Abstract

1. The genus Cinclodes is unique among passerines because it includes two species that can be considered marine/coastal and also includes several species that inhabit freshwater streams or that shift habitats between terrestrial/fresh water and marine habitats. The Cinclodes clade satisfies two criteria of an adaptive radiation: it is monophyletic and it experienced recent speciation accompanied by rapid phenotypic diversification. 2. We focused on the osmoregulatory traits of five Cinclodes species to determine if the clade also satisfies the criterion of adaptive phenotype-environment correlation that characterizes adaptive radiations. We used the δ13C of tissues to estimate reliance on a marine diet. We predicted that δ13C would be positively correlated with the renal traits responsible for urine concentration (relative kidney size, fraction of the kidney comprising medulla, and number of medullary cones per unit of kidney mass). 3. Our analyses confirmed these hypotheses. We concluded that Cinclodes satisfies the adaptive phenotype-environment correlation criterion. Cinclodes seems to represents an example of an avian adaptive radiation in osmoregulatory function. © 2006 The Authors.