Using the BirdTree.org website to obtain robust phylogenies for avian comparative studies: A primer

TLDR

Comparative studies of trait evolution require incorporating phylogenetic history, and birds provide an excellent model system, especially with the comprehensive BirdTree.org database that supplies numerous equally plausible phylogenies for any set of species. This paper outlines how to employ BirdTree tree sets in comparative analyses, either by constructing a consensus tree for standard methods or by using the full set to incorporate phylogenetic uncertainty. The authors recommend that analyses account for phylogenetic uncertainty whenever possible, and they demonstrate this by simulating the impact of varying the number of BirdTree trees on parameter estimates in a bivariate mass‑specific productivity–body mass relationship. Their results show that using at least 1,000 trees yields parameter estimates with coefficients of variation below 0.15%, supporting BirdTree as the future standard for avian comparative studies.

Abstract

Comparative studies of trait evolution require accounting for the shared evolutionary history. This is done by including phylogenetic hypotheses into statistical analyses of species' traits, for which birds often serve as excellent models. The online publication of the most complete molecular phylogeny of extant bird species (www.birdtree.org, BirdTree hereafter) now allows evolutionary biologists to rapidly obtain sets of equally plausible phylogenetic trees for any set of species to be incorporated as a phylogenetic hypothesis in comparative analyses. We discuss methods to use BirdTree tree sets for comparative studies, either by building a consensus tree that can be incorporated into standard comparative analyses, or by using tree sets to account for the effect of phylogenetic uncertainty. Methods accounting for phylogenetic uncertainty should be preferred whenever possible because they should provide more reliable parameter estimates and realistic confidence intervals around them. Based on a real comparative dataset, we ran simulations to investigate the effect of variation in the size of the random tree sets downloaded from BirdTree on the variability of parameter estimates from a bivariate relationship between mass-specific productivity and body mass. Irrespective of the method of analysis, using at least 1,000 trees allows obtaining parameter estimates with very small (< 0.15%) coefficients of variation. We argue that BirdTree, due to the ease of use and the major advantages over previous 'traditional' methods to obtain phylogenetic hypotheses of bird species (e.g. supertrees or manual coding of published phylogenies), will become the standard reference in avian comparative studies for years to come.