Sunday, June 2, 2013

Swainson’s Hawk

Swainson’s Hawks migrate each year from the Great Plains of North America to the pampas of of Argentina, a distance of some 10,000 kilometers. West of the Missouri River in South Dakota, John Holden and I found them to be the most common raptor. The species is in some peril—studies have demonstrated that, in Argentina, thousands of Swainson’s Hawk are poisoned by pesticides (Bechard et al. 2010). I have previously posted on this hawk.

These hawks come in three color phases. Bechard et al. (2010) write that there are light-morph birds, like the hawk in the upper photo, dark-morph ones, and everything else inbetween. The authors appear to claim that a third, rufous-morph exists, like the one John and I found and photographed below. Sibley (2000), however, implies that the rufous birds are simply intermediate between the light and dark birds.

Why do many hawks come in dark and pale morphs? One hypothesis is that the pale birds (usually outnumbering the dark ones) are learned by the prey. Consequently the pale morph declines while the dark one increases—until the prey learns that dark raptors are also dangerous. Then the dark ones decline and the pale ones increase. I do no know if an hypothesis for three morphs exists. The situation with Swainson’s Hawks is complicated. In California, 89% of males were pale. In breeding pairs, males tended to be paler than females (Bechard et al. 2010).

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