DISCUSSION
We show strong spatial and seasonal decoupling for multiple facets of avian diversity across the continental US. The seasonality of species richness expectedly follows a latitudinal gradient, associated with north-south migratory movements of hundreds of bird species, corroborating findings from others (Ng et al. 2022). In the northern US, species richness is expectedly highest in the breeding season, but that temporal pattern reverses along the Gulf of Mexico coast, where species richness instead peaks in winter, likely driven by seasonal influx of short-distance migrants into these wintering grounds (Ng et al. 2022). Across the South, Southeast, Southwest, and California, migrants further contribute strongly to spring and autumn peaks in species richness, with spatial differences between spring and autumn patterns potentially indicative of loop migration (La Sorteet al. 2014).
The seasonality of functional diversity manifests a more complex spatial variation, characterized by a stronger east-west gradient, further latitudinal variation superimposed on it, and some apparent topographical effect. The diametrically opposed temporal patterns of taxonomic and functional diversity are most apparent in the eastern US, where bird assemblages in the breeding season are characterized by a tightly packed functional space that leads to lower overall functional richness, an even distribution of species’ abundances in that space and a wide spread of abundant species in relation to the space centroid. Such a pattern suggests an effective use of the entire range of available resources (Mason et al. 2005) despite the lower overall functional richness of bird assemblages. During winter, functional richness increases despite species loss, suggesting that migratory birds do not contribute disproportionately to the breadth of the functional space occupied by bird assemblages. The remaining most abundant species, however, are confined to one or a few regions of the functional space that are likely associated with the limited resources available during winter. Likewise, along the Gulf of Mexico, multi-faceted functional diversity peaks during the breeding season despite declines in species richness, again suggesting that short-distance migrants do not disproportionately expand the functional breadth of bird assemblages while wintering in that region.
Across the western US, discrepancies in temporal signatures of avian diversity measures are somewhat less pronounced. There, bird assemblages during the breeding season are characterized by high functional richness, and a regular distribution and a wide spread of species’ abundances in the functional space, a pattern that reverses in winter. Superimposed onto this broad pattern in the western US is a latitudinal gradient that reveals the importance of passage migrants, wherein more northerly regions are increasingly characterized by spring and autumn declines in avian diversity. This might partly result from the differential timing of migration, wherein northerly latitudes see earlier departure (in autumn) and later arrival (in spring) dates than southerly regions. Additionally, birds across the western US are known to travel shorter distances as they often combine elevational with latitudinal migration movements, a direct result of high topographic relief (Boyle 2017), which might ultimately lead to a more pronounced north-south gradient.
Stark differences in seasonality of functional richness suggest differential contributions of migratory and resident birds to functional diversity across the east-west gradient. In the western US, migratory birds seemingly play a more important role in maintaining functional diversity by contributing unique trait characteristics outside the trait spectrum represented by resident species. Indeed, dietary and habitat specialization are higher in the western US than in the east (Belmakeret al. 2012) and narrow-ranged, and thus potentially more specialized (Botts et al. 2013; Slatyer et al. 2013), migratory birds make up a greater proportion of avian communities in the western US (Somveille et al. 2013). For example, of 15 species of hummingbirds found in the US, only one—the Ruby-throated hummingbird (Archilochus colubris) —breeds in the eastern US, and is additionally considered a niche generalist compared to hummingbirds of the western US. A seasonal loss of these and other functionally unique species is likely to have an outsized effect on assemblage functional diversity, particularly functional richness. In the eastern US, higher levels of generalization in resident birds (Belmaker et al. 2012) ensure that most regions of the functional space remain occupied during winter, albeit scarcely. There, the primary contribution of migratory birds to functional diversity is through increasing the evenness and dispersion of abundances within the functional space, which ensures a high degree of niche differentiation and thus a more efficient resource use (Mason et al. 2005; Villéger et al. 2008b).
The uncovered complexity of spatiotemporal associations among the different facets of avian diversity illustrates the importance of isolating unique seasonal signals of biodiversity. Our findings, paired with the evidence for strong temporal non-stationarity of the effects of environmental drivers on biodiversity (Wolkovich et al. 2014; Zuckerberg et al. 2016) and seasonally varying projections of regional climate change (Abatzoglou & Redmond 2007; Wuebbles et al. 2014; Pendergrass et al. 2017; Dunn et al. 2020), reinforce the pressing need to place the investigations of biodiversity in an explicitly temporal context to ensure sound forecasting, conservation, and management of biodiversity.