Biology Reference
In-Depth Information
Q Why do I see some species every winter, but other species only every few years?
A
Some food and nesting resources are predictable and found readily year after year, but
some vary greatly. Robins can return to the same yards every spring for nesting, and ex-
cept in extreme droughts can count on a fairly regular supply of worms for feeding on
and mud for nest building. But in winter, the trees of one area can be rich in fruits one
year and barren the next. So they wander widely in winter. Many finches depend on con-
iferseeds duringthewinter.Oneyearconifer cones maybeabundant inonearea, andthe
nextyearbirdsmayhavetotravelhundredsofmilestofindthesamefood.Thebirdsthat
move about feeding on these unpredictable food sources are called “irruptive.”
Q Why do some hawks migrate and some stay through the winter?
A
Different hawks specialize on different prey. Those that eat rodents in large fields can
either head for South America and find rodents in large, summery fields down there, as
Swainson's Hawks do, or stick it out in the north, as Red-tailed Hawks do, grabbing an-
imals in large fields whether they're peeking out from the snow or running across a road.
PeregrineFalconsthatspecializeonshorebirdsoftenflyallthewaytoSouthAmerica;
those that learn to catch city pigeons can spend the entire winter in the north. Some in-
dividual Cooper's Hawks have learned to exploit bird feeders for easy prey. They re-
main north while other individual Cooper's Hawks head to Central America to feed on
warblers, tanagers, and orioles.
Q Every summer most of my hummingbirds disappear for a few weeks and then
suddenly reappear in large numbers. What's going on?
A
Adult male hummingbirds aggressively defend their territories, and if your yard is
within the territory of one, he may drive all other male hummingbirds away during the
