Respiratory and cutaneous water loss of temperate-zone passerine birds

We measured respiratory water loss (RWL) and cutaneous water loss (CWL) of 12 species of passerine birds, all from a temperate environment, and related their CWL to classes of lipids within the stratum corneum (SC). We purposed to gain insight into the generality of patterns of CWL in birds that have been generated mostly from studies on species from deserts, and we addressed the hypothesis that CWL is a passive diffusion process. Despite taxonomic and ecological differences among 12 species of temperate birds, mass-specific RWL and surface-specific CWL were statistically indistinguishable across species. When the birds were dead, CWL was reduced by 16.3% suggesting that CWL is, in part, under physiological control. We found that ceramides, cerebrosides, dioscylceramides, cholesterol, cholesterol sulfate, fatty acid methyl esters, free fatty acid, sterol esters, and triacylglycerol constituted the intercellular lipids of the avian SC. CWL was positively associated with amount of ceramide 3, but this lipid class represented less than 2% of the total SC lipids. Combining direct measurements (n=24) of RWL with indirect estimates (n=25) yielded the equation log RWL (g H(2)O/d)=-0.86+0.73 (log body mass, g).