Most discussion of population limitation in migrants carries the implicit assumption that conditions in wintering areas have no effects on subsequent breeding success, and that conditions in breeding areas have no effect on subsequent winter survival. These assumptions are not entirely justified. The breeding densities of many bird species are determined partly by winter survival, and winter survival can in turn be related to events in the previous breeding season. But more than this, individual birds can carry over effects from one season to the next, and these residual effects, acting through body condition, can explain some of the variation in breeding success and winter survival. For example, among geese and other waterfowl, foraging conditions at wintering and migration sites have long been known to affect subsequent breeding success through the affect of food supplies on body condition. In most such species, conditions on spring staging areas seem more important than conditions in wintering areas, so further discussion is left for Chapter 27. However, in Whooper Swans Cygnus cygnus, the proportions of young in wintering flocks in Sweden each year were correlated with the mean temperature in the preceding winter, implying that winter temperatures influenced subsequent breeding success (presumably though effects on feeding conditions and body reserves) (Nilsson 1979).
Carry-over effects of winter conditions on breeding success have been detected not only in large birds, which carry substantial body reserves to breeding areas, but also in small birds, which carry relatively smaller reserves. Thus, individual Barn Swallows Hirundo rustica breeding in Italy were affected by conditions in wintering areas, as estimated from the satellite-recorded 'normalised difference vegetation index' (NDVI) used to assess the annual state of the vegetation (dependent on rainfall) in Africa (Saino et al. 2004). The arrival dates and egg-laying dates of the same individuals in consecutive breeding seasons were advanced, more second broods were produced, and the total seasonal production of young was increased, after winters with high NDVI in African winter quarters. Other carry-over effects from winter to summer, affecting morphology, migration dates and breeding, were described in another population of Barn Swallows breeding in Denmark (Moller & Hobson 2004).
One important factor contributing to reproductive success among migratory birds is date of spring arrival and commencement of nesting. Within populations, individuals that arrive and start nesting early in the season do better, in terms of habitat quality, territory acquisition and number of young raised, than do those that
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