Phylogenetic signal
Among the available indices available to characterize phylogenetic signals in trait data, Blomberg’s K is the most widespread and is considered to capture the effect of trait evolution (Blomberg and Garland 2003; Münkemüller et al. 2012). This is based on an approach in which the magnitude of independent contrasts has smaller variance if related species are similar to each other in trait character. Observed versus expected contrast variances were compared under a null model created by swapping the tips of the phylogenetic tree to test for significance differences (Blomberg et al. 2003). When K approaches 1, trait evolution follows a mode of evolution that is consistent with Brownian motion (i.e. random walk), whereas for K > 1 and <1, respectively, close relatives are more similar or less similar than expected indicating a strong phylogenetic signal (Blomberg et al. 2003). Using all the traits measured as continuous variables (body mass, body size, bill length, tarsus length, and clutch size), we calculated Blomberg’s K as reported by Münkemüller et al. (2012) using the R package “phylosignal” (Keck et al. 2016). The significance of K (p -value) was calculated by comparison to a null distribution (Yang et al., 2014). We also used Moran’s correlograms, plotted using the function “phyloCorrelogram” from the package “phylosignal” (Keck et al. 2016), to assess how phylogenetic autocorrelation changed across different phylogenetic distances. Originally a measure of spatial autocorrelation, when used in phylogenetic analysis Moran’s I assesses phylogenetic proximity among species to describe the relationship between cross-taxonomic trait variation and phylogeny.
All statistical tests were performed with R software version 3.6.2 (R Core Team, 2019).