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 the
reference group. We found that moult onset is best predicted by the
diet-manipulated condition just before moult, while moult duration is best
predicted by the 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 20-day more variable start, 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 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.