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.