Winter food limits timing of pre‐alternate moult in a short‐distance migratory bird

Abstract

Summary Moult is critical for fitness for many organisms for several reasons: it allows growth and maintains the function of the integument for protection, thermoregulation and communication. Feather moult in birds is costly and therefore typically does not overlap with migration or reproduction. In spring, the rapid succession of pre‐alternate moult, migration (if a migrant) and breeding suggests that timing of moult could constrain the initiation of breeding. A trade‐off between time spent moulting and breeding might also limit moult quality. The proximate basis for the timing of pre‐alternate moult initiation is not well known, although it typically occurs during a resource poor time of year. Food limitation combined with fitness consequences of moulting earlier suggests that plasticity in timing of pre‐alternate moult in response to food abundance should be advantageous. We experimentally tested, for the first time, if food abundance influences the timing of moult in the wild. We conducted a controlled food supplementation experiment on free‐living swamp sparrows (Melospiza georgiana) preceding and during the time of natural pre‐alternate moult (January–March 2009, 2010). Supplemented birds began moulting the body, face and crown earlier than control birds (11, 14, and 8 days earlier, respectively), indicating that food abundance limits the initiation of moult. Along with interannual variation, these results indicate that photoperiod is not the sole cue for initiation of moult. Both control and supplemented birds moulted in sequence, starting with the body, followed by the crown 9 days later, and the face 11 days after the body. The presence of a sequence further suggests energetic limitation of moult or possibly a strategy to moult‐specific regions first to ensure completion or growth at an optimal time. This study provides novel experimental evidence that food abundance can: (i) limit pre‐alternate moult timing and (ii) limit moult timing in the wild. Food limitation of moult timing could allow earlier breeding or production of higher quality feathers and thus cascade through other life‐history stages and affect reproductive success. These results indicate that food availability is a cue for moult initiation, possibly acting secondarily to photostimulation.