Using stable isotopes to estimate migratory connectivity for a patchily distributed, wetland-associated Neotropical migrant

Estimates of migratory connectivity are needed for full annual cycle
population models of migratory bird species experiencing rapid declines in
abundance. One technique to determine migratory connectivity is through
stable isotope analysis. This low-resolution method may be influenced by
how data are calibrated between isotopes measured in precipitation and
those measured in feathers, and can be informed by incorporating relative
abundance into the assignment model. eBird abundance maps are a new tool
combining citizen science data into a predictive species distribution
model. In the Prothonotary Warbler (Protonotaria citrea), a
wetland-associated songbird with a patchy breeding distribution, we sought
to use stable-hydrogen isotope analysis informed by a species-specific
calibration equation and eBird abundance data to determine the strength of
migratory connectivity. We developed a species-specific calibration
equation using known-origin samples from the breeding grounds and found
that stable-hydrogen isotope values measured in precipitation explained
50% of the variation in stable-hydrogen isotope values among feathers. We
found that the assignment model incorporating eBird abundance data
correctly identified the true origins of 66% of individuals, and that the
average assignment area (as a measure of precision) was 64% of the
breeding distribution. These results represented a 7% increase in
precision and a 3% decrease in accuracy when compared to a model that was
not informed by abundance. Based on these models, wintering populations
from 6 countries represented a mix of likely breeding origins, suggesting
low migratory connectivity for Prothonotary Warblers. We found evidence
that wintering latitude was related to likely breeding origin, with
individuals at western wintering locations more likely to have southern
breeding origins, but this relationship was weak. These results
corroborate studies using archival light-level geolocators and
high-resolution genetic markers, which also demonstrated weak migratory
connectivity in this species. For patchily distributed species, eBird
abundance data may not provide a useful increase in precision and accuracy
for isotope assignments.