THE ADAPTIVE SIGNIFICANCE OF BROOD SIZE AND TIME OF BREEDING IN THE DIPPER CINCLUS-CINCLUS (AVES, PASSERIFORMES) AS SEEN FROM POSTFLEDGING SURVIVAL

We assessed the post-fledging survival of dippers Cinclus cinclus from 743 broods in relation to brood size, time of hatching and territory quality. We paid particular attention to assessing whether contrasting breeding performance along unproductive (i.e. acidic) and productive (i.e. circumneutral) rivers represented strategies which optimized the number of surviving young. For all brood sizes, post-fledging surviva l varied significantly through the breeding season, with most survivor s coming from attempts in the peak period of hatching. After correctin g for these seasonal effects, the most common brood size overall, of f our, was also the most productive as seen from post-fledging survival; differences in the frequency of occurrence and survival between brood s of four and five were marginal. Moreover, a change in the modal broo d size from five to four occurred as the season progressed, consistent with a shift in brood productivity. Broods at acidic sites were signi ficantly smaller than at circumneutral sites; while brood size four wa s the most productive at both types of site, brood size three was the second most productive at acidic sites, while brood size five was the second most productive al circumneutral sites. Dippers at acidic sites bred significantly later than at circumneutral sites, but post-fledgi ng survival declined most rapidly through the season at the former. Th ese survival data provide evidence from both seasonal and spatial patt erns that brood sizes in the dipper may be optimized in ways consisten t with the enhancement of productivity. By contrast, delayed breeding at acidic sites contrasted with the patterns expected from optimizatio n, instead reflecting resource scarcity.