Nest attendance by adult Pied-billed Grebes, Podilymbus podiceps (L.)

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Abstract
In 1984, we monitored diurnal attendance by adult Pied-billed Grebes, Podilymbus podiceps (L.), on nests found in the emergent vegetation of ponds located near Minnedosa, Manitoba (55°16′N, 99°50′W). Adults increased attendance on nests over the egg-laying period; by the time four of six to eight eggs had been laid, adults spent roughly 90% of daylight time on nests. Nest attendance remained high during the post-laying period, but declined to between 75 and 85% of monitoring time later in the hataching period. Time of day did not affect the amount of time spent by adults off the nest. Previous visits by observers to nests on a given day, however, had a profound effect in that adults spent less time on "disturbed" than on "undisturbed" nests during the laying period, but more time on disturbed nests during the hatching period. Females accounted for roughly 70% of the total time spent by adults on nests during the hatching period, but were equal to males in this regard during the laying and post-laying stages. Our data suggest that nest attendance by both sexes before clutch completion largely determines onset of incubation and consequently the hatching interval for Pied-billed Grebe broods. Neglect of eggs by adults after the onset of hatching, however, may exaggerate hatching intervals.