Territory acquisition in loons: the importance of take-over.

We examined patterns of territory acquisition and reconnaissance in common loons, Gavia immer, from northern Wisconsin. Among all territory acquisitions, 41.5% occurred through passive occupation of territories left vacant after the death or desertion of a previous resident, 17% constituted founding of new territories and the remaining 41.5% came about through take-over: either usurpation of defended territories or appropriation of territories before the seasonal return of previous owners. Take-overs occurred in both sexes, but individuals acted alone, never in pairs. Displaced breeders usually took refuge on undefended lakes near their former territories; about half of these loons later regained former territories through passive occupation or took possession of new territories elsewhere. As predicted by the reconnaissance hypothesis, usurpations occurred most often in territories that had produced chicks during the previous 12 months, suggesting that loons use the presence or absence of chicks as a cue for territorial usurpation. Large individuals of both sexes held onto territories longer than small individuals, an indication that body size might be correlated with fighting ability. In terms of life history, loons appear to locate good territories through reconnaissance, usurp them in a subsequent year and recover from displacements by reclaiming their original territories or new ones. Copyright 2000 The Association for the Study of Animal Behaviour.