Correlated evolution of beak and braincase morphology is present only in select bird clades.

Complex morphological structures, such as skulls or limbs, are often composed of multiple morphological components (e.g., bones, sets of bones) that may evolve in a covaried manner with one another. Previous research has reached differing conclusions on the number of semi-independent units, or modules, that exist in the evolution of structures and on the strength of the covariation, or integration, between these hypothesized modules. We focus on the avian skull as an example of a complex morphological structure for which highly variable conclusions have been reached in the numerous studies analyzing support for a range of simple to complex modularity hypotheses. We hypothesized that past discrepancies may stem from both the differing densities of data used to analyze support for modularity hypotheses and the differing taxonomic levels of study. To test these hypotheses, we applied a comparative method to 3D geometric morphometric data collected from the skulls of a diverse order of birds (the Charadriiformes) to test support for 11 distinct hypotheses of modular skull evolution. Across all Charadriiformes, our analyses suggested that charadriiform skull evolution has been characterized by the semi-independent, but still correlated, evolution of the beak from the rest of the skull. When we adjusted the density of our morphometric data, this result held, but the strength of the signal varied substantially. Additionally, when we analyzed subgroups within the order in isolation, we found support for distinct hypotheses between subgroups. Taken together, these results suggest that differences in the methodology of past work (i.e., statistical method and data density) as well as clade-specific dynamics may be the reasons past studies have reached varying conclusions.

The Effects of Foraging Ecology and Allometry on Avian Skull Shape Vary across Levels of Phylogeny.

AbstractAvian skull shape diversity is classically thought to result from selection for structures that are well adapted for distinct ecological functions, but recent work has suggested that allometry is the dominant contributor to avian morphological diversity. If true, this hypothesis would overturn much conventional wisdom regarding the importance of form-function relationships in adaptive radiations, but it is possible that these results are biased by the low taxonomic levels of the clades that have been studied. Using 3D morphometric data from the skulls of a relatively old and ecologically diverse order of birds, the Charadriiformes (shorebirds and relatives), we found that foraging ecology explains more than two-thirds of the variation in skull shape across the clade. However, we also found support for the hypothesis that skull allometry evolves, contributing more to shape variation at the level of the family than the order. Allometry may provide an important source of shape variation on which selection can act over short timescales, but its potential to evolve complicates generalizations between clades. Foraging ecology remains a better predictor of avian skull shape over macroevolutionary timescales.

Comprehensive taxon sampling and vetted fossils help clarify the time tree of shorebirds (Aves, Charadriiformes).

Shorebirds (Charadriiformes) are a globally distributed clade of modern birds and, due to their ecological and morphological disparity, a frequent subject of comparative studies. While molecular phylogenies have been key to establishing the suprafamilial backbone of the charadriiform tree, a number of relationships at both deep and shallow taxonomic levels remain poorly resolved. The timescale of shorebird evolution also remains uncertain as a result of extensive disagreements among the published divergence dating studies, stemming largely from different choices of fossil calibrations. Here, we present the most comprehensive non-supertree phylogeny of shorebirds to date, based on a total-evidence dataset comprising 353 ingroup taxa (90% of all extant or recently extinct species), 27 loci (15 mitochondrial and 12 nuclear), and 69 morphological characters. We further clarify the timeline of charadriiform evolution by time-scaling this phylogeny using a set of 14 up-to-date and thoroughly vetted fossil calibrations. In addition, we assemble a taxonomically restricted 100-locus dataset specifically designed to resolve outstanding problems in higher-level charadriiform phylogeny. In terms of tree topology, our results are largely congruent with previous studies but indicate that some of the conflicts among earlier analyses reflect a genuine signal of pervasive gene tree discordance. Monophyly of the plovers (Charadriidae), the position of the ibisbill (Ibidorhyncha), and the relationships among the five subfamilies of the gulls (Laridae) could not be resolved even with greatly increased locus and taxon sampling. Moreover, several localized regions of uncertainty persist in shallower parts of the tree, including the interrelationships of the true auks (Alcinae) and anarhynchine plovers. Our node-dating and macroevolutionary rate analyses find support for a Paleocene origin of crown-group shorebirds, as well as exceptionally rapid recent radiations of Old World oystercatchers (Haematopodidae) and select genera of gulls. Our study underscores the challenges involved in estimating a comprehensively sampled and carefully calibrated time tree for a diverse avian clade, and highlights areas in need of further research.