Data from: Habitat suitability and the constraints of migration in New World warblers

Identifying the factors that influence geographic range limits can
illustrate the various ecological, physiological, and evolutionary
constraints imposed on a species. The range limits of migratory birds are
particularly challenging to study as they occur in disjunct regions at
different times of the year, which can impose different constraints.
Travel between breeding and wintering regions poses a significant
energetic and navigational challenge to birds, although it is not clear
how these movements influence breeding dispersal and range expansion. Here
I ask whether the possible costs of migration limit the breeding ranges of
wood warblers, a group of birds with an extensive history of ecological
and evolutionary studies. I used occurrence records for multiple wood
warbler species, breeding primarily in the boreal forest of North America,
to generate environmental niche models. I tested for over-prediction of
habitat suitability into the western boreal forest, where most these
species do not have occurrence records but where there is presumably
suitable habitat. I found that these vagile taxa, primarily found east of
the Rocky Mountains, also have predicted habitat suitability that extends
into the north and west, where they have little to no occurrence records.
I discuss several possible explanations for this discordance. In
particular, the patterns are consistent with the costs of a long-distance
migration limiting the benefits of range expansion, as migration may
become too onerous as the distance between breeding and wintering areas
increases. These results speak to the process of niche filling more
generally and call attention to an under-appreciated explanation for why
migratory species may not fully occupy their fundamental niche.