The ecology of dippers Cinclus cinclus in relation to stream acidity in upland Wales: breeding performance, calcium physiology and nestling growth

Laying dates were later, clutch and brood sizes smaller, and nestling growth slower along acidic streams by comparison with circumneutral streams. Second clutches were absent from acidic streams, but occurred at 20% of circumneutral sites. Though egg-laying was delayed at higher altitudes, pH had additional significant effects. Pairs began laying later by 13-22 days at acidic sites than circumneutral sites even after removing the effect of altitude. Reduced clutch size at acidic sites in 1986 could be explained solely by delayed laying, but pH had an additional effect in 1985 and in the combined years' data. Smaller broods at acidic sites could be explained solely by reduced clutch size. Reduced daily mass increase in nestlings at acidic sites could be explained by delayed laying, but reduced tarsal growth was affected by pH after accounting for the effects of laying date. Adult mass was significantly lower on acidic streams than on circumneutral streams in the pre-breeding period. The age structure of the population did not differ between the two types of stream, though the sex ratio on acid streams was female biased. Serum Ca was significantly lower in both males and females at acidic sites in the pre-breeding period than at circumneutral sites. At acidic sites only, females had higher serum Ca than males, and showed also elevated serum alkaline phosphatase. -from Authors