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.