Gull-billed Terns nesting in the Valli di Comacchio (Northern Italy) fed their chicks mainly with lizards, while fish and insects were less important. The relative frequency of the various prey changed through the day; prey other than lizards predominated in the early morning and at noon, when lizards were less available. Fish were captured mainly during the hours of low lizard availability. The tern switch to alternative prey in relation to the rhythm of their availability.
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Bogliani et al. (1990) studied this question.