Mapping migratory connectivity
Population structure analyses identified five genetically distinct
breeding populations with weak genetic differentiation, in contrast to a
previous mtDNA analysis that identified only two populations split by a
phylogeographic break (Colbeck et al., 2008). Our delineation of the
Maritime Provinces breeding population in the far northeast portion of
the range corresponds with the Newfoundland population from the mtDNA
analysis. Colbeck et al. (2008) hypothesized that the phylogeographic
separation of Newfoundland and mainland American Redstart populations
was the result of two refugia during Pleistocene glaciations. Our
findings of weak genetic differentiation between the Maritime Provinces
and other breeding populations suggest that there is ongoing gene flow
among these populations. However, the limited admixture of individuals
sampled in Newfoundland supports the notion that geographic separation
of the island provides some barrier to gene flow, which has been
demonstrated in several other avian species . The weakest genetic
differentiation was found among the Western Boreal, Northern Temperate,
and Southern Temperate breeding populations (FST:
0.004-0.006), suggesting limited barriers to gene flow. The Basin
Rockies breeding population had higher genetic differentiation with the
eastern breeding populations than the more northern Western Boreal
population, which corresponds to the Great Plains functioning as a
barrier to gene flow.
Using the five genetically distinct breeding populations allowed us to
document at a fine scale more complex migratory patterns than previously
identified. At the continental scale, our results broadly correspond to
previous stable isotope studies that found eastern breeding American
Redstarts overwintered in the eastern nonbreeding range and western
breeders overwintered in the west . In several other species of
Nearctic–Neotropical migrants, similar patterns of parallel migration
have also been observed . However, in contrast to previous isotope
analyses in American Redstart , our use of genomic data allowed us to
clearly differentiate Maritime Provinces and Northern Temperate breeding
birds and revealed that individuals breeding in the Maritime Provinces
do not follow the parallel migration pattern. Parallel migration would
result in these breeders being found in the far eastern portion of the
nonbreeding range (e.g., Lesser Antilles and Trinidad and Tobago).
Instead, individuals from the Maritime provinces bypass the Caribbean
portion of the nonbreeding range and have a “leap-frog” migratory
pattern to eastern Colombia. One explanation for the discordance of the
Maritime Provinces migratory connectivity patterns from the rest of the
breeding populations is the phylogeographic separation of these regions
documented by Colbeck et al. (2008). Migration routes are influenced by
the historical separation of Pleistocene glacial refugia and in the
American Redstart, an Atlantic Shelf (near the Maritime Provinces) and
eastern continental refugia are hypothesized to have caused the observed
phylogeographic separation of these regions . A previous mtDNA analysis
of American Redstart migratory connectivity only detected several
individuals from the Maritime Provinces population in the Caribbean
islands of the Dominican Republic and Puerto Rico but lacked samples
from South America . Our results suggest that the Maritime Provinces
breeding population has the strongest connectivity with eastern
Colombia, though the full extent of the population’s nonbreeding
distribution throughout South America is unknown.
Our use of genomic data allowed us to characterize migratory
connectivity at a fine scale and identify distinct regions on the
wintering range that separate breeding populations. In southern Central
America, a clear split between the two sampling sites in Costa Rica, a
separation of 360 km, occurs where the northern site predominantly has
individuals from the Western Boreal breeding population and the southern
site has individuals from the Southern Temperate population. This split
corresponds with a biogeographic separation of drier broadleaf forest in
the northern Pacific side of Costa Rica and moist broadleaf forest in
the southern Pacific side (Corrales, Bouroncle, & Zamora 2015). Another
geographic split in breeding origin occurs between the Lesser Antilles
(Southern Temperate) and the Greater Antilles (Northern Temperate) in
the Caribbean. Sampling of American Redstarts in Colombia was limited to
the eastern slopes of the East Andes, and may not represent the wider
Andes, given that the three chains of the Andes that run through
Colombia influence connectivity patterns in the Canada Warbler,Cardellina canadensis Further population assignment studies that
include sampling of American Redstarts from the Central and Western
Andes, and the Caribbean region of Colombia, may identify the Andes
Mountains as another barrier in the nonbreeding region, creating
geographic splits in breeding origin for this species.