Geography Reference
In-Depth Information
feeding, watering, and roosting sites. They are opportunistic—robbing nests,
scavenging on the dead, and ambushing the unsuspecting. I guess Coops are a
close second to the domestic cat in being a songbird's worse nightmare.
Seattle is stuf ed with Coops. They hunt from my backyard tiki torch. I
see them among the high-rise buildings of the city center as well as the lofty
peaks of the high Cascades. They regularly build their large stick nests in the
quieter parts of our neighborhoods and strafe nearby feeders. It takes several
hundred songbirds to raise a brood of hungry hawks each spring, and Stan
wondered what living next to a Coop nest might mean to the main items on
the menu. It turns out that about a third of the places where we counted song-
birds and monitored their reproduction and survival hosted Coop nests; the
other two-thirds did not. Stan took advantage of this information to calculate
the ef ects.
It was indeed dangerous for prey to breed near the nest of a Cooper's hawk.
Stan estimated that the annual survival of robins and Swainson's thrushes—
favorite Coop prey—was about 7 percent lower as a result of nesting near
a hawk nest. Compounded over several years of a bird's expected life, this
greater mortality could reduce a population. The breeding success of prey
was even worse; it dropped from a i fty-i fty chance of l edging young in hawk-
free places to a one-in-three chance of doing so where hawks nested. But this
is where things got a bit more complex, and interesting.
Not surprisingly, nesting success of all songbirds increased with dis-
tance away from an active Coop nest. But this increase only lasted for a dis-
tance of a few city blocks. Beyond that, success again declined. This uptick
in nest failure is probably indirectly related to the hawks' presence. Near
their nests, hawks shield songbird chicks and eggs from predation by other
predators such as jays, crows, squirrels, and chipmunks. These generalist
predators are rare near hawk nests but increasingly common as one travels
away from the nest. As the protective umbrella of the hawk drops away,
