Geography Reference
In-Depth Information
encountered a particular towhee in 2002 and again in 2004, we knew it was alive
though unobserved in 2003. By considering each bird's encounter history—
the year-by-year record of whether we saw the bird or not—Dave calculated
the chances that an average adult and l edgling bird of each species in each
type of landscape we studied lived to see another year.
Because adults move little after they establish a territory, Dave thought
our estimates of their survival were pretty good. By repeatedly checking on
territories, we rarely missed a bird one year that later turned up alive. As a
grand average, Dave calculated that just over half of all adult birds survived
each year. It mattered little what type of bird he analyzed. Robins had the
highest survival—more than seven in ten survived each year in forest reserves.
Wrens, both Bewick's and Pacii c, had the lowest annual rates of survival,
around 50 percent. The chances of survival also varied modestly between
birds inhabiting forest reserves, lands undergoing development, and estab-
lished neighborhoods. We discovered that two classic adapters, the song
sparrow and dark-eyed junco, were most likely to survive the year in develop-
ments. Abundant food, which these species readily exploit at feeders, is likely
a key to their longevity in subirdia. In contrast, classic avoiders, such as the
Swainson's thrush and Pacii c wren, survived best in forested sites, as ex-
pected. The loss of nesting habitat and native foods in developments is costly
to these birds. The mathematics for towhees, robins, and Bewick's wrens sur-
prised us a bit, because these species survived best in reserves, rather than in
developments. These common residents of subirdia may best be thought of as
tenuous adapters that, while able to persist in most settings, thrive only where
there is a good mix of forested and built environments.
Dave's machinations suggested that most adult songbirds have annual
rates of survivorship adequate to balance their productivity in some of the set-
tings that characterize subirdia. But as encouraging as these numbers were,
his assessment of l edgling survival was discouraging. For a population to be
