PREDATION BY RAVENS CORVUS-CORAX ON BRUNNICHS-GUILLEMOT URIA-LOMVIA EGGS AND CHICKS AND ITS POSSIBLE IMPACT ON BREEDING SITE SELECTION

We studied the behaviour of Ravens Corvus corax taking the eggs or chi cks of Brunnich's Guillemots Uria lomvia at East Digges Island, Northw est Territories, Canada, We classified the characteristics of the bree ding sites attacked by Ravens according to their location on the cliff s, the width of the breeding ledge, the number of adjacent rock faces and the density of nearby breeding guillemots, Ravens selectively atta cked peripheral sites near the top of the occupied area, those on narr ow ledges and those with few dose neighbours, The risk of predation at the most vulnerable sites was more than seven times the average, For the most part, breeding success at 419 sites near the cliff top confor med to predictions based on the observations of Raven predation. Howev er, losses observed over 3 years suggested that more eggs Fall acciden tally than are taken by avian predators, We concluded that the risks o f predation and of accidental dislodgement both contribute to choice o f nest sites by Brunnich's Guillemots, The effect of multiple dose nei ghbours in deterring predation by Ravens may be important in determini ng the density of guillemot breeding sites within the colony.