Rate of moult affects feather quality: a mechanism linking current reproductive effort to future survival

Life-history theory proposes that costs must be associated with reproductio n, Many direct costs are incurred during breeding There is also evidence fo r indirect costs, incurred after breeding, which decrease survival and futu re reproductive success. One possible indirect cost identified in birds is that breeding activity in some way compromises plumage quality in the subse quent moult. Here we propose a mechanism by which this could occur. Breedin g activity delays the start of moult. Birds that start to moult later also moult more rapidly an effect of decreasing daylength. Could this result in poorer quality plumage? The kept two groups of male European starlings, Stu rnus vulgaris, one on constant long days and the other on decreasing daylen gths from the start of moult. Decreasing daylengths reduced the duration of moult from 103 +/- il days to 73 +/- 3 days (P < 0.0001). Newly grown prim ary feathers of birds that moulted fast were slightly shorter, weighed less (p < 0.05) and were more asymmetrical. They had a thinner rachis (P < 0.00 5), were less hard (p < 0.01) and less rigid (P < 0.05). They were also les s resistant to wear so that differences in mass and asymmetry increased wit h time. There was no difference in Young's modulus. Poorer quality plumage will lead to decreased survival due to decreased flight performance and inc reased thermoregulator) costs. Thus, reproduction incurs costs through a me chanism that operates after the end of breeding.