‘Information parasitism’ in mixed colonies of western grebes and Forster's terns

Abstract

Of 38 western grebe ( Aechmophorus occidentalis) colonies initiated on the Delta Marsh, Lake Manitoba, in 1973 through 1979, over half (53%) included breeding Forster's terns ( Sterna forsteri). Tern-associated colonies included 12 of the 13 largest colonies, incorporating 83% of the total nesting effort ( N=2821 nests). Such colonies were impossible to approach undetected, because grebes used the tern's aerial alarm system to warn of danger. Playbacks showed that grebes recognized the tern's alarm call, and responded by immediately leaving their nests. This essentially constitutes a form of ‘information parasitism,’ which probably is widespread in many nesting associations. This work suggests a broadening of the ‘information centre’ hypothesis to include transfer of information about any reproductive requisite, not just food.