Dietary complementation by wild birds: Considerations for field studies
Free-living birds must satisfy fluctuating nutrient requirements in diverse and varying environments. Ingesting nutritionally complementary foods may be the most effective means by which wild birds match nutrient ingestion and nutrient needs. Dietary complementation may occur fortuitously when foods chosen in response to non-nutritive factors (e.g. competition, predation risk, food colour), or on the basis of energy density, also fulfill specific nutrient needs (passive dietary complementation). In some environments, especially during productive phases (e.g. reproduction), free-living birds may rely on nutrient appetites to ensure their choice of foods satisfies their nutrient needs (active dietary complementation). Meeting nutrient needs through dietary complementation can be facilitated, complicated, or impeded by any of several environmental or organism determinants of food choice. Nutrient appetites, exogenous food stores, and endogenous nutrient stores are three organismal determinants that are probably the most important in facilitating dietary complementation.