Diverse strategies for tracking seasonal environmental niches at hemispheric scale

Species depend upon a constrained set of environmental conditions, or niches, for survival and reproduction that are being increasingly perturbed or lost under rapid climatic change. Seasonal environments offer an important system to study the range of biological responses to cope with such change, as they require species to either undergo physiological or behavioral changes to sustain altered conditions or to track their niches via movement. Here we use a uniquely well sampled system of 619 New World bird species to identify the tracking of both environmental niche position and breadth and assess its phylogenetic and functional underpinning. We find a diversity of strategies, including the tracking of niche position, niche breadth, both, or neither. Species that track niche position most often also track niche breadth, but nearly 40% only track one component and 26% only track niche breadth and not position. There was only limited phylogenetic determinism to this variation, but a strong association with ecological and functional attributes that differed between niche position vs. niche breadth tracking. The observed diversity in type and strength of niche tracking strategies points to highly differing sensitivity to ongoing climatic change, with narrow trackers of both position and breadth particularly susceptible. The trait associations of niche tracking imply significant functional consequences for communities and ecosystems as impending climate change affects some strategies more strongly than others. Seasonal environments and their diversity of niche tracking strategies offer exceptionally dynamic systems for understanding the biological responses and consequences of climate change. Significance Statement: As climate change progresses, species are increasingly being exposed to novel environmental conditions. Niche tracking, i.e. maintaining exposure to required environmental conditions, is a key mechanism for species persistence under climate change. To date, a general understanding of the causes and consequences of niche tracking across diverse taxonomic groups remains missing. Here, we identify environmental niche tracking strategies in New World birds and assess their phylogenetic and functional underpinnings. Across 619 species – nearly the entire avian diversity of the United States and Canada – we find a diversity of niche tracking strategies that points to highly differing sensitivity to ongoing climatic change, with narrow trackers particularly susceptible, implying significant functional consequences for communities and ecosystems. ### Competing Interest Statement The authors have declared no competing interest.