EFFECTS OF MURRE NEST-SITE CHARACTERISTICS AND WIND CONDITIONS ON PREDATION BY GLAUCOUS GULLS

Glaucous gulls, Larus hyperboreus, are the primary predator of thick-b illed murre, Uria lomvia, eggs at arctic breeding colonies. This study used eggs placed experimentally on murre nesting ledges retest the ef fects on glaucous gull foraging success of (i) murre nesting density; (ii) murre group defence; (iii) nesting ledge width; (iv) timing of mu rre laying; and (v) wind speed. Defensive responses by murres increase d egg survival times, and gull foraging success was reduced by high mu rre nesting density. The timing of murre egg laying and ledge width al so influenced the effectiveness of murre defence within the colony. Eg gs placed following the peak of murre egg laying survived longer than eggs placed on the same nest sites prior to the peak. Murres without e ggs early in the season often abandoned the group when attacked, leavi ng experimental eggs and brooding murres vulnerable to gulls. Gulls ha d difficulty reaching exposed eggs on narrow ledges when foraging on f oot. At wind speeds greater than 15 km/h, however, egg survival was in dependent of ledge width because wind increased the maneuverability of gulls in flight and enabled them to reach narrow ledges. Murre defenc e at high nesting densities was also less effective an narrow ledges. Apparently, murres had difficulty rotating on their nest sites to face attacking gulls without dislodging their eggs. We conclude that moder ate wind speeds enhanced the ability of gulls to reach narrow ledges a nd avoid contact with defending murres while stealing eggs. Consequent ly, the vulnerability of murre nest sites depended on wind conditions at the colony.