The Roles of Soil Characteristics and Toxin Adsorption in Avian Geophagy

Geophagy, the intentional ingestion of soil, is a widespread phenomenon whose function is still debated. Recent studies suggest that consumed soils adsorb dietary toxins and provide sodium. However, quantification of soil choice has been absent from most studies, prohibiting the direct comparison of soil use and characteristics. To determine correlates among bird use and soil characteristics, we analyze physical structure, mineral composition, and alkaloid adsorption (14 soil characteristics in total) for 22 soil samples from a riverbank 'clay lick' used by macaws and parrots (family Psittacidae) in Amazon lowland forests in the Tambopata region of southeastern Peru. We quantified bird preferences through photographic mapping. Mineral concentrations were determined using analyses designed to approximate the mineral availability in the vertebrate stomach. The sodium levels in the soil were much higher than in food plants consumed by parrots in this region. Both adsorption of quinine, the model alkaloid used, and sodium concentration correlated significantly with bird use. Sodium explained a much higher percentage of the variation in bird use than quinine adsorption. However, the analyses were complicated by the high degree of correlation among soil variables: clay percentage and sodium concentration were highly correlated with each other and with bird use, making it impossible to determine which of these may have the greater contribution to bird preference. The results suggest that the soils consumed provide an important source of dietary sodium and adsorb alkaloid toxins.