Evolution
generally has adjusted the timing of avian breeding seasons
to maximize the number of young produced. In the temperate,
subarctic, and arctic zones, the overriding factor is the
availability of food. Abundant nourishment is needed, not
only by growing nestlings and juveniles, but also to meet
increased energy demands of breeding adults. For females
those increased demands include the energetic burden of
producing eggs; males need additional energy to support
vigorous displays and to defend territories. One or both
adults generally participate in the work of building a nest,
foraging for more than one individual (mate or chicks), and
in some cases territorial defense or guarding young from
predators. For most birds the young
hatch and grow when insects are abundant. In the arctic and
subarctic, egg laying is concentrated primarily in May and
June to take advantage of the late June-early July flush of
mosquitoes, blackflies, butterflies, and other six-legged
prey. The supply is rich near the pole, but the season is
short, and birds must court, mate, and nest well before the
risk of frigid storms is over. In fact, geese that nest in
the arctic arrive on the breeding grounds before the snow is
gone, in order to start incubating as soon as nest sites are
clear. The geese depend on reserves of body fat to sustain
them in an initially food-poor environment. In general, the number of
passerine broods raised annually decreases as the poles are
approached. Widely distributed species in North America that
manage to rear only one brood at the northern edge of their
ranges, may rear two or more at their southern limits. In
temperate areas, many passerine species commonly renest if a
clutch or brood is lost; in contrast, many nonpasserines can
produce only one brood. In some nonpasserines, such as
arctic-breeding geese, the reproductive organs begin to
shrink as soon as the eggs are laid. These birds have
neither the energy reserves to lay replacement eggs if a
clutch is lost, nor sufficient time to rear the young of a
second clutch even if one could be produced. In fact, the
young of arctic-breeding geese often do not have time to
mature fully before winter conditions return, and seasons
without successful breeding are common for species such as
Snow and Ross' Geese. Although not the only
factors, assured food supplies and accompanying benign
weather are by far the most common influences that affect
the timing of the avian breeding seasons. To find examples
of other factors, however, we must look outside of North
America. For instance, to reduce predation on eggs and
young, the Clay-colored Robin (which only rarely nests in
South Texas) breeds in the dry season in Panama, when food
is relatively scarce. Fewer losses to predators more than
compensate for the risk of starvation for the
chicks. In addition to such ultimate
causes favoring the evolution of breeding at a particular
time, we must consider environmental changes that are
proximate causes of the triggering of breeding behavior. The
overwhelming majority of bird species living outside of the
tropics sense that it is time to start breeding by the
lengthening of the days as spring approaches. Day length,
per se, has relatively little to do with breeding success,
although, of course, long hours of daylight to forage --
especially for time-constrained bird populations in the Far
North -- can be very important. But evolution seems to have
latched on to day length as a "timer" of activities, since
it is a signal that can be used to forecast future events.
If, for instance, birds that breed in the arctic did not
start to develop their reproductive organs until insects
were abundant, the insects would be gone before the eggs
hatched. The day length cue for development occurs long
before the insects emerge. Other factors, such as weather
(and associated abundance of food), also play important
roles in starting the reproductive process, and especially
in fine-tuning responses to the cues already provided by day
length. For example, if Red-winged Blackbirds are
experimentally provided with abundant food, they will begin
laying their eggs three weeks earlier than birds without
supplemented diets. At least some birds also
have "biological calendars" -- internal timing devices that
are independent of external environmental cues and tell them
when it is time to breed. Consider experiments involving the
Short-tailed Shearwater, a Southern Hemisphere species that
"winters" in the summer off the Pacific coast of North
America but breeds on islands near Australia. Birds were
kept in a laboratory for over a year and subjected to a
constant light regime, 12 hours of light and 12 hours of
darkness, for the entire period. In spite of this constancy,
their reproductive organs developed and their feathers
molted at the same time as those of Short-tailed Shearwaters
in the wild. The physiological basis for biological clocks
and calendars -- the mechanisms by which they function --
remains one of the great mysteries of biology. SEE: Metabolism;
Polyandry
in the Spotted Sandpiper;
Variation
in Clutch Sizes. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.