The adults of most birds leave the young when they are grown. This gives the parents an opportunity to rest and renew their plumage before starting for winter quarters. The young are likely to move south together ahead of their parents. This has been documented in a number of species including our mourning dove, the common swift of Europe, and storks. Mueller and Berger (1967) found an age-specific migration pattern in sharp-shinned hawks passing through Wisconsin. The immatures were much in evidence during mid-September while the adults came through a month later. Far to the south in the Antarctic, young Adelie penguins depart for northern wintering grounds much earlier than adults.
[Illustration: _Figure 7. Average number of species captured daily in mist nets during spring migration at Eilat, Israel, in 1968. The number of species passing through an area on migration will rise and fall similar to the number of birds counted in the area. In this case two major movements came through about 1 month apart._]
In a few species, adults depart south before the young. Adult golden plovers, Hudsonian godwits, and probably most of the Arctic breeding shorebirds leave the young as soon as they are capable of caring for themselves and set out for South America ahead of the juveniles. Likewise, data for the least flycatcher indicate adults migrate before the young, but Johnson (1963) did not find this segregation in the Hammond's flycatcher. In Europe, adult red-backed shrikes are known to migrate ahead of their young.
[Illustration: Cretzschmar's Bunting Ortolan Bunting Black-headed Bunting]
[Illustration: _Figure 8. Average number of three species of buntings captured daily in mist nets during spring migration at Eilat, Israel, in 1968. Closely related species that migrate through the same area often appear at different times. Thus species that may eat the same foods do not compete with each other._]
In contrast to this loss of parental concern, geese, swans, and cranes remain in family groups throughout migration. The parent birds undergo a wing molt that renders them flightless during the period of growth of their young so that both the adults and immatures acquire their flight capabilities at the same time and are able to start south together. Large flocks of Canada geese, for example, are composed of many families banded together. When these flocks separate into small V-shaped units it is probably correct to assume an old goose or gander is leading the family. After female ducks start to incubate their eggs, the males of most species of ducks flock by themselves and remain together until fall. When segregation of the sexes such as this occurs the young birds often accompany their mothers south. Murray and Jehl (1964) concluded from mist-netting many thousands of migrant passerines at Island Beach, New Jersey, that adults and juveniles travel at approximately the same time.