Timing of arrival in the breeding area is repeatable and affects reproductive success in a non-migratory population of blue tits

Events in one part of the annual cycle often affect the performance (and subsequently fitness) of individuals later in the season (carry-over effects). An important aspect of this relates to the timing of activities. For example, many studies on migratory birds have shown that relatively late spring arrival in the breeding area reduces both the likelihood of getting a mate or territory and reproductive success.
In contrast, relatively little is known about movements of individuals in non-migratory populations during the non-breeding season. Few studies have investigated the timing of arrival at the breeding area in such species, possibly due to the assumption that most individuals remain in the area during the non-breeding season.
In this study, we used four years of data from a transponder-based automated recording system set up in a non-migratory population of blue tits (Cyanistes caeruleus) to describe individual variation in arrival at the breeding site. We investigated whether this variation can be explained by individual characteristics (sex, body size, or status), and we assessed its effect on aspects of reproductive success in the subsequent breeding season.
We found substantial variation in arrival date and demonstrate that this trait is individual-specific (repeatable). Females arrived later than males, but the arrival dates of social pair members were more similar than expected by chance, which suggests that individuals may mate assortatively depending on their arrival in the breeding area. Arrival predicted both whether an individual would end up breeding that season, and several aspects of its breeding success.
Our study suggests that individuals of non-migratory species leave the breeding area during the non-breeding season. Hence, it may be useful to consider variation in the scale of movements between breeding and non-breeding sites, rather than using a simple dichotomy between “resident” and “migratory” species. We conclude that the timing of pre-breeding events, in particular arrival date, may be an overlooked, but important, fitness-relevant trait in non-migratory species.

年度生活周期内某一阶段发生的事件，常会影响个体在当季后续阶段的表现，进而波及自身适合度（fitness），这类效应统称为跨季遗留效应（carry-over effects）。此类效应的一个关键研究维度，与个体活动的时序安排息息相关。举例而言，众多针对候鸟的研究已证实，春季抵达繁殖地的时间越晚，个体成功觅得配偶或领地的概率就越低，同时其繁殖成功率也会随之下降。
与之形成鲜明对比的是，学界对于非迁徙种群个体在非繁殖季的移动模式仍知之甚少。针对这类物种抵达繁殖地的时序展开的研究更是寥寥无几，这或许是因为学界此前普遍假设：多数个体在非繁殖季会留守于繁殖地内。
本研究依托部署于青山雀（Cyanistes caeruleus）非迁徙种群中的基于应答器（transponder）的自动化记录系统，收集了四年的监测数据，以此刻画个体抵达繁殖地的时序差异。我们首先探究了这类时序差异是否可通过个体特征（性别、体型或社会地位）加以解释，随后评估了抵达时序对后续繁殖季各项繁殖成功率指标的影响。
本研究发现，个体的抵达日期存在显著差异，且证实该性状具有个体特异性（即可重复性）。雌性个体的抵达时间晚于雄性，但配对的社会配偶双方的抵达日期却比随机预期更为相似，这表明个体或许会根据自身抵达繁殖地的时序进行选型交配。抵达时序不仅能够预测个体当季是否能成功繁殖，还能对其多项繁殖成功率指标产生影响。
本研究结果表明，非迁徙物种的个体在非繁殖季会离开繁殖地。由此可见，相较于简单地将物种划分为“留居型”与“迁徙型”的二元分类，考虑繁殖地与非繁殖地之间的移动规模差异或许更具研究价值。我们最终得出结论：繁殖前活动的时序，尤其是抵达繁殖地的日期，在非迁徙物种中或许是一项被忽视却至关重要的、与适合度相关的性状。