ABSTRACT Optimizing breeding phenology, an important aspect of fitness, is complex for migratory species as they must make key timing decisions early, and remotely, from breeding sites. We examined the role of weather (locally and cross-seasonally), cavity availability, and competitive exclusion in determining among-year variation in breeding phenology over 17 yr for 2 migratory, cavity-nesting birds: Mountain Bluebirds (Sialia currucoides; n = 462 nests) and Tree Swallows (Tachycineta bicolor; n = 572) using natural tree cavities in British Columbia, Canada. We assessed weather effects within the winter and migratory range and at our study sites. We quantified competition as the proportion of cavities occupied by European Starlings (Sturnus vulgaris) (for both species) and Mountain Bluebirds (for Tree Swallow only) in each year. For 229 bluebird and 177 swallow nests with known fates, we tested whether late years resulted in reduced productivity. Although the effects were small, heavy rainfall and strong diurnal westerly winds during migration were associated with breeding delays for Mountain Bluebirds. However, cavity availability (earlier breeding with increases) had a 5–8 × greater effect on timing than migratory conditions. There was no evidence that starling competition delayed bluebirds. In Tree Swallows, greater local daily rainfall was associated with delayed breeding, as was starling abundance (the effect of starlings was 1.4 × smaller than that of rainfall). Neither bluebird abundance nor cavity availability changed swallow phenology. Neither species showed reduced productivity in late breeding years. In both species, individuals that bred late relative to conspecifics within-year had smaller clutches and greater probability of nest failure. We conclude that breeding ground conditions, particularly cavity limitation and local rainfall (for swallows), are important drivers of breeding phenology for our focal species, but that the productivity cost of late years, at least for Tree Swallows, is minimal.

中文翻译：

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降雨和巢址竞争延迟了山蓝鸟和树燕的繁殖，但不会影响生产力

摘要 优化繁殖物候学是适应度的一个重要方面，对于迁徙物种来说很复杂，因为它们必须在繁殖地点及早远程做出关键的时间决定。我们研究了天气（局部和跨季节）、空腔可用性和竞争排除在确定 17 年间 2 只候鸟、空巢筑巢鸟类的繁殖物候变化时的作用：山蓝鸟（Sialia currucoides；n = 462巢）和树燕（Tachycineta bicolor；n = 572）使用加拿大不列颠哥伦比亚省的天然树洞。我们评估了冬季和迁徙范围内以及我们研究地点的天气影响。我们将竞争量化为每年欧洲椋鸟（Sturnus vulgaris）（两种物种）和山地蓝鸟（仅树燕）占据的空腔比例。对于已知命运的 229 只蓝鸟和 177 只燕窝，我们测试了晚年是否会导致生产力下降。尽管影响很小，但迁徙期间的强降雨和强烈的昼夜西风与山地蓝鸟的繁殖延迟有关。然而，腔可用性（早期繁殖增加）对时间的影响比迁徙条件大 5-8 倍。没有证据表明椋鸟的竞争延迟了蓝鸟。在 Tree Swallows 中，当地日降雨量增加与繁殖延迟有关，椋鸟的丰度也是如此（椋鸟的影响比降雨量小 1.4 倍）。蓝鸟的丰度和腔的可用性都没有改变燕子物候。这两个物种在繁殖后期都没有表现出生产力下降。在这两个物种中，与同种相比，在同一年内繁殖较晚的个体具有较小的离合器和较大的巢失败可能性。我们得出的结论是，繁殖地条件，特别是空洞限制和当地降雨（对于燕子），是我们重点物种繁殖物候的重要驱动因素，但晚年的生产力成本，至少对于树燕来说，是最小的。