RED-NECKED GREBES BECOME SEMICOLONIAL WHEN PRIME NESTING SUBSTRATE IS AVAILABLE

Abstract

Abstract During 1995–1998 we studied the dynamics of nesting dispersion in a population of 160–200 Red-necked Grebes (Podiceps grisegena) on Lake Osakis, a large, windblown lake in central Minnesota. Breeding grebe pairs arriving on the lake were highly attracted to several large floating cattail (Typha angustifolia) mats that had broken off and drifted away from the mainland to become “islands” snagged within rooted emergent beds. These mats provided prime nesting habitat protected from waves and predators that was chosen by nearly 40% of the breeding population. Most pairs establishing nests on the islands averaged 5–10 simultaneously active neighbors within 50 m, compared to pairs that established territories along the shoreline, most of which had no neighbors within 50 m. The overall success of the two strategies varied widely from year to year, depending on the water levels, nesting habitat availability, timing of windstorms, and predator activity. During most years, however, pairs that established n...