Geography Reference
In-Depth Information
Keeping longevity records is enjoyable; nothing beats a day in the i eld
when I glimpse a bird I've watched for several years. When I do, my history
with that individual l oods forth as I recall past nests, mates, of spring, trav-
els, and tribulations. I feel as if I really do
know
another creature. But to know
whether a population of birds is sustainable—able to persist on its own accord
without the need for input from populations that live elsewhere—I would have
to determine the likelihood that the average adult and the average l edgling
would survive the coming year. A little algebra helps.
We usually do the math for the average female and try to determine whether
she is able to l edge at least one daughter that in turn lives long enough to l edge
a daughter of her own, and so on. Such production would keep the number of
mother birds—and by extension all birds—in a population constant from year
to year. The number of birds in next year's population equals the survival of
this year's adults, plus the number of young they produce discounted by their
ability to survive the coming year. So, for a population to remain constant—
the ratio of this year's population to next year's equal to 1—requires that a
mother's survival rate plus her annual production of l edglings multiplied by
their annual survival rate equals at least 1. For a mother Pacii c wren that
produces only 0.7 females per year on average in a neighborhood under con-
struction, replacing herself requires that she and her l edgling both have
a 59 percent chance of living to breed another year. That is unlikely. But for a
mother junco who produces 1.7 females per year in the same construction
zone, to replace herself requires only that she and her l edgling have a 37 percent
chance of living another year. For a small bird, that is more than reasonable.
Let's see whether the birds of subirdia can hit that mark.
Dave Oleyar is a rare breed, a person who is equally adept in both the
wild and the computer lab. For his dissertation research, he determined the
sustainability of our marked birds, beginning with an estimation of survivor-
ship in subirdia. This work required him to determine the probability that a
marked bird is both alive and observed each year. For example, when we
