Body mass and geographic distribution determined the evolution of the wing flight-feather molt strategy in the Neornithes lineage

Abstract

The evolutionary history of many organisms is characterized by major changes in morphology and distribution. Specifically, alterations of body mass and geographic distribution may profoundly influence organismal life-history traits. Here, we reconstructed the evolutionary history of flight-feather molt strategy using data from 1,808 Neornithes species. Our analysis suggests that the ancestral molt strategy of first-year birds was partial or entirely absent, and that complete wing flight-feather molt in first-year birds first evolved in the late Eocene and Oligocene (25-40 Ma), at least 30 Myr after birds first evolved. Complete flight-feather molt occurred mainly at equatorial latitudes and in relatively low body mass species, following a diversification of body mass within the lineage. We conclude that both body mass and geographic distribution shaped the evolution of molt strategies and propose that the evolutionary transition towards complete juvenile molt in the Neornithes is a novel, relatively late adaptation.

Figure 1

The evolutionary history of molt strategies during the first year of life among modern birds (Neornithes). Ancestral trait reconstruction analysis (1,808 species; continuous-time Markov chain, all rates different model) using a comprehensive phylogeny of modern birds^,. Pie charts at the nodes denote the posterior probabilities for each of three molt strategies. Our results indicated that the ancestral molt during the first year of life for all modern birds is probably partial or absent molt and that the transition to complete wing flight-feather molt occurred independently several times in the history of this group, mainly among passerines (order Passeriformes). The scale (bottom left) represents 10 Myr.

Figure 2

Ancestral trait reconstruction analysis (continuous-time Markov chain) among 84 modern birds families using two phylogenetic trees published by Jetz et al. (left) and by Prum et al. (right). Pie charts at the nodes denote the posterior probabilities for each of three molt strategies. Lines in the center connect the same branches on the two different trees, with the color gradient reflecting the order on the left tree, from top (red) to bottom (blue).

Figure 3

The evolutionary history of bird body mass in modern birds (Neornithes) in relation to molt strategy (complete, partial or absent molt) during the first year of the bird's life. Ancestral trait reconstruction analysis indicated that the mean body mass of the modern bird’s ancestor was slightly less than 1,000 g. Body mass diversification, mainly the declining body mass of later taxa during the evolutionary history of modern birds, took place in parallel to the evolution of complete wing flight-feather molt in yearling birds. The scale (top left) represents 25 Myr. The boxed inset (top right) displays the fitted phylogenetic logistic regression in red (complete vs absent molt; P < 0.001, n = 1,591 species).

Figure 4

The molt strategy during the first year of the bird's life (complete vs absent molt) in relation to the mid-distribution latitude (°) of modern birds (Neornithes; n = 1,591 species). Our results indicated that complete wing flight-feather molt is more common in lower latitudes than in higher ones (P < 0.001; fitted phylogenetic logistic regression in red).