Aim: Across a variety of taxonomic scales species diversity is unevenly distributed among its constituent units, and clades with few species are more common than expected assuming homogeneous rates of speciation and extinction among lineages. In order to explain the prevalence of species poor families among a global and speciose radiation of passerine birds, we test whether these groups share common eco-morphological, geographic and phylogenetic attributes.

Location: Global

Time period: Late Oligocene to the present day

Major taxa studied: The Corvides (ca. 790 species)

Methods: We obtained 10 linear measurements of external morphology for 782 species of corvoid passerines. Using these measurements as a proxy for species ecology, we assessed the positioning of corvoid families in eco-morphological trait space, and how these factors are associated with their species richness and rates of lineage diversification. Subsequently, we compared these same characteristics (species richness, morphological positioning and rates of lineage diversification), between families that are currently endemic to the Australasian ancestral area of the Corvides, with those that have dispersed and diversified throughout other continental and insular landmasses.

Results: Families with low species richness and rates of diversification tend to occupy the most peripheral positions in eco-morphological trait space, with almost all of these groups being endemic to Australasia. The peripheral eco-morphological positioning of the Australasian groupings is generally greater than expected upon accounting for differences in phylogenetic isolation and heterogeneity in rates of trait evolution, implying that species poor corvoid families commonly evolved towards marginal areas of morphospace.

Main conclusions: The overrepresentation of species poor clades across diverse sets of organismal groups is consistent with their evolution towards, and the maintenance in, marginal areas of ecological niche space. The evolution of peripheral eco-morphological characters represents a potentially significant limit to rates of range expansion and lineage diversification.

Morphological measurements of museum study skins of the Corvides

In order to assess morphological variation among the Corvides we measured 10 linear variables representing different aspects of avian functional morphology. These morphological measurements were chosen as the respective traits have been illustrated to reflect dispersal ability, foraging strategy, habitat use and climbing adaptations in passerines (Miles and Ricklefs 1984; Miles et al. 1987; Tobias et al. 2014; Pigot et al. 2016). All measurements were taken by Petter Zahl Marki between July and October 2014. For all species, we attempted to measure six adult male specimens following the IOC world bird species list v.2.7 (Gill and Donsker, 2010). In order to reduce the potential influence of intra-specific variation, for each species, we focused on measuring specimens belonging to the same subspecies, or that otherwise had been collected from a restricted geographical area. However, for species poorly represented in the visited collections, we supplemented the measurements with those from females, unsexed specimens, and/or specimens belonging to different subspecies/geographical locations. We used the mean values of these measurements per species in all analyses presented in the main article, because using ANOVA we found that 99% of the variation in the traits was between rather than within species.

Specimens from the following institutions were measured:

Natural History Museum of Denmark, Copenhagen, Denmark (ZMUC) courtesy of Jon Fjeldså and Jan Bolding Kristensen

American Museum of Natural History, New York, USA (AMNH) courtesy of Joel Cracraft and Paul Sweet.

Natural History Museum, Tring, UK (NHM) courtesy of Robert Prys-Jones and Hein van Grouw

Museum für Naturkunde, Berlin, Germany (MFN) courtesy of Sylke Frahnert and Pascal Eckhoff

Below we provide a description of the individual linear measurements taken. Unless otherwise stated measurements were taken to the nearest 0.1 mm using a caliper.

Wing measurements: We took two wing measurements, flattening, but not stretching the wing (“Svensson’s method 2”; Svensson 1992). (1) We measured the length from the carpal joint to the longest primary feather, reflecting wing length, and (2) the length from the carpal joint to the first secondary feather, reflecting a measure of wing aspect (see Claramunt et al. 2011, Fig.2 for a diagram of this approach). Both measurements were taken to the nearest 1 mm using a wing ruler.

Tail measurements: Tail maximum and minimum length were taken from the base of pygostyle to the tip of the outer and central retrices respectively. These measurements were taken by inserting the ruler (or pointed tip of the caliper) between the central retrices until this stops upon contacting the pygostyle. Both measurements were taken to the nearest 1 mm using a ruler/caliper.

Bill measurements: Bill length was measured from the tip of the bill to the base of the skull (total culmen, Baldwin et al. p. 13). Bill depth and width were measured at the level of the proximal edge of the nostrils.

Tarsometatarsus (tarsus) length: We measured tarsometatarsus length from the tibiotarsus joint to the base of the toes, which is represented by the last undivided scute.

Hind toe: As a measure of the hind toe, we measured the hallux on dorsal side, both with and without the claw.

References

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