Are all species moving poleward? Distributional shifts in Ohio’s breeding birds and potential drivers of change

Background/Question/Methods

Research on the effects of environmental changes on the distributions of plants and animals is essential to understanding how these forces will influence ecosystems and biological processes. Several recent studies have reported poleward shifts in bird distributions which are likely associated with a warming climate.  However, only a few such studies have used fine-scaled regional data such as that generated by Breeding Bird Atlas projects (e.g. Great Britain, Central Europe), only one of which is from North America (New York State). The Midwestern region of North America presents different species assemblages and landscapes relative to these previous studies.  For example, approximately half of Ohio’s land is in intensive agricultural use. Our study provides an opportunity to test whether poleward trends in avian distributions observed in other regions transcend these ecological differences. We used detailed grid-based data collected during two Ohio Breeding Bird Atlas projects (1982-1987, 2006-2011) to quantify changes in latitudinal extent, center of occurrence, and occupancy in 94 species within Ohio over ~25 years.

Results/Conclusions

Individual species demonstrated dramatic latitudinal changes in their distributions, with the centers of occurrence of 53% of species examined shifting north or south by more than 10km. Despite these results, our analyses did not show a significant poleward shift in distributions across species, although, on average, northern extent and center of occurrence of southerly species did shift north by 4.6km and 8.6km, respectively (p>0.10). Additionally, we found evidence of southward shifts in northerly species, with southern extents and center of occurrence shifting on average 19.6km and 6.5km south, respectively (p<0.05). Although northerly and southerly species did not differ significantly in occupancy changes, we found that for southerly species, the change in occupancy was positively associated with the proximity of a species’ distributional range boundary to our study area (p<0.05). This suggests that species along the northern periphery of their range gained more blocks than species for which Ohio is more central in their distribution, as would be expected with a warming climate.

Poleward shifts in avian distributions may be more difficult to detect in our study because additional factors such as land cover change may affect distributions more strongly on this finer scale or within Ohio’s largely human-utilized landscape. Our future analyses will attempt to partition variance in both climate and land cover change to elucidate environmental determinants of the changes we observed in Ohio’s breeding bird distributions.