Wintering birds serve as vital climate sentinels, yet they are often
underrepresented in comprehensive surveys and overlooked in studies of
avian diversity change. Here, we provide a continental-scale
characterization of change in multiple facets of wintering avifauna and
examine the effects of climate change on these dynamics. We reveal a
strong functional reorganization of wintering bird communities marked by
a distinct east-west gradient in functional diversity change, along with
a superimposed north-south gradient in trait composition change.
Assemblages in the eastern US saw an expansion of the functional space
and increases in functional originality, evenness, and divergence, while
the western US saw contractions of the functional space. Shifts in
functional diversity were underlined by significant reshuffling in trait
composition, particularly pronounced in the northern US. Finally, we
find strong contributions of climate change to this functional
reorganization, underscoring the importance of wintering birds in
tracking climate change impacts on biodiversity.