The skeleton flight apparatus of North American bluebirds (Sialia): phylogenetic thrushes or functional flycatchers?

To better understand ecological traits of organisms, one can study them from two, not necessarily mutually exclusive perspectives: how the traits evolved, and their current adaptive utility. In birds, foraging behavior and associated morphological traits generally are explained by a combination of adaptive and phylogenetic predictors. The avian skeleton and more specifically, the skeletal flight apparatus is under well-known functional and phylogenetic constraints. This is an interesting area to partition the relative contributions of adaptive correlated evolution and phylogenetic constraint to species clustering in morphological space. A prediction of convergent evolution is that nonphylogenetic morphological clustering is a characteristic of ecological similarity. We tested this using representatives of North American birds from two clades, one with a mixture of foraging modes (Turdid thrushes, solitaires, and bluebirds) and one with more canalized foraging behaviors (Tyrannid flycatchers). Nine characters on the skeletal flight apparatus from 19 species were used to characterize the morphological space and test for ecomorphological clustering. When body size and phylogeny are considered, the three bluebird species and Townsend's solitaire cluster with the ecologically similar flycatchers rather than with their phylogenetic close relatives. Furthermore, sit-and-wait foragers tend to exhibit relatively long distal elements and a long keel while active ground foragers have deeper keels and a longer humerus. Distal elements, expected to be relatively shorter and more bowed in the flycatchers and bluebirds, were actually longer and narrower. A reduction of distal element mass may be more important for facilitating maneuverability than surface area for insertion of wing-rotational musculature.