Geography Reference
In-Depth Information
thrushes were extremely mobile, averaging movements of one to four city
blocks per day. Their mobility likely rel ects their migratory tendencies and
reliance on distant, patchy foods such as fruiting shrubs and concentrations
of insects. Robins in particular sought out lawns and pastures that were rich
in earthworms. Young towhees and song sparrows made a beeline for the
nearest bird feeders, typically covering less than a block a day.
Although the vast majority of young birds lived to see the next day, we
would expect fewer than half to live a full year. Extrapolating from the prob-
ability of surviving one day to the probability of surviving a full year means
that we assume the chances of death remain constant as a bird gains experi-
ence, encounters its i rst winter, migrates, and searches for a breeding terri-
tory. That assumption is certainly not true, but it gives us a starting point.
And for song sparrows, Swainson's thrushes, and towhees, it provides a more
realistic estimate of survival than did our approach of banding and trying to
resight young birds. Kara estimated that nearly half of sparrows and thrushes
and a third of towhees survived their i rst year. The other two dozen studies
that have tried to estimate survival during a bird's i rst year found similar
rates. Suburban bird populations with juvenile survival nudging 50 percent
could be sustained.
Young robins are an important source of food for most everything that has
teeth or talons and crawls, runs, or l ies through the forest. If we extend the
50 percent mortality rate during our nine-week-long study to a full year, we
would expect less than 1 percent of young robins to survive. Our banding
study showed that this prediction is incorrect; as dii cult as they are to i nd a
year later, we found one of every three robins born and banded in subirdia. I
suspect that the high mortality Kara observed is fairly typical of a robin's i rst
few weeks; they are plump, conspicuous, and clumsy just out of the nest.
Those that survive this vulnerable time gather in great l ocks that cut their
risk of predation and increase their ability to track the ebb and l ow of food.
This social behavior likely lessens mortality considerably relative to their
