Why some rails have white tails: the evolution of white undertail plumage and anti-predator signaling

Abstract

Conspicuous plumage patches have evolved in birds as conspecific signals for mate attraction and assessment, intersexual competition or to signal alarm. Signals may alternatively be directed at potential predators to discourage pursuit. Rails (Family Rallidae) are ground-dwelling birds, many of which inhabit wetlands, while others occur in forests and grasslands. They are renown for their secretive nature and the tendency to flick their tails when observed. This behavior is more conspicuous in species with white undertail coverts that contrast sharply with darker body plumage. Using species comparisons and controlling for phylogeny, we investigated four hypotheses for the evolution of white undertail coverts in rails. We found little support for the hypothesis that white tails are sexually selected: white tails were not more common in species with polygamous as opposed to monogamous mating systems, species with sexual dimorphism, nor species that display their tails in courtship. Nor did our results support the hypothesis that white tail plumage evolved for intersexual competition during territorial interactions. Instead, we found that species that flock for at least part of the year and species found in open as opposed to concealing habitats were significantly more likely to have white undertail coverts. Rail species inhabiting concealing habitats are less commonly gregarious and more likely selected for crypsis. Using phylogenetically-controlled statistical inference we found that adaptation to open wetland habitats significantly precedes the evolution of white undertails, whereas gregariousness likely evolved later in some lineages. The inferred order of trait evolution suggests that this plumage characteristic could have been selected primarily for enhancement of an anti-predator signal rather than a social signal for conspecifics.

Mitochondrial Genomes from New Zealand’s Extinct Adzebills (Aves: Aptornithidae: Aptornis) Support a Sister-Taxon Relationship with the Afro-Madagascan Sarothruridae

“…The Aptornithidae-Sarothruridae clade was suggested by our analysis to form a clade with the finfoots (Heliornithidae), comprising the sister-group of the "true" rails (Rallidae) (Figures 1-3). This unexpectedly close relationship between some of the largest and smallest ralloids-adzebills and flufftails, respectively-is not the only instance of discordance between morphological and molecular based phylogenies of Ralloidea [22,33,55,75,88]. For example, Sarothrura was long considered a member of Rallidae, and Sarothrura ayresi was originally described in the Rallidae genus Coturnicops [22,88,89].…”

“…However, it is plausible that other "rail" taxa may yet be revealed as heliornithoids rather than rallids. Genera currently lacking molecular data include Gymnocrex (East Indonesia and New Guinea) and Rougetius (East Africa), both of which have been considered early-diverging members of the Rallidae based on phylogenetic analyses of morphological data [22,88,90]. In addition, Rallicula from New Guinea, which also lacks molecular data, has been simultaneously considered a close relative of Sarothrura and Rallina (incompatible with molecular phylogenies that suggest Sarothrura and Rallina are only distantly related).…”

Correlated Evolution of White Spots on Ears and Closed Habitat Preferences in Felids

“…However, the factors driving the evolution of white marks in mammals are expected to be similar to those in birds. Thus, habitat is known to exert an effect on the evolution of contrasting white marks in at least some species of birds that commonly fall prey to other birds or mammals, as these marks facilitate anti-predator signaling in gregarious species that inhabit open habitats (Stang and McRae 2009). While it is unknown whether habitat has a similar effect on the evolution of white marks in predatory birds, it is likely that habitat also has an effect on the presence of white marks in mammals.…”

Análise filogenética de ralídeos Neotropicais (Aves: Rallidae) com base em caracteres osteológicos

“…As partes inferiores, em alguns gêneros, são listradas (ex: Porzana) ou uniformes (ex: Aramides). Em galinhas-d'água, carquejas e frangos-d'água, as coberteiras inferiores da cauda são brancas, de grande contraste com o restante da plumagem (Taylor & Perlo, 1998;Stang & McRae, 2009).…”