Grit selection in waterfowl and how it determines exposure to ingested lead shot in Mediterranean wetlands

Abstract

Waterfowl ingest lead shot because they confuse it with grit, but there has been limited study of differences among species and locations. The spatial and interspecific variation in the quantity and size composition of ingested grit and in the ingestion of lead shot by eight waterfowl species in the three main wintering areas in the western Mediterranean (Doñana, Ebro Delta and Camargue) was investigated. Variation in the mass of grit in the gizzard was related to bird species, whereas size composition of ingested grit was more closely related to locality and less to species. Birds with a large proportion of vegetation in their diets had more grit in the gizzard. Grit size composition was related to prevalence of lead shot ingestion. Thus, the quantity of grit in the gizzard is an attribute of species, and grit size composition (which largely determines the risk of ingestion of lead shot) is more affected by local conditions. This conclusion is supported by a meta-analysis of previous studies of the incidence of lead shot ingestion in 51 locations and 27 waterfowl species in North America and Europe. The prevalence of lead shot ingestion in a given waterbird species was highly variable between localities, and was not consistently different between dabbling, grazing and diving species.