density dependent (as found in some migratory species, Newton 1998b), mortality on autumn migration could be offset by reduced winter losses, and have no effects on subsequent breeding numbers. In contrast, mortality on spring migration leaves little or no time for any such compensation to occur before breeding begins.

The above models concern effects at the population level, but events on migration can also influence the subsequent performance of individuals, as carryover effects. An important element of competition among migrants concerns the timing and speed of the journey, especially in spring. Within populations, those individuals that arrive in breeding areas in the early part of the arrival period usually take territories in the best habitat (identified as such from previous work in the area), begin breeding first, and show the highest nest success (Chapter 14). Later arrivals are relegated to poorer habitat, and may even fail to acquire a territory or a mate, so cannot reproduce that year. Similar pressures may be assumed to affect arrival in wintering areas, if the first arrivals take the best habitats, and thereby have greater chance of surviving the non-breeding season (for American Kestrel Falco sparverius see Smallwood 1988, for Ruddy Turnstone Arenaria interpres see Whitfield, in Wernham et al. 2002). These various considerations give rise to the proposed model of stopover ecology depicted in Figure 27.2, which links individual migratory performance to subsequent survival or breeding success. In due course, changes in the performance of individuals may lead to change in population size, the most difficult link to establish in migratory birds.

Figure 27.2 Flow diagram showing the hypothetical links between spring food supply, migratory performance, arrival in breeding area and subsequent breeding success. The two columns show the extremes in a spectrum of variation between individuals. From Newton (2005).
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