Some General Principles

If we accept that, for various reasons, the total carrying capacities of the breeding and non-breeding habitats of particular populations need not necessarily correspond, then two scenarios are possible when birds return to their nesting areas each year and compete for territories or nest-sites:

1. Too few birds are left at the end of the non-breeding season to occupy all nesting habitat fully, so that practically all individuals of appropriate age and condition can breed (Figure 26.1A). In this case, breeding numbers would be limited by whatever factors operate in the non-breeding areas, and alleviation of these factors (such as the extent or carrying capacity of non-breeding habitats) would

Figure 26.1 Model showing seasonal changes in total bird numbers in relation to the carrying capacity of the breeding area (thick line). In the lower curve (A) numbers left at the end of the non-breeding season are fewer than the nesting habitat could support, and in the upper curve (B) numbers are greater than the nesting habitat could support, leading to a surplus of non-territorial, non-breeders. In (A), breeding numbers are limited by conditions in the area occupied in the non-breeding season, and in (B) by conditions in the area occupied in the breeding season. From Newton (1998b).

Figure 26.1 Model showing seasonal changes in total bird numbers in relation to the carrying capacity of the breeding area (thick line). In the lower curve (A) numbers left at the end of the non-breeding season are fewer than the nesting habitat could support, and in the upper curve (B) numbers are greater than the nesting habitat could support, leading to a surplus of non-territorial, non-breeders. In (A), breeding numbers are limited by conditions in the area occupied in the non-breeding season, and in (B) by conditions in the area occupied in the breeding season. From Newton (1998b).

be needed before breeding numbers could rise. For convenience, the breeding numbers in such populations could be described as 'winter-limited'. 2. More birds are left at the end of the non-breeding season than the available nesting habitat can support, producing a surplus of non-territorial, non-breeders (Figure 26.1B). In this case, alleviation of the factors that influence the extent or carrying capacity of the nesting habitat would be needed before breeding numbers could rise. For convenience, the breeding numbers of such populations could be described as 'summer-limited'.

The same two types of scenario could hold at the end of the breeding season, as birds return to their non-breeding quarters. At that time, the total numbers of adults and young might be insufficient to use the resources of non-breeding areas to the full, leading to good survival through the non-breeding season; alternatively, the total numbers may exceed the carrying capacity of non-breeding areas, leading to intense competition and poor survival.

Because bird reproduction is by definition confined to the breeding range, numbers there might be 'summer-limited' in a different way, namely if breeding success were so poor that subsequent breeding numbers could not reach the level necessary to fill either the available breeding habitat or the available wintering habitat. This situation could arise even with relatively good year-round survival. In other words, while failure to occupy all wintering habitat must be due to events on breeding or migration areas, failure to occupy all breeding habitat could be due to events on either breeding, migration or wintering areas. Understanding the limiting mechanisms in any one population may not be easy, especially if the situation changes from year to year. There is no reason why a species might not be summer-limited in one year or area and winter-limited in another year or area (for examples see Newton 1998b).

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