Migration And Deferred Breeding

In some long-lived bird species which do not breed until they are several years old, differences in wintering areas are apparent between several successive age groups, as individuals move progressively less far from the breeding areas as they age, or spend shorter periods in the more distant parts of the wintering range (for Herring Gull Larus argentatus see Coulson & Butterfield 1985, for Western Gull L. occidentalis see Spear 1988). This pattern is shown for the Herring Gull in Figure 15.6, based on month-by-month analysis of ring returns from breeding colonies around the Great Lakes in North America (Moore 1976). In November, when the weather was mild, all age classes were recovered near the breeding areas. But as winter progressed, the age classes became increasingly separated, with the youngest birds moving furthest south. Then, as conditions improved again from March, all age classes moved northwards towards the breeding places, where most of the population spent the summer. Nevertheless, some first- and second-year birds remained well south of the nesting areas during the breeding season.

The difference between age groups in date of return to breeding areas is greatest in those (mostly large) species that do not breed until they are several years of age, and in which the immatures are therefore under less pressure to return early to the breeding areas in spring. In some such species, the young migrate much later than the adults, and take longer over the journey. In the various eagle species that pass through Israel each spring, the age groups migrate in order of oldest first to youngest last, but there is considerable overlap between them (Shirihai et al. 2001). The youngest age groups of some eagles and vultures return to breeding areas up to several weeks later than adults, and too late to attempt breeding that year. Radio-tracking revealed that immature Lesser Spotted Eagles Aquila pomarina and Steppe Eagles A. nipalensis migrate more slowly and later in spring, and arrive in breeding areas up to 10 weeks later than adults (Meyburg et al. 1995c, 2001, Meyburg & Meyburg 1999). As they make no attempt to nest, the immatures suffer no obvious penalty by arriving late. The same held for Steller's Eagles Haliaetus pelagicus, some juveniles of which migrated a shorter distance

Figure 15.6 The seasonal distribution of Herring Gulls Larus argentatus breeding in the Great Lakes region of North America. Left: Monthly proportion of ring recoveries from each age class south of the Great Lakes region. Right: Mean monthly distance of ring recoveries from each age class. Based on 6949 recoveries of juveniles, 1900 of one-year-olds, 879 of two-year-olds, and 2956 of older birds (adults) obtained during 1929-1971 from birds ringed at colonies mainly within 44-46°N. Year classes separated at 31 May each year, the approximate date of peak hatch. From Moore (1976).

Figure 15.6 The seasonal distribution of Herring Gulls Larus argentatus breeding in the Great Lakes region of North America. Left: Monthly proportion of ring recoveries from each age class south of the Great Lakes region. Right: Mean monthly distance of ring recoveries from each age class. Based on 6949 recoveries of juveniles, 1900 of one-year-olds, 879 of two-year-olds, and 2956 of older birds (adults) obtained during 1929-1971 from birds ringed at colonies mainly within 44-46°N. Year classes separated at 31 May each year, the approximate date of peak hatch. From Moore (1976).

and spent the summer in areas well south of their natal areas (Ueta et al. 2000, McGrady et al. 2003).

In three species studied in Japan by satellite-tracking, namely Steller's Eagle Haliaeetus pelagicus, Black-faced Spoonbill Platalea minor and White-naped Crane Grus vipio, the immatures on spring migration stayed, on average, about twice as long on stopover sites, and took about twice as long over the whole journey as adults. In this study, the two age groups showed no difference in the mean distances between stopover sites or in the overall distance of migration (Ueta & Higuchi 2002). The longer stopovers of immatures may have reflected a lack of urgency in reaching breeding areas, as well as a lower feeding efficiency and rate of fuel accumulation.

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