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.