Do brown-headed cowbird hatchlings alter adult Yellow Warbler behavior during the hatching period?

Early hatching and intense begging by nestling Brown-headed Cowbirds (Molot hrus ater) have been hypothesized to reduce host incubation attentiveness, thereby extending the incubation periods of host eggs and ultimately increa sing the competitive advantage of the cowbird chick. We tested this hypothe sis by adding newly hatched cowbirds to Yellow Warbler (Dendroica petechia) clutches one day prior to their predicted hatch date, and observing whethe r the presence of the cowbird nestling brought about changes in parental be havior during the hatching period. The presence of a cowbird nestling did n ot significantly alter female nest attentiveness relative to control clutch es. The presence of a cowbird nestling did not significantly increase the r ate at which either the female or male Yellow Warbler delivered food to the nest. Female Yellow Warblers tended to feed food delivered to the nest by the male to a cowbird nestling more frequently than to a warbler nestling, but the difference only approached significance. The presence of a cowbird nestling resulted in significantly longer Yellow Warbler incubation periods for only the third egg to hatch in the clutch. Host clutches with a cowbir d nestling experienced decreased hatching success and increased frequency o f egg disappearance, but the difference in nestling mortality only approach ed significance. Our data indicate minimal alteration of host behavior by t he cowbird nestling during the hatching period. Although the detrimental ef fect of the cowbird nestling is measurable by the end of the hatching perio d, its full effect, especially for small hosts, does not manifest itself un til later in the nestling and fledgling periods.