Breeding biology and prey availability in the White-throated Dipper in Northern Iberia

The breeding biology of a White-throated Dipper Cinclus cinclus population in the Atlantic region of the Iberian Peninsula was studied. Breeding was monitored in February–May 2015 at 215 nest-boxes and naturally occurring nests along 28 rivers in the five main river basins (Oiartzun, Urumea, Igara, Oria and Urola) in Gipuzkoa (N Spain). Of the 65 recorded nesting attempts, the average laying date was late March, average (±SD) clutch size was 4.3 ± 0.8 eggs, average productivity was 3.8 ± 1.0 chicks, and breeding success was 72%. In 18.5% of nest boxes, there were second clutches. The causes of nesting failure included predation, human disturbance and flooding. In 22 randomly selected occupied nest-boxes, we also studied the effect of food availability on breeding performance by assessing the macroinvertebrate communities in the nesting territories. The amount of macroinvertebrates had no significant effect on clutch size or productivity. Compared to other European populations, Dippers in Gipuzkoa had smaller clutches, which was compensated for by greater breeding success. This suggests that this population breeds in a relatively favourable environment that allows for higher survival rates, thereby reducing pressure on offspring clutch size.