Determinants of lead shot, rice, and grit ingestion in ducks and coots

We investigated the relationships between lead shot ingestion, grit size selection, bill morphology, and diet in a community of 8 duck species and common coot (Fulica atra) wintering in the Ebro Delta Spain. There were no intraspecific differences related to sex or age in grit composition, lead shot, and rice-grain ingestion. Strong interspecific differences were recorded for all these variables and for the density of bill lamellae. The proportion of grit of size >1 mm (especially >2-3 mm) was positively correlated with the prevalence of lead shot ingestion, as well as with rice ingestion. Rice ingestion was also positively correlated with the prevalence of lead shot ingestion. Those duck species feeding on rice had larger grit and higher prevalences of lead shot than herbivorous species. Contrary to the predictions of a straining model for food or grit ingestion, lamellar density did not explain interspecific differences in grit selection, rice ingestion, or prevalence of lead shot ingestion. These findings contradict previous claims in the literature, and suggest that mechanisms other than straining are used by ducks for grit selection and lead shot ingestion.