Geography Reference
In-Depth Information
l edged one baby bird for every egg laid. Adding up these dif erences means
that on a nest-by-nest basis, Swainson's thrushes produce nearly one more
l edgling than do robins. And they do so especially where they were most
common—in forested reserves.
This result is where most studies of nesting success stop, and while thrushes
af ord us a large sample of nests with visible output, counting just one batch of
eggs or l edglings is far from the full story. Because robins are short-distance
migrants, often wintering near where they nest, they can start breeding earlier
and continue longer than can Swainson's thrushes, which winter thousands
of miles away in Central and South America. In Seattle, robins begin nesting
in late March—four to six weeks before Swainson's thrushes. And robins nest
well into August when Swainson's thrushes are already busy fueling up to l y
south, not to lay more eggs. Female robins are also phenomenal egg machines,
nesting again rapidly after one nest fails and occasionally succeeding at l edg-
ing two or more full broods within a single year. Swainson's thrushes nest
again only after early failures and in our experience have only once success-
fully raised two broods in a single season. Scoring our two thrushes' season-
long production reveals that both species are equally successful. Each pair of
birds typically produce two young per year.
The range in annual productivity of robins and Swainson's thrushes and
its composite measures—number of eggs laid, success of hatching, number of
l edglings, and frequency of breeding—were typical of the birds we were able
to study in detail. Although we found far fewer nests of other species, some
were especially amenable to observation outside the nest, something much
trickier for thrushes. Rather than risk attracting predators or mistakenly tram-
pling upon nests of Wilson's warblers, Pacii c and Bewick's wrens, dark-eyed
juncos, song sparrows, and spotted towhees, we watched for signs of breeding
as we mapped out a pair's territory. We noted courtship and carrying of nesting
material and nestling food, and we listened for the raspy begging of a hungry,
young l edgling as it trailed its parents. This protocol allowed us to determine
