Geography Reference
In-Depth Information
must be i ne. At present, getting the message out is winning, and pitches are
on the rise, just as they are in urban birds, such as great tits, that also balance
bravado and clarity. In frogs, however, the change in pitch proceeds much
more slowly than it does in birds because while big frogs may be able to raise
their voices above the trai c din, this ability is ultimately constrained by size.
Big frogs have deep voices because their bodies resonate at lower frequencies
while croaking. Change is also slow in roadside grasshopper populations, be-
cause it requires either evolution of the nervous system or the toothed comb-
like structure on the insects' hind legs that are raked against the forewing
to “sing.” Evolution of physical characteristics, such as body size or leg
form, proceeds slowly across generations, but birdsong can evolve cultur-
ally through social learning within a generation. Big birds learn to sing so-
prano much more quickly than big frogs or leggy grasshoppers can evolve
new instruments.
Possessing the cultural wherewithal to quickly and persistently adjust be-
havior to new environmental challenges is one reason that many birds have
adapted to cities. An inability to do so is part of the reason why other animals
have succeeded less well. Flexibility benei ts birds, but much of their success
is deeply rooted in their DNA, essentially preadapting them to the fast-paced,
urban life.
Wings allow birds to avoid massive road mortality that culls the urban
herd of mammals and herps. Grounded on four legs, dispersing animals and
migrating amphibians are slaughtered by the millions, especially at night.
Snakes, unable to generate body heat internally, as we mammals do, seek
roads for the warmth they radiate. On cool evenings they bask on the tarmac
and are killed en masse. The full magnitude of roadkill is dii cult to estimate,
but collisions with large animals are certainly on the rise. In a 2008 report to
Congress, the Western Transportation Institute noted that in 2004, one in
twenty reported vehicle collisions in the United States—some three hundred
