Fecal calcium levels of bird nestlings as a potential indicator of species-specific metal sensitivity

Sensitivity of bird species to environmental metal pollution varies but there is currently no general framework to predict species-specific sensitivity. Such information would be valuable from a conservation point-of-view. Calcium (Ca) has antagonistic effects on metal toxicity and studies with some common model species show that low dietary and circulating calcium (Ca) levels indicate higher sensitivity to harmful effects of toxic metals. Here we measured fecal Ca and five other macroelement (potassium K, magnesium Mg, sodium Na, phosphorus P, sulphur S) concentrations as proxies for dietary levels in 66 bird species to better understand their interspecific variation and potential use as an indicator of metal sensitivity in a wider range of species (the main analyses include 39 species). We found marked interspecific differences in fecal Ca concentration, which correlated positively with Mg and negatively with Na, P and S levels. Lowest Ca concentrations were found in insectivorous species and especially aerial foragers, such as swifts (Apodidae) and swallows (Hirundinidae). Instead, ground foraging species like starlings (Sturnidae), sparrows (Passeridae), cranes (Gruidae) and larks (Alaudidae) showed relatively high fecal Ca levels. Independent of phylogeny, insectivorous diet and aerial foraging seem to indicate low Ca levels and potential sensitivity to toxic metals. Our results, together with information published on fecal Ca levels and toxic metal impacts, suggest that fecal Ca levels are a promising new tool to evaluate potential metal-sensitivity of birds, and we encourage gathering such information in other bird species. Information on the effects of metals on breeding parameters in a wider range of bird species would also help in ranking species by their sensitivity to metal pollution.