Effects of stream acidity on non‐breeding dippers cinclus cinclus in the south‐central highlands of Scotland

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Abstract
Macroinvertebrate and Dipper Cinclus cinclus abundances and pH were assessed on four rivers in the south‐central highlands of Scotland in early winter 1990 to examine the general applicability of Dipper/acidity relationships established during the breeding season (Ormerod et al., 1985; Vickery, 1991). The most acid waters had the lowest invertebrate biomasses and Dipper densities. Birds on these rivers spent significantly more of the morning foraging and less time at rest, apparently reflecting reduced prey capture rates and hence the greater time required to replace overnight mass losses. Analysis of published data on seasonal changes in Dipper population densities and individual energetic demands suggests that even accounting for the higher numbers of Dippers in all habitats during autumn and winter, energetic requirements both of individuals and populations are greatest during breeding, when the costs of self maintenance are coupled with the additional demands of chick rearing. However, from the patterns found here and elsewhere during autumn it appears that the effects of acidity on Dipper populations are important generally during both the breeding and non‐breeding seasons.