Effects of Brown-headed cowbird parasitism on provisioning rates of Swainson's Warblers

We studied the effects of brood parasitism by Brown-headed Cowbirds (Molothrus ater) and other factors on food provisioning rates of Swainson's Warblers (Limnothlypis swainsonii), a secretive and poorly understood species of conservation concern. We used time-lapse video systems to collect provisioning data at 25 nests, nine of which were parasitized by Brown-headed Cowbirds. We found strong relationships between feeding rate and brood size with increases from 2.0 feeding visits/hr for nests with a single Swainson's Warbler nestling to 3.0/hr for broods of four. We also found an effect of nestling age with 1.9 visits/hr early and 3.2 visits/hr late in the nestling period. The relationships between cowbird parasitism and provisioning were complex. Nests with brood size of one or three that contained a single cowbird nestling had greater provisioning rates (2.5 and 3.3 visits/hr, respectively) than non-parasitized nests (2.0 and 2.6 visits/hr, respectively). Nests with two cowbirds and no Swainson's Warbler young had greater provisioning rates (3.4 visits/hr) than those with two warbler young (2.4 visits/hr), or one warbler and one cowbird (2.6 visits/hr). The increase in provisioning rate with nestling age was more pronounced with two cowbird nestlings present than in nests with zero or one cowbird nestling. These results suggest that parasitized nests, especially those with multiple cowbird nestlings, can impose greater energetic demands on parents. Food limitation may constrain the ability of Swainson's Warblers to adequately care for both their own nestlings and cowbird young.