Data from: Cost of reproduction: a comparison of survival rates of breeding and non-breeding male ortolan buntings

The cost of reproduction is expected to influence survival or future reproduction. Most previous studies have assessed cost of reproduction in relation to natural and experimental variation in number of offspring produced. The ortolan bunting Emberiza hortulana is a passerine bird species with biparental care, and the Norwegian population of the species has an extraordinarily skewed sex ratio with only about half of the males attracting a female, and therefore provides a rare opportunity to compare survival of males that have paired and bred with that of non-breeders (unpaired males), which have not paid a cost of reproduction. Results showed that survival rates of paired (65.0%) and unpaired (64.2%) males did not differ. However, when comparisons were restricted to paired males that definitely had nestlings, their survival rate (76.8%) was significantly higher than that of unpaired males, and the same was the case when comparisons were further restricted to paired males that had offspring recruiting to the population the next year (76.8% survived). Males breeding successfully are likely to be a biased subset of high quality males. In analyses of a subset of males that had bred successfully when young, there was no difference in survival of paired and unpaired individuals when these males were older. In conclusion, breeding male ortolan buntings did not appear to pay a cost of reproduction in terms of reduced survival to the next year compared to non-breeding males. These results may be explained by non-breeding males also incurring extra costs during the breeding season, and that costs of reproduction are not shared equally among sexes in the ortolan bunting and other bird species with biparental care.

繁殖代价被认为会影响个体存活或未来繁殖能力。既往多数研究均围绕子代数量的自然变异与实验操控，探讨繁殖代价相关问题。圃鹀（ortolan bunting, Emberiza hortulana）是一种具有双亲育幼（biparental care）行为的雀形目鸟类（passerine bird），其挪威种群的性比极度偏倚——仅约半数雄性可获得配偶，这为对比配对繁殖雄鸟与未繁殖（未配对）雄鸟的存活率提供了难得契机，因为后者无需承担繁殖代价。结果显示，配对雄鸟（存活率65.0%）与未配对雄鸟（64.2%）的存活率无显著差异。不过，当仅纳入确实育有雏鸟的配对雄鸟时，其存活率（76.8%）显著高于未配对雄鸟；当进一步限定为次年子代成功招募至种群中的配对雄鸟时，该结果同样成立（此类雄鸟存活率为76.8%）。成功繁殖的雄鸟大概率是高质量雄性的偏倚亚群。对早年成功繁殖的雄鸟亚群进行分析时，年长个体中配对与未配对雄鸟的存活率仍无差异。综上，与未繁殖雄鸟相比，繁殖期的圃鹀雄鸟并未表现出以次年存活率下降为代价的繁殖成本。该结果可通过以下机制解释：未繁殖雄鸟在繁殖季同样会承担额外成本，且圃鹀及其他双亲育幼鸟类的繁殖代价并未在两性间均等分摊。