The pattern discovered in the East is particularly puzzling since it implies that the overall diversity of birds’ ecosystem contributions is highest in periods when a large number of migratory species are gone.

“This tells us that, probably, migratory birds in the East versus the West have very different functional contributions to assemblages,” said lead author Marta Jarzyna, an assistant professor of Evolution, Ecology, and Organismal Biology at OSU. “It suggests that resident birds in the East have a wider array of functional characteristics than migratory birds, and in the West, it’s the opposite: migratory birds contribute more to functional richness than resident birds.”

Besides quantifying species richness, the experts also incorporated four characteristics to help them estimate functional diversity: the birds’ body mass, diet, foraging niche, and activity time. “Different species obviously have different characteristics and different traits, and contribute in different ways to the community composition and ecosystem functioning,” Jarzyna explained.

“You can have 10 species in a community that just eat seeds, or 10 species in a community, five of which eat seeds and five of which eat insects. The community with more diverse attributes will have more functional diversity, even though in terms of species richness they might be exactly the same.”

While previous efforts to describe bird functional diversity have focused only on one breeding season – a summer in the Northern Hemisphere – this study is the first to investigate diversity as the abundance of birds changes across the country over an entire year. Better understanding how functional diversity among birds changes over space and time is highly important for informing forecasting, conservation, and management of biodiversity.

However, further research is needed to understand the causes of the puzzling differences observed between the eastern and western U.S. “Why is it in the East that in the winter, when we are seeing so many species leaving those regions, we see an increase in functional richness? It didn’t make a lot of sense that you would gain this other dimension of diversity while losing something else,” Jarzyna said. “It’s not the case in the West, where we are seeing both the highest species richness and highest functional richness in the summer.”

“We still don’t even know about individual species’ contributions to functional diversity and whether there is, indeed, a difference between migratory and resident birds.”

The study is published in the journal Current Biology.