Geography Reference
In-Depth Information
early life, invalidating our naïve extension of early daily mortality rates to the
full year.
Using our most realistic estimates, it seems that survival of young birds
approaches what is necessary to sustain our most productive breeding popu-
lations. The nearly 50 percent annual survival of song sparrows would bal-
ance their high annual productivity. Slightly lower survival of young robins,
Swainson's thrushes, and towhees may be adequate in the settings where
adult survival is highest or productivity exceptional. We remain less certain
about wrens and juncos, because transmitter studies were not feasible and
our limited ability to i nd the young birds we banded suggested unreasonably
low survivorship across most lands. Nonetheless, we have estimates of the
three main demographic rates—adult survival, juvenile survival, and annual
productivity—that allows us to more formally appraise sustainability.
Dave made a stew of suburban bird vital rates—the essentials of survivor-
ship and reproduction—and cooked them with linear algebra to estimate the
expected annual change in population size year after year, something demog-
raphers call “lambda.” He did this with the average survivorship and produc-
tivity values as well as with values reasonably expected in the best of times.
The latter helped account for the fact that we know we underestimated pro-
duction and survival of young birds. He even bumped up i rst-year survival
with Kara's projections. A lambda value of at least 1 indicates a stable popu-
lation able to maintain itself with reproduction. Populations characterized
by a lower lambda value are often called “sinks,” because they would dwin-
dle to extinction over time without replenishment from other places, known
as “sources.”
Changing lands, where forest is cleared, streets paved, and houses built
for suburban families, are at best moderate sinks for birds. Under the most
favorable demographic conditions, robins, Bewick's wrens, and song spar-
rows may be sustained in these areas, but Pacii c wrens, towhees, and
Swainson's thrushes appear unsustainable in this rapidly changing world.
 
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