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Swainson's Hawks on the Laramie Plains, Wyoming

Sidney W. Dunkle
The Auk
Vol. 94, No. 1 (Jan., 1977), pp. 65-71
Stable URL: http://www.jstor.org/stable/4084890
Page Count: 7
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Swainson's Hawks on the Laramie Plains, Wyoming
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Abstract

The biology of Swainson's Hawk (Buteo swainsoni) was studied during the breeding season of 1964 on a $1,554\ {\rm km}^{2}$ area west of Laramie, Wyoming. The hawks show a statistically significant preference for mates of the same color phase. The study tract supported 55 pairs and 5 single adult Swainson's. Based on various considerations, the area within 0.8 km of possible nesting sites added up to $401.5\ {\rm km}^{2}$ of Swainson nesting habitat to give a population density of 1 pair or unmated adult per $6.7\ {\rm km}^{2}$. Eggs were laid in 49 nests located with no significant preference for dry grass, irrigated meadows, or the edge between those habitats. The 31 probably complete egg clutches seen contained two or three eggs (mean 2.55). Young birds were fledged by 33 pairs for an average of 2.06 young per pair, or 1.24 young per pair for each of the 55 pairs in the study area. In the 33 nests fledging young, 15% of the eggs were lost to infertility, death of the embryo, and unknown causes. No second sets of eggs were laid after destruction of the first set. One-third of the nests in which eggs were laid were destroyed by crows, owls, the wind, and unknown causes. Food used by the Swainson's included 115 small mammals, 42 birds, 11 other vertebrates embracing aquatic vertebrates, and a few insects. Notes on nest placement, behavior, causes of mortality, numbers of other raptors in the study area, and relationship with other large birds are given.

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