

A study of the diet of chicks in a Phillip Island colony showed that Australian anchovies (Engraulis australis) and jack mackerel (Trachurus declivis) were the most popular items on the menu. Anchovies also made up the main part of the diet of little penguins (Eudyptula minor). (Hold the pizza.) So when the anchovy population declines, so do those of the terns and penguins. Add to that the commercial fish catch. In bad years that's a lot of competition for a limited resource.
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Chiaradia, A., Dann, P., Jessop, R. and Collins, P. (2002). The diet of Crested Tern (Sterna bergii) chicks on Phillip Island, Victoria, Australia. Emu 102 (4): 367–371.