Feathers are essential for flight and thermoregulation as well as for communication. However, producing and maintaining feathers is costly. As with any outer covering, wear and tear necessitates their replacement on a regular basis. The shedding of worn feathers and their subsequent replacement happens through a process called moult. Moulting, especially when it occurs over the entire body at once, incurs a high energetic cost. Not only is it costly to produce the feathers themselves, but birds also sustain additional costs: their thermoregulatory costs rise because of skin exposure and their ability to fly can also be impaired. Thus, especially in temperate climates where the availability of energy is seasonally limited, moult rarely coincides with other energetically costly life stages such as migration and breeding. In most species moulting is deferred until after the breeding season, often late in the summer, to repair damage sustained during reproduction as well as to ensure adequate plumage coverage before the winter. However, species that produce showy breeding plumage also need to moult before reproduction (pre-alternate moult), so that they are equipped to display their readiness to breed.

The factors that trigger initiation of the pre-alternate moult have received little attention to date. Changes in season, primarily indicated by changing photoperiod, are likely to be the main drivers of moult timing but other possible factors include breeding and energy availability. It was the latter hypothesis that led a team of American researchers, headed by Raymond Danner of Virginia Tech University and the Smithsonian Migratory Bird Center to initiate a study into the patterns and timing of moulting in free-living swamp sparrows (Melospiza georgiana) in coastal North Carolina. Specifically, they sought to test whether the pre-alternate moult was limited and/or initiated by energy availability.

The team established four research plots, separated by several kilometres and provided a rich supply of food – in the form of millet and mealworms – on two of the plots to determine the effects of energy availability on moult timing. Then they monitored when the birds dropped their feathers prior to developing their breeding plumage. Food-supplemented birds were able to initiate the pre-alternate moult earlier than birds from the more impoverished plots, potentially allowing earlier migration to breeding grounds and an early start to the breeding season. The researchers also discovered that there was a distinct pattern to the moulting sequence in all individuals, regardless of treatment. The birds first lost feathers from their bodies, before losing them from the head and then the face, ensuring that the energetic costs are spread out over a long period of time and that high-priority areas (such as the body) moult first and that regions of the body that are important for sexual signalling (such as the head and face) moult closest to breeding.

These findings confirm that energy availability is indeed a trigger for moulting. The study also has implications for lab-based studies, where the animals are often fed freely, as well as for songbird populations that experience fluctuations in the availability of food as a result of well-stocked backyard feeders, invasive species and climate change. It remains to be seen how the availability of food triggers the initiation of moult and whether this is characteristic of all species that exchange their plumage prior to breeding.

R. M.
R. S.
J. E.
J. R.
Winter food limits timing of pre-alternate molt in a short-distance migratory bird
Funct. Ecol.
doi: 10.1111/1365-2435.12322