Seasonal, environmental and individual determinants of double‐brooding in the White‐throated Dipper ( <i>Cinclus cinclus</i> )

In many birds, productivity can be boosted by producing more than one brood during the breeding season. This multi‐brooding strategy may become more common as a response to environmental change, especially in those species for which the timing of breeding is advancing or the breeding season is lengthening. Understanding the factors affecting the likelihood and outcome of multi‐brooding is therefore crucial for predicting population change. Here, we investigate double‐brooding in a declining riverbird species, the White‐throated Dipper Cinclus cinclus . Using data collected between 2013 and 2021 from a long‐term study population in the UK, we found that double‐brooding was common, with an average of 29% of pairs per year having a second brood, and that reproductive output was significantly higher in double‐brooded pairs than single‐brooded pairs. The best predictor of double‐brooding was the timing of breeding for the first brood; the earlier this occurred, the higher the likelihood of having a second brood. This suggests that seasonal changes in environmental conditions and food resources may determine opportunities for breeding. The probability of double‐brooding was also higher in older males, indicating that individual quality probably plays a role too, although there was no effect of female age. Clutch size and the number of chicks fledged were lower in second broods than in first broods, and the best predictor of breeding success in second broods was nestbox use. The probability of young fledging was almost twice as high in boxes, suggesting that nestbox provision may be an important conservation measure. Together, these results provide evidence that double‐brooding is driven by favourable environmental conditions, but further work is needed to better understand individual and adaptative variation in this response.