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 acquisitio ns, 41.5% occurred through passive occupation of territories left vacant af ter 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 b efore the seasonal return of previous owners. Take-overs occurred in both s exes, but individuals acted alone, never in pairs. Displaced breeders usual ly took refuge on undefended lakes near their former territories; about hal f of these loons later regained former territories through passive occupati on or took possession of new territories elsewhere. As predicted by the rec onnaissance hypothesis, usurpations occurred most often in territories that had produced chicks during the previous 12 months, suggesting that loons u se the presence or absence of chicks as a cue for territorial usurpation. L arge individuals of both sexes held onto territories longer than small indi viduals, an indication that body size might be correlated with fighting abi lity. In terms of life history, loons appear to locate good territories thr ough reconnaissance, usurp them in a subsequent year and recover from displ acements by reclaiming their original territories or new ones. (C) 2000 The Association for the Study of Animal Behaviour.