Breeding Biology of Swainson's Warblers in a Managed South Carolina Bottomland Forest

Abstract

Conservation plans for the southeastern U.S. have identified Swainson's Warblers (Limnothlypis swainsonii) to be among the more vulnerable Neotropical migrants. However, a lack of life history information, in particular breeding, jeopardizes sound management decisions. My study examined a population in Britton's Neck, South Carolina on timberland owned by International Paper Co. From 1999-2001, I conducted a breeding biology study that examined nesting, territory, vegetative, and landscape characteristics.

I discovered 98 SWWA nests, 63 of which were active. SWWA nesting success equaled 60% using the Mayfield success estimator. Contrary to the notion of SWWA requiring mature forests to breed, SWWA bred successfully in 20-year old regenerating clearcuts. Multiple brooding, where a pair initiates nesting attempts after their first is successful, was detected in 20% of the pairs. An individual's seasonal fecundity was doubled by multiple brooding, increasing the young per female from 1.5 to 3.7. Brown-headed Cowbirds (Molothrus ater) parasitized only 10% of nests. No SWWA chicks were observed fledging in the presence of Cowbirds.

Apparent population density was 17 territories/km2, equaling the highest density ever recorded for SWWA. One-fifth the site contained 59 territories/km2, compared to the remaining four-fifths with 12 territories/km2. I examined vegetative differences between nests built in high- versus low-density areas using a discriminant function analysis (DFA). The DFA results indicated understory thickets accounted for the largest difference between high- and low-density nest sites. Nests in the high-density area were more associated with understory thickets.