Migration
Birds are directed in their travels mostly by instinct. Exactly how they find their way, often covering thousands of miles, is a study in itself. Many of our popular songbirds—including most thrushes, flycatchers, vireos, warblers and orioles—are termed "Neotropical migrants," which means they migrate to Missouri from the tropics for the summer-nesting season only. Others are transients, meaning they merely pass through in spring and fall. Still others, such as the dark-eyed juncos and American tree sparrows, are with us only during the winter months. Finally, some of Missouri's most familiar birds—crows, cardinals, goldfinches, chickadees and most woodpeckers—remain here as permanent, year-round residents.
A Typical Nesting Cycle: The American Robin
Having survived the winter in nomadic flocks, robins show up in our parks and gardens during early March. Each male stakes out a nesting territory by singing and chasing other male robins from an area that satisfies family needs. Females arrive about a week later and are courted by the males, who display by strutting and shaking their wings. The female is believed responsible for mate selection. Nesting behavior varies considerably among birds. In the case of robins, both sexes share in building the nest, which is composed of a mixture of mud and grass. Incubation is done solely by the female. This is typical of species in which the male is more colorful and the female more drab. Presumably, the drabness is less conspicuous to nest predators. To ensure that all four to six eggs hatch together, she does not begin to incubate until her entire clutch has been laid. The eggs hatch after only two weeks of incubation.
Newly hatched robins are blind, featherless and require intense parental care. Approximately two-thirds of Missouri's birds, and all our songbirds, have helpless young that are called altricial. In contrast, birds such as northern bobwhites, killdeers and ducks incubate for nearly a month, and they hatch chicks that are clothed in down and ready to leave the nest and feed. These young are termed precocial.
Nestling robins, like those of all birds, do nevertheless grow rapidly. Fed and brooded by both parents, they are fully fledged and ready to take wing after only two weeks in the nest. By this time, male robins take over the supervision of fledglings while their mates lay another clutch of eggs, often in the same nest. Many species of songbirds typically produce more than one brood per year, though not always in the same nest.
Nesting Strategies for Survival
As with wild creatures in general, a species' annual production of young birds reflects the species' annual mortality rate. That is, if the number of individuals of a species is stable, the number that die each year must roughly equal the number that are added by reproduction. Northern bobwhites, which can nest twice in a season with 12 to 15 eggs per nest, therefore are expected to have a high mortality rate. They are vulnerable to many hazards, including nest predation. Red-tailed hawks, by contrast, nest only once a year and rear two to three young each time. Their long life expectancy compensates for their relatively low reproductive rate. In short, nesting behavior among birds does more than simply entertain us. It is a strategy unique to that species that helps to ensure survival and the passage of genes to subsequent generations.