Weak breeding seasonality of a songbird in a seasonally arid tropical environment arises from individual flexibility and strongly seasonal moult

In some tropical birds, breeding seasonality is weak at the population level, even where there are predictable seasonal peaks in environmental conditions. It therefore remains unclear whether individuals are adapted to breeding at specific times of the year or flexible to variable environmental conditions. We tested whether the relative year-round breeding activity of the Common Bulbul Pycnonotus barbatus arises due to within-individual variability in breeding dates. We collected data from 827 birds via mist-netting over two years with corresponding local weather data. We used a combination of climate envelope and generalised linear mixed models to explore how the timing of breeding is influenced by time of year, individual variation, rainfall and temperature in a West African savannah where seasonal precipitation determines annual variation in environmental conditions. We also pooled 65 breeding records from 19 individuals recorded between 2006 and 2017 based on brood patch occurrence and behavioural observation to compare within individual and population variability in breeding dates. We show that the breeding dates of individuals may be as variable as the population as a whole. However, we observed a seasonal peak in juvenile occurrence that varies significantly between years. Models suggest no relationship between nesting and moult, and within-year variation in rainfall and temperature, and birds were unlikely to breed during moult but may do afterwards. Moult was very seasonal, correlating strongly with day length. We suggest that because environmental conditions permit year-round breeding, and because reproductive output is subject to high predation risk, there is probably a weak selection for individuals to match breeding with variable peak conditions in the environment. Instead moult, which always occurs annually and successfully, is probably under strong selection to match variable peak conditions in the environment so that long term survival ensures future reproduction.