SYNCHRONY IN EGG-LAYING AND REPRODUCTIVE SUCCESS OF NEIGHBORING COMMON MURRES, URIA AALGE

Common murres (Uria aalge) are highly colonial; pairs often breed at t he highest possible densities, in bodily contact with neighbors. At Bl uff, a colony in western Alaska, we tested for synchrony in egg laying at various spatial scales and found little evidence for higher synchr ony, either within study plots of 15-195 pairs, or within subplots con taining several pairs, than among plots in a 5-year study. Egg laying of neighbors generally was more synchronous than expected based on ove rall frequency distributions in laying dates, however. Breeding succes s was positively correlated with the number of breeding neighbors and the number of neighbors tending eggs or nestlings at the time of egg l aying. Breeding success of pairs with neighbors was positively related to the breeding success of neighbors. Pairs that produced eggs synchr onously with at least one neighboring pair had higher success than tho se that began breeding either before or after their neighbors. Most re productive failures at Bluff are due to accidental egg loss and predat ion on eggs by common ravens, Corvus corax, soon after laying. By occu pying space where a raven might otherwise land and defending their own eggs, active breeding neighbors locally reduce the probability of egg predation. Active breeding neighbors also are less likely to flush an d accidentally dislodge nearby eggs when disturbed than are nonbreeder s. Murres breeding synchronously with neighbors have the highest assur ance of the presence of active breeding neighbors both at the time of egg laying and throughout their reproductive attempts. Groups of neigh boring murres can be considered small ''selfish herds,'' demonstrating by-product mutualism through their continued presence and defense of their own eggs and nestlings. Despite the advantages of breeding synch ronously with neighbors, early breeding may often be favored, however.