Factors affecting the survival of
Birds of Conservation Concern

by David Leech

5.1.3 Climate

• Several studies have indicated that unusually cold winters frequently result in a reduction in the population sizes of a wide range of bird species, particularly passerines, during the following breeding season (Dobinson and Richards, 1964; Cawthorne and Marchant, 1980)

• Increased energy expenditure necessary to maintain body temperature in cold conditions may potentially lead to deterioration in body condition and ultimately mortality. O'Connor (1980) reported that air temperatures in November and January were positively correlated with the size of great tit populations.

• Greenwood and Baillie (1991) found that the duration of winter snow cover was positively correlated with a reduction in the population sizes of a variety of common breeding birds, particularly ground-feeding species, suggesting that decreased food availability and not air temperature per se may be the cause of observed population declines.

• The incidence of frosts in late winter, which may also affect food availability, has been shown to correlate negatively with the population size of song thrushes in the following year (Baillie, 1990).

Graph showing the negative relationship between song thrush abundance, as indicated by the Common Bird Census index, and the number of freezing days in the preceeding January.

• The mortality rates of first-year and adult song thrushes (Thomson et al., 1997) and lapwings (Peach et al., 1994), both species of conservation concern, have been observed to increase in response to cold winters.

Graph showing the positive relationship between first-year song thrush survival probability and mean daily temperature during the winter months.

Graph showing the relationship between the survival probability of adult lapwings and mean daily temperature during winter.

• Pinowski and Pinowska (1985) observed that the survival rates of Polish tree sparrows, inferred from the proportion of occupied nest boxes at the start of the breeding season, were negatively correlated with the duration of snow cover during the winter period.

Tree sparrows survive less well in winters with heavy snowfall.

• The survival rate of British wrens (Troglodytes troglodytes) has also been shown to decrease as the duration of snow cover increases (Peach et al., 1999). The same study found that treecreeper (Certhia familiaris) survival was negatively related, not to snow cover, but to the amount of rainfall during the winter months, which may affect both foraging opportunity and temperature regulation. Whilst neither of these species is of conservation concern, these results are of interest as they are likely to reflect a more general relationship between weather conditions and survival rates across passerine species.

Treecreeper survival rates are lower in wet winters.

• However, the survival of first-year reed buntings was found to be unrelated to winter conditions (Peach et al., 1999). The relationship between weather conditions and survival may therefore differ between species

• Decreases in the size of British sedge warbler (Acrocephalus schoenobaenus), whitethroat (Sylvia communis) and sand martin (Riparia riparia) populations have all been linked with periods of severe drought on their African wintering grounds (Baillie and Peach, 1992), suggesting that climatic conditions experienced in other countries should also be considered when investigating population size changes of migratory species in Britain. Again, these species are not themselves of conservation concern. However, these studies are still of interest as they may aid in the interpretation of population declines of other migrant passerines for which less data are available.

Drought conditions on their African wintering grounds may lead to a decrease in the size of European whitethroat populations.

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