Males and females of some species may migrate either simultaneously or separately. In the latter case it is usually the males, rarely the females, that arrive first. Sometimes great flocks of male red-winged blackbirds reach a locality several days before any females; this is particularly the rule in spring. The first robins are usually found to be males, as are also the first song sparrows, rose-breasted grosbeaks, and scarlet tanagers. In Europe, the three buntings mentioned previously are also segregated as to sex during migration. Figure 8 shows two prominent peaks for both the Cretzschmar's and Ortolan buntings; during passage the first peak was primarily males while the second peak consisted mostly of females. This early arrival of males on the breeding grounds is associated with territorial possession whereby the male selects the area where it intends to breed and each individual attempts to protect a definite territory from trespass by other males of his own kind, while announcing his presence to rival males and later arriving females by song or other display. The female then selects the site where she wishes to nest. The long-billed marsh wren is a noteworthy example; the males may enthusiastically build several nests before the females arrive. In the fall, common and king eiders are sexually segregated during migration. During July, flocks crossing Point Barrow are composed almost entirely of males, while after the middle of August the flocks are almost all females (Thompson and Person 1963). In the Chicago area, Annan (1962) reported that some males, such as the hermit thrush, Swainson's thrush, gray-cheeked thrush, and veery, arrive before any females and predominate during the first week of occurrence. In a few species the males and females apparently arrive at the breeding grounds together and proceed at once to nest building. In fact, among shorebirds, ducks, and geese, courtship and mating often takes place in whole or in part while the birds are in the South or on their way north, so that when they arrive at the northern nesting grounds they are paired and ready to proceed at once with raising their families. Mallards and black ducks may be observed in pairs as early as December, the female leading and the male following when they take flight. Naturally, these mated pairs migrate north in company, and it was largely to protect such pairings that duck shooting in spring was abolished by Federal law. In the coastal subspecies of the western flycatcher, the sexes appear to migrate in synchrony during the spring in contrast to migration of Hammond's flycatcher in which the adult males usually precede the females (Johnson 1973). Both sexes of the common blackcap of Europe appear to migrate together at least across the eastern end of the Mediterranean during the spring (Fig. 9). [Illustration: Blackcap]