Behavioral Responses of European Blackbirds and Australasian Silvereyes to Varying Acid and Sugar Levels in Artificial Grapes

Abstract

Diminishing acid concentrations have long been thought to be one of the effects of ripening grapes that leads to increased bird pressure approaching harvest. Blackbirds (Turdus merula) and silvereyes (Zosterops lateralis) were offered in a field context varying concentrations of tartaric and malic acids in artificial grapes, where sugar and all other ripening grape parameters were controlled. No linear response of consumption to varying acid concentration was found for either species. A response to rising sugar was confirmed, but diminishing acid concentrations in ripening grapes appear not to be a contributing factor to increasing bird pressure approaching harvest.