Geography Reference
In-Depth Information
to be the case for cardinals. But to enhance subirdia's riches with species that
may survive, but only rarely thrive, functional connections with nearby re-
serves are required.
Observing the actual connections among populations of birds is in prac-
tice dii cult. Kara Whittaker's radio telemetry research gave us a i rst glimpse.
In the future, new genetic methods hold promise to advance her insights to
help us uncover past movements recorded in the DNA of the birds present
today. In theory, by comparing the genetic signature of an individual with the
average signature of various populations, one can determine whether a partic-
ular bird is most like those it lives among or most like those from a dif erent
location. The former would suggest that the bird is of local origin, and the
latter would suggest that it is an immigrant. In practice, there is much uncer-
tainty, especially considering the high mobility of birds within a city. Thomas
Unfried likes a good challenge, so he pricked the wing veins of hundreds of
sparrows and analyzed their genetic code, looking for likely relatives in dis-
tant places. He found some, even at sites separated by miles of urban land.
And he detected a pattern among our study sites where sparrows were more
likely to emigrate from productive source populations and immigrate into
less productive sinks. Song sparrow sources—in our study the developed
lands of subirdia—not only provided vibrant local populations, they stocked
the more challenging places where local sparrow populations would decline
to extinction if left alone. Connectivity was especially important for these
marginal sinks.
The potential to sustain birds in subirdia is no guarantee of continued
vibrancy. Reproduction and survivorship vary tremendously from year to
year depending on climate, food, predator habits, and luck. This random
l uctuation in demography is often the last nail in a species' coi n. It is why we
no longer have ivory-billed woodpeckers, heath hens, passenger pigeons, and
seaside sparrows among us. Even the sustainable vital rates we observed for
