COS 96-2 - Northward shifts in latitudinal optima for North American birds over the last five decades

Abstract: Changes in climate can alter environmental conditions faster than most species can adapt. Thus, many species shift their geographical distribution, tracking environmental conditions conducive for survival. As average temperatures generally decrease towards the poles, a primary prediction for a warming climate is that species will shift their distributions poleward through time. While many studies focus on range shifts, species’ abundances, including shifts in optima, can occur without detectable changes at their margins. We quantified shifts in latitude abundance optima for each of 209 bird species over the last 55 years, using annual data from the North American Breeding Bird Survey. We predicted that species’ optima would shift northward, on average, and would be related to thermal preferences of individual bird species. Analyses were done at a continental scale, and also separately for eastern and western routes. The latitudinal optimum (m) for each bird in each year was estimated using a bespoke flexible non-linear zero-inflated model of species’ abundances versus latitude that allowed asymmetric, flattened and/or peaked response curves. Linear models with autocorrelated errors were used to quantify temporal trends in the optimum (m) for each species. Results were synthesized across species in a meta-analysis, including thermal preference as a moderator. At a continental scale, we estimated an overall poleward trend in latitudinal optima for North American birds of ~1.4 km/yr (± 0.7 se), corresponding to a northward shift of ~77 km (± 39 se) per species over the last five decades. Northward shifts were associated with a preference for colder temperatures, indicating a warming climate as the likely driver. At a regional scale, trends were variable, with a greater poleward shift detected in the western region (2.2 km/yr ± 0.79 se), while shifts in the eastern region were more heterogeneous (some northward, some southward). Thus, different processes may drive spatial re-distributions of birds through time – thermal physiology may be a primary driver for western populations, while eastern populations may be affected by land-use change, habitat loss, and/or competition with invasives or endemics on the move. Displacement of species from their historical realized optimal niches, caused by climate change and other drivers, has potentially severe ecological consequences, with altered local abundance affecting intra- and inter-specific interactions. Effective conservation strategies should consider within-range abundance shifts when planning management initiatives; areas currently deemed “optimal” might not remain so.