Brood parasitism of Hooded Warblers by Brown-headed Cowbirds: Severe impact on individual nests but modest consequences for seasonal fecundity and conservation

Abstract

Brood parasitism by Brown-headed Cowbirds (Molothrus ater) often has pronounced negative effects on host nests. However, the extent to which parasitism reduces annual reproduction and presents conservation challenges for host species is unclear. We address this issue with data from a color-banded population of Hooded Warblers (Setophaga citrina) in Pennsylvania, where Hooded Warblers have increased dramatically despite frequent nest parasitism. Our analysis is based on both an extensive dataset (8 years, 847 nests) on the per-nest impacts of cowbird parasitism, and female-based stochastic simulations that accurately reflect the reproductive biology and parasitism rate (30%) of our study population. Cowbird parasitism has multiple negative consequences for Hooded Warbler nests, including (1) reduced host clutch size; (2) increased nest abandonment; (3) increased risk of complete failure due to predation; and (4) in surviving nests increased egg loss, hatching failure, and nestling mortality. We estimate that parasitism reduces success of Hooded Warbler nests 68%, from 1.29 to 0.41 fledglings per nest. For females and populations, however, the consequences of nest parasitism are considerably less extreme; female annual fecundity decreases 25% for each nesting attempt parasitized, and population-level fecundity drops 5.6% for each 10% increase in the frequency of parasitism. These more modest impacts are attributable to (1) steep declines in rates of cowbird parasitism as the nesting season progresses, (2) rapid re-nesting following abandonment or failure of parasitized nests, and (3) regular double brooding, with second broods initiated in late June and July when the incidence of cowbird parasitism is low. Our results help resolve the paradox of how cowbird parasitism can have both severe consequences for individual host nests but more modest and sustainable conservation impacts on the seasonal fecundity of females and populations. They further underscore the importance of determining population-level effects of brood parasites before investing in costly management efforts.

Stabilizing selection on a plumage-based foraging adaptation: hooded warblers with average-sized white tail spots live longer

“…Variation in tail white had no significant effect on lifetime fledgling production once duration of reproductive lifespan was statistically controlled (table 2; electronic supplementary material, figure S11). This finding was not unexpected, as nesting success in hooded warblers is determined primarily by nest predation [16] and nest parasitism by brown-headed cowbirds ( Molothrus ater ) [22], not by the ability of adults to forage efficiently and provision nestlings [16]. Thus, variation in tail white probably influences lifetime reproductive success primarily via effects on survival and reproductive lifespan, not via effects on nesting success.…”

“…Intensive efforts were made May–August each year to completely census the breeding population of 45–70 pairs, capture and individually colour-ring all breeding adults, and locate and monitor the reproductive success of all nests. Complete details on general field methods and hooded warbler breeding biology are available elsewhere [16,21,22].…”