Diet‐manipulated body condition affects onset and speed of moult in common bulbuls in a tropical environment

Resource acquisition and allocation are central to life history theory, explaining the diversity of strategies among species as well as the distribution of events over the annual cycle. Moult is a major phase in the annual cycle of birds, but explanations for moult scheduling are heavily biased towards temperate systems with seasonal breeding patterns. Our research on a year‐round breeding tropical bird, the common bulbul Pycnonotus barbatus (bulbul), tests whether moult depends primarily on stored body reserves (capital) or on resources acquired throughout the moult period (income). Making this distinction elucidates trade‐offs between moult, and other annual cycle events, and responses to environmental change. We estimated moult start date and duration in captive bulbuls whose body condition we experimentally manipulated by feeding them fruits or invertebrates 6–3 and 3–0 months before moult, and fruits or a mixed diet during moult. We studied free‐living bulbuls as reference group. We found that moult onset is best predicted by diet‐manipulated condition just before moult, while moult duration is best predicted by diet‐manipulated condition during moult. Specifically, invertebrate‐fed bulbuls started moult 33 days later than fruit‐fed bulbuls. In addition, once invertebrate‐fed bulbuls were switched to a mixed diet, they moulted 52 days quicker than fruit‐fed bulbuls, albeit still 36 days slower than free‐living bulbuls on average. Males started moult 15 days earlier and had a more variable start of 20 days, but did not moult quicker than females on average. Our findings indicate that moult in bulbuls is both income‐ and capital‐dependent, with moult initiation determined by individual body reserves and feather growth still occurring on a fruit‐only‐diet, but is significantly improved by dietary proteins from a mixed diet and in field bulbuls. In this year‐round breeding bird, moult seasonality is maintained in the absence of breeding, but heavily influenced by foraging conditions.