Data from: Consequences of breeding system for body condition and survival throughout the annual cycle of tidal marsh sparrows

An individual’s body condition and probability of survival can change throughout the annual cycle, based on the combined effects of many factors, including reproductive investment during breeding, colder temperatures during winter, and elevated risks during migration. We evaluated body condition and survival during breeding and non-breeding periods in two closely related species with notably different reproductive systems. Male and female saltmarsh sparrows (Ammodramus caudacutus) represent extremes in parental care: males perform none, leaving females to do everything from build nests to care for fledglings. In contrast, male and female seaside sparrows (A. maritimus) have bi-parental care and similar levels of reproductive investment, intermediate between male and female saltmarsh sparrows. Our results are consistent with the idea that females experience non-lethal effects of reproduction, and that differences between the breeding season and winter affect condition. In both species, females had lower scaled mass index (SMI) values than males during both breeding and non-breeding seasons, and female saltmarsh sparrows had lower SMI values than female seaside sparrows. Females carried more fat than males during the breeding season, and female, but not male, fat and muscle scores decreased over time, which is consistent with the adaptive mass hypothesis. In winter, all groups carried more fat and had higher muscle scores than when breeding, despite having lower SMI scores. Although we observed variation in body condition, within-season survival was uniformly high in both seasons, suggesting that sex, species, season, body size, and body condition have little impacts on within season survival. Comparisons with previously-published estimates of annual adult survival suggest that most mortality occurs during migration, even in these short-distance migrants. The importance of considering multiple aspects of body condition, multiple seasons, and difficult-to-monitor events, such as migration, should not be ignored when thinking about the events and processes that cumulatively determine population dynamics.

个体的身体状况与存活概率会随年度周期发生动态变化，其变化受诸多因素共同调控，包括繁殖期的繁殖投入、冬季的低温环境，以及迁徙过程中升高的生存风险。本研究针对两种亲缘关系相近但繁殖系统差异显著的鸟类，评估了其繁殖期与非繁殖期的身体状况及存活情况。盐沼麻雀（Ammodramus caudacutus）的雌雄个体在亲本抚育行为上处于两个极端：雄性完全不参与抚育，从筑巢到育雏的所有工作均由雌性承担。与之相对，海滨麻雀（A. maritimus）则采用双亲抚育模式，雌雄两性的繁殖投入水平相近，其投入程度介于盐沼麻雀的雄性与雌性之间。本研究结果支持以下假说：雌性个体需承受繁殖带来的非致死性生理代价，且繁殖季与冬季的环境差异会影响身体状况。在两个物种中，无论繁殖期还是非繁殖期，雌性的标准化身体质量指数（scaled mass index, SMI）均低于雄性；且盐沼麻雀雌性的SMI值也低于海滨麻雀雌性。繁殖期雌性的脂肪储备量高于雄性，且仅雌性的脂肪与肌肉评分随时间推移而下降，这一结果符合适应性体重假说（adaptive mass hypothesis）。冬季时，所有组别个体的脂肪储备与肌肉评分均高于繁殖期，尽管其SMI值更低。尽管我们观测到身体状况存在个体差异，但两个时期的季内存活率均处于较高水平，这表明性别、物种、季节、体型与身体状况对季内存活率的影响极小。结合已发表的年度成体存活率估算结果来看，即便对于这些短距离迁徙鸟类，绝大多数死亡事件也发生在迁徙过程中。在探究共同决定种群动态的各类事件与过程时，综合考量身体状况的多个维度、多个季节以及迁徙这类难以监测的事件的重要性，不应被忽视。