Limited movement of an avian hybrid zone in relation to regional variation in magnitude of climate change

Studies of natural hybrid zones can provide documentation of range shifts
in response to climate change and identify loci important to reproductive
isolation. Using a temporal (36–38 years) comparison of the black-capped
(Poecile atricapillus) and Carolina (P. carolinensis) chickadee hybrid
zone, we investigated movement of the western portion of the zone (western
Missouri) and assessed whether loci and pathways underpinning reproductive
isolation were similar to those in the eastern portion of the hybrid zone.
Using 92 birds sampled along the hybrid zone transect in 2016 and 68 birds
sampled between 1978 and 1980, we generated 11,669 SNPs via ddRADseq.
These SNPs were used to assess movement of the hybrid zone through time
and to evaluate variation in introgression among loci. We demonstrate that
the interface has moved ~5 km to the northwest over the last 36–38 years,
i.e., at only one-fifth the rate at which the eastern portion (e.g.,
Pennsylvania, Ohio) of the hybrid zone has moved. Temperature trends over
the last 38 years reveal that eastern areas have warmed 50% more than
western areas in terms of annual mean temperature, possibly providing an
explanation for the slower movement of the hybrid zone in Missouri. Our
results suggest hybrid zone movement in broadly distributed species, such
as chickadees, will vary between areas in response to local differences in
the impacts of climate change.