To avoid energy allocation conflicts, birds generally separate breeding, migration and moult during the annual cycle. North American passerines typically moult on the breeding grounds prior to autumn migration. However, some have evolved a moult-migration strategy in which they delay moult until stopping over during autumn migration. Rohwer et al. (2005) proposed the 'push-pull hypothesis' as an explanation for the evolution of this moult strategy, but it has not been empirically tested. Poor conditions on the breeding grounds at the end of the summer would push birds to depart prior to moult, while productive stopover locations would pull them. We tested for a relationship between moult-migration and breeding grounds aridity as measured by the normalized difference vegetation index. Our results strongly support the 'push' aspect of the push-pull hypothesis and indicate that arid breeding grounds, primarily in western North America, would drive species to evolve stopover moult-migration, although this relationship may depend upon migration distance.

中文翻译：

---

蜕皮迁徙的发展与北美雀形目繁殖地的干旱直接相关。

为了避免能量分配冲突，鸟类通常在年度周期中将繁殖，迁移和换羽分开。北美雀形目通常在秋季迁徙之前在繁殖地上换羽。但是，有些人已经发展了换羽策略，在这种策略中，他们将换羽时间推迟到秋季迁徙期间停止。Rohwer等。（2005年）提出了“推挽假说”来解释这种换羽策略的演变，但尚未经过经验检验。夏季末繁殖场的恶劣条件将迫使鸟类在换羽前出发，而高产的中途停留地点则会将它们拉扯。我们测试了归一化差异植被指数所测得的迁徙与繁殖地干旱之间的关系。我们的结果强烈支持“推动”