Terrestrial birds living on marine environments: does dietary composition of Cinclodes nigrofumosus (Passeriformes: Furnariidae) predict their osmotic load?
Feeding on saline marine foods may be especially challenging for passerine birds that lack functional salt glands and have a limited ability to concentrate urine. To reduce the salt load imposed by consumption of marine food these birds may select food with low salt contents and/or increase their intake of freshwater. The genus Cinclodes is particular among passerines because it includes species that inhabit both inland and maritime shores. We analyzed the diet of Cinclodes nigrofumosus and explored the possible relationships between dietary composition and salt load at mesic and arid coastal sites of Chile. From a biogeographical perspective, we hypothesized that freshwater availability is critical to the relationship between diet composition and osmotic load of C. nigrofumosus. Our analysis of prey abundance and distribution indicated that the diet of C. nigrofumosus is composed mainly by marine prey, in spite of the availability of terrestrial insects at both sites. Stomach content osmolality was higher in the arid site, but it was not correlated with prey type, which suggests that Cinclodes nigrofumosus avoid a high osmotic load, drinking fresh water when available. Thus physiological constraints are modulating niche breath of Cinclodes