Abstract
BackgroundThe trade-off between current and residual reproductive values is central to life history theory, although the possible mechanisms underlying this trade-off are largely unknown. The ‘molt constraint’ hypothesis suggests that molt and plumage functionality are compromised by the preceding breeding event, yet this candidate mechanism remains insufficiently explored.Methodology/Principal FindingsThe seasonal change in photoperiod was manipulated to accelerate the molt rate. This treatment simulates the case of naturally late-breeding birds. House sparrows Passer domesticus experiencing accelerated molt developed shorter flight feathers with more fault bars and body feathers with supposedly lower insulation capacity (i.e. shorter, smaller, with a higher barbule density and fewer plumulaceous barbs). However, the wing, tail and primary feather lengths were shorter in fast-molting birds if they had an inferior body condition, which has been largely overlooked in previous studies. The rachis width of flight feathers was not affected by the treatment, but it was still condition-dependent.Conclusions/SignificanceThis study shows that sedentary birds might face evolutionary costs because of the molt rate–feather quality conflict. This is the first study to experimentally demonstrate that (1) molt rate affects several aspects of body feathers as well as flight feathers and (2) the costly effects of rapid molt are condition-specific. We conclude that molt rate and its association with feather quality might be a major mediator of life history trade-offs. Our findings also suggest a novel advantage of early breeding, i.e. the facilitation of slower molt and the condition-dependent regulation of feather growth.
Highlights
The trade-off between current reproductive effort and future reproductive value is of outstanding concern in life history theory [1,2]
Embedded in the molt constraint scenario, fitness accrual can be mediated by body condition if birds with greater energy capital can better fuel the demands of molting to grow better-quality feathers, even if molt is compromised
Groups did not differ in scaled mass index (SMI) (LM, F1,47 = 0.10, P = 0.76), indicating that photoperiod manipulation had influenced molt speed [24] without deteriorating the mean body condition of birds
Summary
The trade-off between current reproductive effort and future (residual) reproductive value is of outstanding concern in life history theory [1,2]. Compromised molt (1) impinge fundamental adverse effects on flight [21,22,23] and thermoregulation [3,20] due to lower-quality feathers, (2) diminishes reproductive success because of breakage or altered pigmentation and/or microstructure of feathers involved in visual displays ([24] and refs therein, [25]), or (3) might have multiple costs through starvation–predation and thermoregulation–predation trade-offs [26,27] This cause–effect cascade implies that the molting process might link past (or current) and future life history events. Embedded in the molt constraint scenario, fitness accrual can be mediated by body condition if birds with greater energy capital can better fuel the demands of molting to grow better-quality feathers, even if molt is compromised (i.e. show higher developmental homeostasis) This aspect has only been addressed indirectly [17] or investigations have been limited to feather growth rate [39,40] and molt–immunity trade-off [14]. An interaction between treatment and body condition is predicted if good-quality birds can cope better with a faster molt rate
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