Conservation implications
Describing migratory connectivity is essential for informing effective wildlife conservation and management decisions involving migratory species Our delineation of five breeding populations, and their linkages to wintering regions, provides the necessary information to prioritize regions for conservation and improve our understanding of the underlying drivers of abundance. While our demographic analysis highlights that the species is declining overall from 1966 – 2021, there is wide variation in trends among the breeding populations. The Northern Temperate population has the largest population of American Redstarts on the breeding grounds and is increasing in abundance. One potential explanation for the increase in abundance is that birds in the southern portion of the breeding range have shifted their breeding latitude northward in response to climate change, as has been documented in other Nearctic-Neotropical migrants . However, our results do not depict a correspondingly large decline in the Southern Temperate breeding population which would be the source population of northward movement. Given that the Northern Temperate breeding population has strong connectivity with the Greater Antilles archipelago in the Caribbean, efforts aimed at conserving the greatest proportion of the global distribution of American Redstarts could focus on the Northern Temperate-Greater Antilles migratory population.
The Maritime Provinces population had the second highest abundance of American Redstarts and the highest density of individuals. Despite being geographically adjacent to the Northern Temperate breeding population, Maritime Provinces individuals were detected almost exclusively outside the Caribbean, along the eastern slopes of the Andes of Colombia. Our demographic analysis highlighted the Maritime Provinces to be the second fastest declining population. Thus, future research into the stressors driving this decline could focus on the breeding region as well as stationary nonbreeding region of eastern Colombia. Notably, other populations of long-distance migratory birds connected to the Eastern Andes are also experiencing declines, including populations of Canada Warbler (Wilson et al., 2018) and Cerulean Warbler, Setophaga cerulea (Raybuck et al., 2022). Additionally, in species such as the Canada Warbler, migration routes between North and South America can concentrate in small regions of Central America which can also affect population trends . Given the phylogeographic split of the Maritime Provinces breeding population with the mainland (Colbeck et al., 2008), conservation of this migratory population may also be important for preserving genetic diversity within the species. The Western Boreal population, ranging from Alaska to Saskatchewan, was characterized by the demographic analysis as having the third highest abundance and density, with population declines larger than the range-wide decline. Strong migratory connectivity with Mexico and Central America highlights the need for conservation efforts to focus on the most western portion of the range for this migratory population.
The Southern Temperate breeding population is unique in American Redstarts, in that nonbreeding individuals were sampled in both the far eastern Caribbean as well as in Central America. Our lack of sampling between these regions in northern South America precludes our ability to describe whether there is a migratory divide within the Southern Temperate population, or individuals are spread across this portion of the nonbreeding range. While weak connectivity across a large nonbreeding distribution could promote resilience from stressors on any single portion of the nonbreeding distribution , this makes targeting regions for conservation difficult.