In those bird species in which pair formation occurs in the wintering or migration areas, as in some waterfowl, the sexes arrive together in their breeding areas. However, in most bird species pair formation occurs in the breeding areas, with males arriving first to establish territories in which they then attract females (for White-crowned Sparrow Zonotrichia leucophrys see Figure 15.1). The sex difference in mean arrival dates can vary from a few days to a few weeks, depending on species, year and area. Among 18 species of North American warblers studied over many years at Prince Edward Point in Ontario, the sex difference in mean arrival dates was greater in species that arrived early in the season than in those that arrived later, and within species, the sex difference in arrival dates was greatest in years when males arrived earliest (Francis & Cooke 1986). These findings were typical of those from many other species studied elsewhere (Cramp 1988, Morgan & Shirihai 1997).
Because in most species males compete for territories (or mates), they may gain more advantage than females in staying on or near their nesting territories
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