NEST DISTANCE MEDIATES THE COSTS OF COLONIALITY IN EARED GREBES

We examined the behaviors and reproductive characteristics of 4 fared Grebe (Podiceps nigricollis) colonies located at the Malheur National Wildlife Refuge in eastern Oregon that varied in nesting density. In 1 993, we studied 2 colonies found on 2 different lakes, whereas in 1994 we conducted a within-lake comparison of 2 subcolonies. During both y ears, nearest neighbor distances differed significantly between the co lonies investigated. Breeding pairs were significantly more aggressive at those colonies with smaller distances among nests. Within each yea r, there was a greater rate of intraspecific brood parasitism and egg loss at the higher density colony. Infanticide occurred at high rates when nests were very densely packed, whereas a relatively high rate of predation by American Coots (Fulica americana) resulted when nests we re quite dispersed. Thus, in spite of the distinct breeding characteri stics of fared Grebes, many disadvantages associated with nesting dens ity in seabirds also have an impact on these colonially breeding water birds.