Predation by RavensCorvus coraxon Brunnich's GuillemotUria lomviaeggs and chicks and its possible impact on breeding site selection

Abstract

We studied the behaviour of RavensCorvus coraxtaking the eggs or chicks of Brunnich's GuillemotsUria lomviaat East Digges Island, Northwest Territories, Canada. We classified the characteristics of the breeding sites attacked by Ravens according to their location on the cliffs, the width of the breeding ledge, the number of adjacent rock faces and the density of nearby breeding guillemots. Ravens selectively attacked peripheral sites near the top of the occupied area, those on narrow ledges and those with few close 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 conformed to predictions based on the observations of Raven predation. However, losses observed over 3 years suggested that more eggs fall accidentally than are taken by avian predators. We concluded that the risks of predation and of accidental dislodgement both contribute to choice of nest sites by Brunnich's Guillemots. The effect of multiple close neighbours in deterring predation by Ravens may be important in determining the density of guillemot breeding sites within the colony.