RESULTS
Spatiotemporal variation in avian diversity. We identify three
Principal Components (PCs) that together explain 65% of weekly variance
for the five taxonomic and functional diversity metrics across the
continental US (Fig. S1). The first Principal Component (PC1; 47%
variance explained; Fig. S1), separates the breeding (positive score)
from the wintering (negative score) season, with two distinct, lesser
peaks likely associated with the temporary addition of transient species
during seasonal migration (Fig. 1a). The second Principal Component
(PC2, 11% variance explained; Fig. S1) further isolates migration
(primarily, spring migration; positive score) from periods of wintering
and breeding (negative score; Fig. 1a). The third Principal Component
(PC3, 8% variance explained; Fig. S1) further emphasizes the signal of
autumn migration (positive score; Fig. 1a). Each subsequent PC explains
< 5% of the total variance (Fig. S1) and captures mostly
stochastic fluctuations, without a clear seasonal signature (Fig. S3).
Seasonal patterns of avian diversity can thus be approximated for each
grid cell as the weighted combination of three principal modes:
breeding/winter season (PC1), spring (PC2), and autumn (PC3) migration,
where the weights (i.e.,
importance of each PC at
different locations) are shown by the PC loading maps (Fig. 1b).
Avian diversity shows clear spatial patterns in the strength (loading)
of temporal variation (score) patterns, but there are notable
differences among the five diversity metrics in how these patterns are
expressed (Figs. 1,2a,S4). Seasonality of species richness demonstrates
a strong latitudinal gradient, with the northern US (n=693,160 grid
cells, ~74% of study region) characterized by high
breeding season SR and low winter SR (strongly positive PC1 loadings;
Figs. 1,2a). Along the Gulf of Mexico coast, SR instead peaks during
winter (n=48,251, ~5%; strongly negative PC1 loadings).
Known migratory staging areas across the South, Southeast, Southwest,
and California (n=159,452, ~17%) experience spring and
autumn peaks in SR (strongly positive PC2 and PC3 loadings; Figs. 1,2a),
with some spatial differences between PC2 (spring) and PC3 (autumn)
loading. A few patches in the Southeast (n=32,298, ~4%)
instead experience autumn troughs in SR (Figs. 1,2a). Raw functional
richness, FRic, expectedly shows fairly strong spatiotemporal congruence
with SR, with only small deviations for parts of Texas and the Midwest
where FRic peaks during the breeding season and autumn migration,
respectively (Fig. 1,2b).
In stark contrast to the strongly correlated SR and FRic, corrected
functional richness, cFRic, peaks during winter across most of the East,
Midwest, and the Pacific Coast (strongly negative PC1 loadings;
n=265,851, ~29%; Figs. 1,2c). This temporal signature
implies that the total breadth of functional space occupied by a bird
assemblage is higher during winter than would be expected after
accounting for seasonal declines in species richness due to migration,
but lower during summer when migrants are back on their breeding
grounds. In contrast, breeding season peaks in cFRic (strongly positive
PC1 loading) are common across northern Michigan, Florida, Texas, the
Rocky Mountains, and California’s Central Valley (n=240,285,
~26%; Figs. 1,2c). For Florida and Texas in particular,
this suggests that the influx of short-distance migrants from northern
latitudes during winter leads to assemblage-wide declines in functional
richness. Importantly, passage migrants strongly influence the
seasonality of cFRic. Specifically, the high plateaus of the
Intermountain West experience spring troughs in cFRic (strongly negative
PC2 loadings; n=144,797, ~15%; Figs. 1,2c), while parts
of the Southwest, Southeast, and New England see spring and autumn peaks
(strongly positive PC2 and PC3 loadings; n=188,824,
~20%; Figs. 1,2c).
Seasonality of functional evenness, FEve, broadly displays an east-west
gradient (Figs. 1,2d), with breeding season peaks (strongly positive PC1
loadings) common east of the Rocky Mountains, in parts of the Great
Basin, and along the Pacific Coast (n=642,027, ~69%;
Figs. 1,2d). Such a temporal pattern indicates an even distribution of
species’ abundances in the functional space during the breeding season.
In winter, however, species’ abundances amass in a few regions of the
functional space. Passage migrants influence seasonality of FEve
particularly strongly in high elevation and topographically varying
regions. The Intermountain West high plateaus, parts of the Rocky
Mountains, and the Sierras experience irregular distribution of species’
abundances within the functional space (low FEve) during spring
(negative PC2 loadings; n=108,813, ~12%) and the
breeding season (negative PC1 loadings; n=34,122, ~4%),
while parts of the Rocky, Chisos, Ozark, and Appalachian Mountains
(n=100,862, ~11%) see functional evenness peak in
spring and autumn (Fig 1,2d).
Strong breeding-wintering seasonality characterizes functional
dispersion, FDis, across most of the continental US (n=745,939;
~80%; Figs. 1,2e), with peaks typically observed during
the breeding season (strongly positive PC1 loadings). This temporal
signature implies that during the breeding season abundant species are
spread further away from the centroid of the functional space relative
to rare species, but in winter they are positioned near the centroid.
The only regions with winter peaks in FDis (negative PC1 loadings) are
the northern Rocky Mountains and lower Peninsular Florida (n=32,980,
~4%; Figs. 1,2e). Passage migrants strongly increase
the dispersion of species’ abundances in the functional space in spring
and autumn (strongly positive PC2 and PC3 loadings) in the Appalachian
Mountains, the Great Lakes region, and upper Peninsular Florida
(n=52,619, ~6%), but lower it (strongly negative PC2
and PC3 loadings) in the Intermountain West high plateaus, the Sierras,
and the Midwest (n=101,588, ~10%; Figs. 1,2e).
Spatial congruence in seasonality of avian taxonomic and
functional diversity. We identify seven distinct spatiotemporal
clusters (Figs. 3,S2). Broadly, Clusters 1 (n=140,846,
~15% of continental US) and 2 (n=140,967,
~15%) represent locations where species richness and
functional diversity peak during the breeding season and migration,
except for cFRic, which alone declines during the breeding season (Fig.
3b). Together, Clusters 1 and 2 cover much of the eastern US (Fig. 3a),
with Cluster 1 representing the higher elevation Appalachian region and
southeastern plateaus and Cluster 2 representing the low lying plains
and prairies of the Lower Great Lakes. Parts of California and the
Pacific Northwest also show characteristics of Clusters 1 and 2 (Fig.
3a).
Cluster 3 (n=120,658, ~13%) is broadly defined by
breeding season peaks in avian diversity, across nearly all measures,
and declines during winter, spring, and autumn (Fig. 3b). cFRic is again
an exception to this pattern as it instead peaks in spring and plummets
in autumn (Fig. 3b). Cluster 3 comprises high elevation, cold winter,
forested regions of the Rocky Mountains, New England, upstate New York,
and the Upper Great Lakes (Fig. 3a). Cluster 4 (n=99,342,
~11%) is characterized by low SR but high functional
diversity during the breeding season (Fig. 3b) and covers the
southernmost regions of Texas, Florida and inland California (Fig. 3a),
areas typically characterized by warm winters.
Broadly, Clusters 5 (n=190,523, ~21%) and 6 (n=115,681,
~12%) experience peaks in avian diversity during the
breeding season and troughs during winter, spring, and autumn (Fig. 3b)
and cover medium (Cluster 5) to high (Cluster 6) elevation plateaus of
the western US (Fig. 3a). Finally, Cluster 7 (n=123,113,
~13%) identifies locations where SR peaks during both
the breeding and migration seasons and functional diversity peaks in the
breeding season (Fig. 3b). Cluster 7 comprises mostly the deserts of the
Southwest and extends into the prairies along the Front Range (Fig. 3a).
Remarkably, we find a close agreement between these emergent
spatiotemporal clusters and Bird Conservation Regions (BCRs; Fig
3a,c)—independently and qualitatively derived regions that are
ecologically distinct in terms of their bird communities, habitat types,
and resource management issues (Babcock et al. 1998).