Losing the edge: trends in core versus peripheral populations in a declining migratory songbird

Knowledge of rangewide variation in abundance, and the processes driving such variation, may be the key to predict species tolerance to large-scale changes in climate and land use. Processes influencing population trends have been shown to vary between the edge and the core of a species’ range. Here, we examined shifts in occurrence and abundance of a declining species, the Wood Thrush (Hylocichla mustelina), toward the northern edge of its breeding range over two to three decades. We examined those shifts at both coarse (breeding-bird atlases) and fine (individual forest fragments) spatial scales in Ontario and Québec, Canada. We hypothesized that populations would be less stable at the periphery of the range than in the core, as a result of combined effects of lower habitat quality and lower immigration rates in the periphery. At the fine spatial scale in the core of the Canadian distribution, we predicted that populations would show turnovers between years, but occurrence would remain stable at the landscape scale. As expected, Wood Thrush occurrence remained relatively stable in fragmented forests in the core of the breeding range (at both coarse and fine spatial scales), in contrast to peripheral regions. The influence of mature forest cover varied between ecoregions and spatial scales. The instability of coarse-scale occurrence patterns in peripheral versus core populations is consistent with the prediction that large-scale declines are more likely to be detected at the periphery of a species’ range. However, populations declined within the core as well, albeit at a lower rate. The greater stability of core populations probably results from higher habitat quality and higher immigration rates. Yet, local recruitment within the core is expected to be low, owing to the negative effects of habitat fragmentation on productivity documented elsewhere.