The conflict between moult and migratory fat deposition:A photoperiodic experiment with bluethroats

The post-breeding moult and fat deposition of adult male bluethroats, Luscinia svecica, were studied in captivity. One group (Control group) was held under light conditions simulating the natural light regime of their moulting grounds at 66°N latitude. The light regime of another group (Shifted group) was similar to the Control group, but shifted 1 month forward, simulating light conditions further into autumn. All birds had food ad libitum. In addition, comparisons were made with free-living bluethroats. There were no differences in moult performance (start and speed) between the Control group and the Shifted group, and moult was similar to that of free-living birds. The start of moult may be endogenously controlled and set earlier in the season so the change in light regime for the Shifted group may have come too late to influence moult performance. The rate of moult may also be inflexible, in spite of the imposed time stress. In addition, bluethroats may moult so fast that further acceleration would hamper flight manoeuvrability and feeding efficiency. Whereas the Control group birds first started to deposit fat when moult was finished, as free-living birds do, the Shifted group birds started fat deposition in mid-moult. Also, the fat deposition rate following completion of moult was higher in the Shifted group. Thus, adult bluethroats can adjust the timing and speed of fat deposition in relation to moult and time of season. Such flexibility should be advantageous for migratory birds moulting at high latitudes where time available between breeding and migration is short