (birds = 91.1%; mammals = 8.9%), the proportion of birds relati
ve to mammals in diets progressively decreased from incubation through
fledging. Taxa of birds in the diet were consumed in proportion to th
eir occurrence in the most abundant of three different habitats surrou
nding nests, whereas some mammalian taxa were consumed in greater prop
ortion than their relative ''availability'' in these habitats. This su
ggested that Sharp-shinned Hawks foraged opportunistically for birds,
but may have selectively foraged for mammals. Differences in the habit
s of mammals (e.g., fossorial vs. terrestrial behavior), and hence the
ir relative availability, may explain the apparent selection for certa
in species of mammals by