Getting the Migration Timing Right - Page 2

There seems to be a premium on getting to nesting areas as soon as possible. In fact, many kinds of waterfowl begin move northward as soon as the lakes and ponds are released from the grip of ice. But at the same time there may be a heavy price to pay if the birds arrive too early -- weather is less dependable in early spring.

Though it is possible to delineate the migration period of every species, the situation is actually quite complex. In may species, birds of different age and sexes tend to migrate at somewhat different times. In spring, the males of many species of songbirds precede the females, presumably because it is to a male's advantage to arrive in its breeding are early and stake out its territory before its rivals do.

In fall, when a large part of a species's population may be young birds on their first migration, the older and younger birds may migrate at different times. In many shorebirds, for example, the parents abandon their precocial young on the breeding grounds and head south in mid- to late summer. The young follow weeks or months later, so that within the migration period of a given species there is one pulse of adult birds and a second pulse of first-time migrants. These kinds of patterns typify what is known as differential migration of age and sex classes.