Geography Reference
In-Depth Information
by open nesters. Nonnative exploiters were even better; fewer than one in
twenty of the house sparrow and starling nests we watched failed.
The ability of adapters and exploiters to reproduce in homes, yards, and
even the nooks and crannies of nature that remain during major construction
activities appears important to their ability to take advantage of the warmth,
landscaping, and abundant food that subirdia of ers. The inability of some
avoiders, such as the Swainson's thrush, to do likewise may coni ne them to
larger reserves rather than outright suburban settings.
My colleagues across North America and Europe are discovering much
the same. Although individual bird nests often sustain other creatures higher
on the food chain, species able to adapt to and exploit our yards and parks
such as cardinals, house wrens, catbirds, magpies, chai nches, blue tits, and
mockingbirds make up their losses over the course of the summer. Only three
of the ten studies that report annual reproductive output found it to be lower
in urban than nonurban landscapes. By breeding early and often, urban birds
regularly overcome handicaps such as smaller clutches of eggs, weaker nest-
lings, and frequent encounters with predators, parasites, and contaminants
that lower a single nest's production.
Watching thousands of birds reproduce among us also makes it obvious
that breeding is only part of what it takes to sustain a population. Pacii c wrens
avoided subdivisions, but when present there, they actually reproduced quite
well; only the consummate adapters—dark-eyed juncos, song sparrows, and
Bewick's wrens—did better. Northern l ickers, the most common woodpecker
in subdivisions, were the least successful primary cavity nesters, l edging young
from just over half of their nests—partially because of the l ickers' interactions
with nonnative European starlings (see Chapter 4).
To thrive in cities, birds must harvest the resources we humans provide
and avoid the obstacles that surround them. For adapters and exploiters the
path is rather straightforward: i nd the feeder and avoid the feline.
