There is widespread evidence that species distributions are shifting in response to climate change. Warming temperatures and climate niche constraints are hypothesized drivers of northward shifts in temperate migratory bird breeding distributions, but heterogeneity in the direction of distribution shifts suggests that the climate niche hypothesis does not explain all changes in distributions. We propose that: 1) changes in migration costs and benefits related to dampened seasonal differences between breeding and winter areas, 2) sensitivity to supplemental cues that affect duration of migration and onset of reproduction, 3) a latitudinal mismatch‐driven fitness gradient, or a combination of these drivers may explain southward distribution shifts. We examined latitudinal shifts in breeding distribution centroids for 73 species of migratory birds from 1994 to 2017 across eastern, central and western regions of North America using Breeding Bird Survey data and tested if life history characteristics related to the above hypotheses and population status were associated with shift patterns. We found that 44% of regional centroid shifts were southward, 55% were northward, and several species shifted in different directions in different regions. Migratory strategy and protandry predicted breeding distribution centroid shifts, although they tended to be more predictive of northward shifts than southward shifts. There was evidence that supplemental cues explained some southward shifts because herbivorous birds tended to shift southward compared to insectivores, or raptors that shifted northward. Shifts in centroids were not explained by trends in abundance, suggesting that centroid shifts were not attributable to population declines or increases at distribution margins. Our results show the prevalence of heterogeneous breeding distribution shifts, including often overlooked southward shifts, and suggest that more work is needed to develop alternative hypotheses that would explain southward shifts in distributions.

中文翻译：

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北美候鸟异种繁殖分布转移的模式和机制

有广泛的证据表明，物种分布正在响应气候变化而变化。假设温度升高和气候生态位约束是温带候鸟繁殖分布向北移动的驱动因素，但分布变化方向的异质性表明，气候生态位假设不能解释分布的所有变化。我们建议：1）与变种和冬季地区之间季节性季节性差异相关的迁徙成本和收益变化; 2）对影响迁徙持续时间和繁殖开始的补充线索的敏感性; 3）纬度不匹配驱动的适应度梯度;或这些驱动因素的结合可以解释向南的分布变化。我们使用种鸟调查数据研究了1994年至2017年北美东部，中部和西部地区73种候鸟的繁殖分布质心的纬度变化，并检验了与上述假设和种群状况有关的生活史特征是否与转变模式。我们发现44％的区域质心向南移动，55％的向北移动，并且几种物种在不同区域向不同方向移动。迁徙策略和protandry预测了育种分布的质心偏移，尽管它们往往比北向更能预测北移。有证据表明，补充提示可以解释一些向南移动，因为与食草动物或猛禽向北移动相比，食草鸟类倾向于向南移动。质心的变化不能用丰度趋势来解释，这表明质心的变化不是归因于人口减少或分布边际增加。我们的结果表明异种繁殖分布变化的普遍性，包括经常被忽视的向南变化，并表明需要更多的工作来开发可以解释分布向南变化的替代假设。