Endogenous Circannual Rhythmicity in Body Mass, Molt, and Plumage of Great Knots (Calidris tenuirostris)

Four Great Knots (Calidris tenuirostris) were kept for six years in a constant-temperature indoor aviary. For two of those six years, they were kept under photoperiodic conditions that mimicked natural changes in daylength for wild birds, followed by four years under a constant photoperiod (light:dark cycle 12:12 h). Under cyclical “natural” photoperiods, three of the four birds maintained cycles of bodymass and contour and flight-feather molt somewhat comparable to that of free-living birds, though the multiple mass peaks characteristic of northward migration were replaced by a single period of high body mass; the mass peaks for southward migration appeared to be absent. Contour-feather molts between nonbreeding and breeding plumages were delayed, and the period of wing molt was longer than in free-living birds. Under constant photoperiods, clear circannual phenotype cycles were maintained. The length of the period with elevated body mass tripled but was partly compensated by a shortening of the duration of wing molt (which never coincided with high body masses). Nevertheless, total cycle lengths were >13 months. Perhaps most interestingly, under constant photoperiod, there was evidence that two components of what is normally considered an integrated phenotypic event, the prebasic molts of contour and wing feathers, were desynchronized. This suggests that the underlying organizational structure of traits is modular to some extent. Such modularity would increase the flexibility and versatility of the cyclic phenotype in evolutionary contexts.