Conservation of migratory species depends on an understanding of the drivers causing population declines across the annual cycle. Since it is difficult to track the entire annual cycle for long-distance migrants, measurements from part of the cycle may provide insights into overall drivers. Barn Swallows (Hirundo rustica) in Canada have undergone steep and long-term population declines. Using Breeding Bird Survey data and breeding performance data collected at field sites in three regions across Canada, we examined long- and short-term Barn Swallow population trends in Canada, differences in breeding performance across regions, if average breeding performance measures or colony size predicted regional population growth rates and if local breeding performance is driven by density dependence and/or recruitment. We found that long-term negative population trends were generally stronger in the east and west, and weaker in central Canada. Short-term population trends were generally weakly negative or neutral. Average breeding performance and colony size were poor predictors of regional growth rates observed in the subsequent year. We found little support for density dependence affecting local breeding performance; however, local breeding success, presumably through subsequent recruitment of young, was positively correlated to changes in local colony size in the following year. Since we found juveniles recruited into colonies located in the same region, but no link between average breeding performance and estimated regional population trajectories, our results suggest that extrapolating from local-scale studies to broader populations is challenging, and other demographic rates (i.e., survival) may need to be considered. Overall, our study suggests that contemporary Barn Swallow populations are below carrying capacity in Canada, further suggesting that food limitation is not currently regulating populations. Further study is needed to understand what factors are limiting and regulating Barn Swallow populations in Canada, across the annual cycle.

中文翻译：

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加拿大燕子种群轨迹的区域变异不是通过繁殖表现来预测的

迁徙物种的保护取决于对导致整个年度周期人口下降的驱动因素的理解。由于很难追踪长途移民的整个年度周期，从周期的一部分进行测量可能会提供对整体驱动因素的洞察。加拿大的家燕 (Hirundo rustica) 经历了长期的急剧下降。使用在加拿大三个地区的野外现场收集的繁殖鸟类调查数据和繁殖性能数据，我们检查了加拿大的长期和短期家燕种群趋势、不同地区繁殖性能的差异，如果预测平均繁殖性能指标或群体大小区域人口增长率以及当地繁殖性能是否由密度依赖和/或招募驱动。我们发现东部和西部的长期人口负增长趋势普遍较强，而加拿大中部较弱。短期人口趋势普遍呈弱负面或中性。平均繁殖性能和菌落大小不能很好地预测随后一年观察到的区域增长率。我们发现密度依赖性影响当地繁殖性能的支持很少；然而，当地繁殖成功，大概是通过随后招募幼崽，与次年当地群体规模的变化呈正相关。由于我们发现幼鱼被招募到位于同一地区的殖民地，但平均繁殖性能与估计的区域人口轨迹之间没有联系，我们的结果表明，从本地规模的研究外推到更广泛的人群具有挑战性，可能需要考虑其他人口统计率（即生存率）。总体而言，我们的研究表明，当代家燕种群在加拿大低于承载能力，进一步表明食物限制目前并未对种群进行调节。需要进一步研究以了解在整个年度周期中限制和调节加拿大家燕种群的因素。