IntroductionLife-history theory predicts that organisms trade off survival against reproduction. However, the time scales on which various consequences become evident and the physiology mediating the cost of reproduction remain poorly understood. Yet, explaining not only which mechanisms mediate this trade-off, but also how fast or slow the mechanisms act, is crucial for an improved understanding of life-history evolution. We investigated three time scales on which an experimental increase in body mass could affect this trade-off: within broods, within season and between years. We handicapped adult skylarks (Alauda arvensis) by attaching extra weight during first broods to both adults of a pair. We measured body mass, immune function and return rates in these birds. We also measured nest success, feeding rates, diet composition, nestling size, nestling immune function and recruitment rates.ResultsWhen nestlings of first broods fledged, parent body condition had not changed, but experimental birds experienced higher nest failure. Depending on the year, immune parameters of nestlings from experimental parents were either higher or lower than of control nestlings. Later, when parents were feeding their second brood, the balance between self-maintenance and nest success had shifted. Control and experimental adults differed in immune function, while mass and immune function of their nestlings did not differ. Although weights were removed after breeding, immune measurements during the second brood had the capacity to predict return rates to the next breeding season. Among birds that returned the next year, body condition and reproductive performance a year after the experiment did not differ between treatment groups.ConclusionsWe conclude that the balance between current reproduction and survival shifts from affecting nestlings to affecting parents as the reproductive season progresses. Furthermore, immune function is apparently one physiological mechanism involved in this trade-off. By unravelling a physiological mechanism underlying the trade-offs between current and future reproduction and by demonstrating the different time scales on which it acts, our study represents an important step in understanding a central theory of life-history evolution.
Timing is crucial in seasonal environments. Passerine birds typically use a combination of physiological mechanisms and environmental cues to ensure that breeding, moult and migration occur without major temporal overlap and under the most favourable conditions. However, late in the breeding season some individuals initiate additional clutches , whereas others initiate moult. Such alternative strategies are thought to reflect trade-offs between reproductive benefits and timely investment in maintenance and survival. The degree of seasonal plasticity differs between species, depending on the mechanisms that govern their annual routine. Migrants are generally under pressure to complete breeding and moult before the autumn departure and often show little plasticity. We studied seasonal plasticity of breeding and moult schedules in the European Stonechat Saxicola rubicola. This species, an obligate short-distance migrant in Central Europe, sometimes initiates late clutches after typically at least two earlier breeding attempts. Based on life-history theory and on observations in captivity, which revealed photoperiodic regulation of breeding and moult, we predicted relatively little seasonal plasticity in Stonechats. We further predicted that reproductive gains of late breeders should be offset by reduced survival. These predictions were tested on long-term field data, using Underhill-Zucchini models to estimate moult. Late breeding occurred in c. 40% of pairs and increased their reproductive success by a third. Both sexes modified moult timing but in different ways. Late breeding females postponed moult approximately until chick independence without compensating for delay by faster moult. Males started moult on time and overlapped it with breeding, associated with markedly slowed plumage change. Sex differences in moult score increased with lay date, but due to their respective modifications, both sexes delayed moult completion. Nonetheless, we could not detect any evidence for survival costs of late breeding. Breeding and moult of European Stonechats appear relatively flexible, despite migratory schedules and photoperiodic programs for seasonal timing. Individuals can modify seasonal behaviour in late summer, presumably depending on their condition, and may profit considerably from extended breeding.
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