Multidimensional environmental influences on timing of breeding in a tree swallow population facing climate change

Most phenological traits are extremely sensitive to current climate change and advances in the timing of important life-history events have been observed in many species. In birds, phenotypic plasticity in response to temperature is thought to be the main mechanism underlying yearly adjustment in the timing of breeding. However, other factors could be important and interact to affect the levels of plastic responses between and/or within-individuals. Here we use long-term individual-based data on Tree swallow (Tachycineta bicolor) to identify the spatial and environmental drivers affecting plasticity in laying date and to assess their importance at both population and individual levels. We found that laying date has advanced by 4.2 days over 10 years, and that it was mainly influenced by latitude and an interaction between spring temperature and breeder density. Analyses of individual plasticity showed that increases in temperature, but not in breeder density, resulted in within-individual advances in laying date. Our results suggest that females can adjust their laying date as a function of temperature, but that this adjustment will be partly constrained in habitats with lower breeder densities. Such potential constraint is especially worrying for the broad array of species already declining as a result of climate change.

绝大多数物候性状（phenological traits）对当前气候变化极为敏感，诸多物种的重要生活史事件发生时间均已被观测到呈现提前趋势。在鸟类中，针对温度响应的表型可塑性（phenotypic plasticity）被认为是繁殖时间年度调整的核心机制。然而，其他因素或同样关键，且可通过交互作用影响个体间乃至个体内的表型响应强度。本研究基于树燕（Tachycineta bicolor）的长期个体水平数据，明确影响产卵日期可塑性的空间与环境驱动因子，并评估其在种群及个体两个层面的重要性。研究结果显示，10年间产卵日期提前了4.2天，且其主要受纬度，以及春季温度与繁殖个体密度（breeder density）的交互作用影响。个体可塑性分析表明，温度升高可促使个体自身产卵日期提前，但繁殖个体密度的变化并无此效应。本研究结果表明，雌性个体可依据温度调整产卵日期，但在繁殖个体密度较低的生境中，这种调整能力会受到一定限制。对于诸多已因气候变化而种群数量下降的物种而言，这类潜在的限制效应尤其令人担忧。