Examples Of Species Affected By Events In Breeding Or Wintering Areas

In some species, year-to-year changes in overall population levels have been clearly driven largely by conditions in breeding areas. On the North American prairies, rainfall varies greatly from year to year, and influences the amount of wetland habitat available to nesting waterfowl. In wet periods, populations increase, and in dry periods they decline. So important are these prairie wetlands as nesting habitat that they influence the entire continental wintering populations of several species, including the American Coot Fulica americana (Figure 26.3). They show how year-to-year conditions in the breeding areas can largely determine year-to-year fluctuations in total numbers.

In contrast, the numbers of several migratory songbird species counted each spring on their European breeding areas have fluctuated according to rainfall (and hence food supplies) in their African wintering areas. Examples include the Sedge Warbler Acrocephalus schoenobaenus, Sand Martin Riparia riparia and Purple Heron Ardea purpurea (den Held 1981, Peach et al. 1991, Bryant & Jones 1995, Szep 1995). They show how conditions in wintering areas can largely determine year-to-year fluctuations in total populations, although it is seldom certain whether most mortality occurs during the wintering period or during migration (see Chapter 24).

Other examples of summer-influenced and winter-influenced population changes in migrants, involving more than 67 different populations of 56 species, are given in Table 26.1. In most populations, the evidence is entirely circumstantial, based on long-term or year-to-year correlations between changes in breeding numbers and changes in either: (1) conditions in breeding or wintering areas, or in (2) associated

123456789 Number May ponds (millions)

123456789 Number May ponds (millions)

Figure 26.3 Relationship between the numbers of American Coots Fulica americana shot each year in the USA (reflecting total population size) and the number of ponds on the prairies in the preceding summer (reflecting the habitat and feeding conditions). Significance of relationship: b = 0.21, r2 = 0.54, P < 0.001. Redrawn from Alisauskas & Arnold (1994).

Table 26.1 Migratory bird species in which temporal changes in breeding density have been linked with changes in previous winter conditions (A), with changes in summer conditions (B), or with changes in both winter and summer conditions (C)

Species Location Long-term Annual Source upward or fluctuations downward trend

A. Associated with change in winter conditions/survival

Twite Carduelis flavirostris

Germany-Netherlands +

Dierschke (2002)

Snow Bunting Plectrophenax nivalis

Germany-Netherlands +

Dierschke (2002)

Shore Lark Eremophila alpestris

Germany-Netherlands +

Dierschke (2002)

Sedge Warbler Acrocephalus schoenobaenus

England

+ Peach et al. (1991)

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