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

(1) We compared the laying dates, clutch size, brood size and nestling growth of dippers (Cinclus cinclus) breeding on acidic and circumneutral Welsh streams in 1985 and 1986. Serum calcium and serum alkaline phosphatase concentrations were measured in nestlings and adults from the wild population in 1988-89. (2) Laying dates were later, clutch and brood sizes smaller, and nestling growth slower along acidic streams by comparison with circumneutral streams. All differences were significant. Second clutches were absent from acidic streams, but occurred at 20% of circumneutral sites. (3) 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. (4) 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. (5) Smaller broods at acidic sites could be explained solely by reduced clutch size. (6) Reduced daily mass increase in nestlings at acidic sites could be explained by delayed laying, but reduced tarsal growth was affected by pH at P = 006 after accounting for the effects of laying date. (7) 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 biassed. (8) Serum calcium 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 calcium than males, and showed also elevated serum alkaline phosphatase. Nestling serum calcium was lower at acidic sites only up to 8 days after hatching. (9) We discuss our results in relation to the reduced availability of calcium in dipper prey along acidified streams.