Drivers of partial migration in the reddish egret Egretta rufescens

The reddish egret Egretta rufescens is North America’s rarest Ardeidae and
is listed as ‘Near Threatened’ by the IUCN, as endangered in Mexico, and
as a species of conservation concern throughout much of their range in the
United States. Little is known about the migratory behavior of the reddish
egret. Individuals that were banded during the breeding season in Texas,
USA, have been resighted away from breeding areas; however, records are
limited and the extent of migration is unknown. Using GPS transmitters, we
found reddish egrets breeding in southern Texas exhibited a partial
migration strategy, with 39% of the marked population migrating from
breeding sites. We assessed the dominance, body size, and arrival time
hypotheses to better understand the drivers of partial migration. We did
not find support for the dominance or body size hypotheses; both males and
females migrated, and migratory status of individuals did not change
across the years of study. Long-distance migrants were also larger than
resident individuals. We did find some support for the arrival time
hypothesis. Residents began breeding earlier than long-distance migrants
and had moderately greater nest success. However, within long-distance
migrants, an earlier arrival to breeding areas did not necessarily equate
to earlier nesting or greater nest success. This study is the first to
examine the migratory behavior of adult reddish egrets and assesses the
dominance, body size, and arrival time hypotheses as explanations for
partial migration in this species. Further, the results of this study
emphasize the need for international conservation efforts.

红鹭（Egretta rufescens）是北美最为稀有的鹭科（Ardeidae）鸟类，被国际自然保护联盟（International Union for Conservation of Nature, IUCN）评定为近危物种，在墨西哥被列为濒危物种，且在美国其绝大多数分布区内均被纳入保育关注物种范畴。目前学界对红鹭的迁徙行为尚缺乏系统了解。此前仅记录到在美国德克萨斯州繁殖季佩戴环志的个体曾在繁殖地以外的区域被重新观测，但相关记录极为有限，迁徙的具体范围仍未明确。本研究通过GPS追踪器（GPS transmitters）开展调查，发现德克萨斯州南部的繁殖种群红鹭采取部分迁徙策略，其中39%的标记个体自繁殖地迁出。研究团队围绕优势地位假说（dominance hypothesis）、体型假说（body size hypothesis）与抵达时间假说（arrival time hypothesis）展开评估，以期阐明该物种部分迁徙现象的驱动机制。研究未发现支持优势地位假说与体型假说的证据：无论雄性还是雌性个体均会发生迁徙，且个体的迁徙状态在整个研究周期内未发生改变；长距离迁徙个体的体型显著大于留居个体。研究为抵达时间假说提供了部分支撑证据：留居个体的繁殖起始时间早于长距离迁徙个体，且巢成功率略高。不过在长距离迁徙个体群体中，更早抵达繁殖地并不必然对应更早的筑巢时间或更高的巢成功率。本研究是首个针对成年红鹭迁徙行为开展的系统性调查，并首次评估了上述三种假说对该物种种群部分迁徙现象的解释力。此外，本研究结果凸显了开展跨国保育合作的必要性。