INTRODUCTION
Pinpointing the mechanisms responsible for maintenance of biodiversity
relies on accurate evaluations of biodiversity patterns (Pereiraet al. 2013; Jetz et al. 2019), but those often reflect
the static conditions of one, often breeding, season (Callaghan et
al. 2021; Jarzyna et al. 2021; Hughes et al. 2022) and
ignore seasonal variability inherent in the full annual cycle of species
(Marra et al. 2015). The availability of resources, however,
shows strong intra-annual variability, particularly in temperate
regions, that causes many animals to engage in seasonal short- and
long-distance migration or hibernation (Somveille et al. 2013),
ultimately leading to seasonal patterns of species richness and
abundance (Ng et al. 2022).
No other taxonomic group shows stronger intra-annual geographic
redistribution of species occurrence and abundance than birds. Each
year, billions of individuals (Hahn et al. 2009; Dokter et
al. 2018) of an estimated 1,855 species (19% of all extant bird
species; Kirby et al. 2008, a percentage that strongly increases
with latitude; Somveille et al. 2013) migrate toward lower
latitudes in autumn and higher latitudes in spring in response to
seasonal fluctuations in resource availability (Strong et al.2015; Thorup et al. 2017; Ng et al. 2022) and unfavorable
weather conditions given some species’ physiological limitations
(Pfeifer et al. 2018). These migratory movements produce seasonal
patterns of biomass, abundance, and species richness (Ng et al.2022). Unexplored, however, are the implications of this seasonal
redistribution of bird occurrence and abundance for other facets of
biodiversity, such as functional diversity, the diversity and
distribution of functional traits within an assemblage of species (Safiet al. 2011; Cardoso et al. 2014; Gagic et al.2015). Functional diversity is vital for understanding community
assembly (Cavender-Bares et al. 2009; Kraft et al. 2015),
niche packing, expansion, and complementarity (Pigot et al. 2016;
Pellissier et al. 2018), ecosystem functioning, services, and
stability (Cadotte et al. 2011; Jarzyna et al. 2022), and
conservation prioritization (Thuiller et al. 2015; Pollocket al. 2017; Mazel et al. 2018).
Seasonality of avian functional diversity is likely to deviate from that
of taxonomic diversity because migratory and resident birds likely
present with different trait characteristics. For example, migratory and
resident birds have been shown to differ in traits such as body mass
(Hein et al. 2012; Soriano-Redondo et al. 2020) and clutch
size (Jetz et al. 2008). Migrants often show stronger habitat
(Martin & Fahrig 2018; Zurell et al. 2018; but see Reif et
al. 2016), diet (Boyle et al. 2011; Fristoe 2015), and climate
(Gómez et al. 2016; but see Dufour et al. 2020)
specialization than partial migrants or resident species, who are often
generalists that possess a broader spectrum of trait values conferring
tolerance to harsh winter conditions of temperate regions. Furthermore,
seasonality in resource availability leads to a disproportionate
decrease in the occurrence of species that depend on resources typically
unavailable during winter (Somveille et al. 2015) such as
obligate insectivores (Fristoe 2015). Such disproportionate
redistribution of trait characteristics likely leads to decoupling of
spatiotemporal patterns of avian functional from taxonomic diversity,
but the magnitude and direction of such decoupling have not been
previously elucidated.
Here, we present the first broad-scale assessment of commonalities among
seasonal patterns in avian taxonomic and functional diversity. We
leverage abundance estimates during the full annual cycle for
> 600 North American bird species from eBird Status and
Trends (Fink et al. 2021) to quantify taxonomic diversity
(species richness, SR). We combine these with avian trait databases
(Wilman et al. 2014) to quantify three independent and
complementary components of functional diversity—functional richness
(FRic), functional evenness (FEve), and functional dispersion (FDis)
(Villéger et al. 2008a). A species-corrected version of
functional richness (cFRic) that better reflects true breadth of
occupied functional space is included, raising the total metrics
considered to five, measured across the continental US at weekly
temporal resolution. Our study offers, to our knowledge, the first
comprehensive characterization of the seasonality in taxonomic and
functional diversity for any taxonomic group, with an unprecedented
level of detail that can yield invaluable insights into spatiotemporal
dynamics of avian communities.