FIGURE CAPTIONS
Figure 1. Intra-annual variability in avian diversity—measured as
species richness (SR), raw functional richness (FRic), species
richness-corrected functional richness (cFRic), functional evenness
(FEve), and functional dispersion (FDis)—is well captured by three
primary modes (Principal Components, PCs). The scores (temporal seasonal
patterns) are illustrated in (a). The seasonal patterns of scores of the
first mode (PC1) captures differences in avian diversity between
breeding (high score) and wintering (low score) season; the second mode
(PC2) separates migration (particularly, spring migration; high score)
from periods of wintering and breeding (low score); the third mode (PC3)
further emphasizes the signal of autumn migration (high score). (b) PC
loading maps show how strongly, positively (red hues) or negatively
(blue hues), the temporal pattern given by scores for each PC is
expressed at a given location: PC1 (left column), PC2 (middle column),
and PC3 (right column). The loading maps demonstrate strong and
contrasting spatial variation in seasonality of each avian diversity
measure.
Figure 2. The ability of the first three Principal Components (PCs) to
explain the intra-annual variability in avian species richness (SR; a),
raw functional richness (FRic; b), species richness-corrected functional
richness (cFRic; c), functional evenness (FEve; d), and functional
dispersion (FDis; e) varies across space and among diversity metrics. We
computed an empirical reconstruction of true avian diversity time series
(dependent variable) as a linear model of the time series recreated by
each PC’s scores and loadings (independent variable; see Materials and
Methods for more details). The coefficient of determination,
R2, of these reconstructed diversity models was used
as a measure of the variation explained by each PC. In purple are
regions where PC1 explains the most variation in avian diversity. In
green and yellow are regions where PC2 and PC3, respectively, explain
the most variation in avian diversity. Dark and light hues indicate
positive and negative PC loadings, respectively. Inset boxes indicate %
of the study area falling within each category. Avian diversity at
select sites (right panels) show seasonal patterns broadly consistent
with those recreated by the PC that explains the most variation for that
location.
Figure 3. We identified seven unique spatiotemporal clusters that are
characterized by similar temporal patterns of avian diversity (a). These
are indicated on the map by colour, unique to each cluster. Cluster
identities are listed in (b) where the cluster colour is indicated by
the colored column at the left edge of each cluster’s bar graph. The
spatiotemporal clusters coincide strongly with the Bird Conservation
Regions (BCRs), indicated by white boundaries and a numbered tag in (a).
Box plots in (b) show distribution of loadings for each Principal
Component (PC) and each avian diversity metric (species richness, SR;
raw functional richness, FRic; species richness-corrected functional
richness, cFRic; functional evenness, FEve; and functional dispersion,
FDis) for locations that fall within each cluster; blue-and-red heat
maps in (b) summarize the direction of PC loadings, with red (blue)
indicating those loadings/avian diversity metric combinations whose
interquartile range is positive (negative) and does not overlap zero,
and grey indicating that the interquartile range overlaps zero. Bar
graphs in (c) show proportion of each cluster falling within each BCR.
BCRs in (a) follow numbering consistent with their official
designation41 and are as follows: 5) Northern Pacific
Rainforest, 9) Great Basin, 10) Northern Rockies, 11) Prairie Potholes,
12) Boreal Hardwood Transition, 13) Lower Great Lakes/St. Lawrence
Plain, 14) Atlantic Northern Forests, 15) Sierra Nevada, 16) Southern
Rockies Colorado Plateau, 17) Badlands and Prairies, 18) Shortgrass
Prairie, 19) Central Mixed Grass Prairie, 20) Edwards Plateau, 21) Oaks
and Prairies, 22) Eastern Tallgrass Prairie, 23) Prairie Hardwood
Transition, 24) Central Hardwoods, 25) West Gulf Coastal
Plain/Ouachitas, 26) Mississippi Alluvial Valley, 27) Southeastern
Coastal Plain, 28) Appalachian Mountains, 29) Piedmont, 30) New
England/MidAtlantic Coasts, 31) Peninsular Florida, 32) Coastal
California, 33) Sonoran and Mojave Deserts, 34) Sierra Madre Occidental,
35) Chihuahuan Desert, 36) Tamaulipan Brushlands, 37) Gulf Coastal
Prairie.