Breeding Season

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.