In many species, adults trapped at stopover sites put on weight more rapidly, achieved greater body reserves, and stayed for shorter periods than first-year birds; and within age groups, males put on weight more rapidly, achieved greater body reserves, and stayed for shorter periods than females.1 These differences may have arisen because young birds were at a competitive disadvantage in the presence of older ones, and females in the presence of males. Juveniles may also have foraged less efficiently than adults, resulting in generally slower fuel accumulation and migratory progress, with longer or more frequent stopovers (for Dunlin Calidris alpina see Rösner 1990, for cranes and raptors see Ueta & Higuchi 2002). In some species in which females are bigger than males, the sex-related difference in fuelling rates was reversed, as females at particular sites tended to have shorter stopovers (Figuerola & Bertolero 1998, Butler et al. 1987).
Such age and sex differences could influence the travel speeds, departure and arrival dates, and survival chances of individuals during migration. It is hard to tell how much they result from competition acting here and now at stopover sites, or from genetic age-linked and sex-linked differences in migration strategies that have evolved as a result of competition and other differential selection pressures in the past. However, age and sex differences are not apparent in all species, or at all stopover sites (Morris et al. 1996, Maitav & Izhaki 1994). Occasionally, juveniles are found to be heavier than adults on average (Butler et al. 1987, Alerstam & Lindström 1990, Woodrey 2000), possibly because juveniles migrate at a more favourable time of year (earlier in autumn or later in spring), so may have the advantage of a better food supply than adults.
In conclusion, increasing bird densities at stopover sites have been shown to intensify competition, reducing food availability through depletion or interference, lowering food intake and fat deposition rates, and thereby slowing migration. Some individuals are affected more than others. Interference competition may also redistribute birds among habitats, with younger less experienced migrants forced into sites where feeding rates are lower. While competition for food may limit the number of birds that can fatten simultaneously at particular sites, it may have influence on the overall population level only if displaced birds find no alternative places to fatten, or migrate more slowly with adverse consequences.
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