Attempted Nest Parasitism of the Marsh Wren by a Brown-Headed Cowbird

Abstract

The Brown-headed Cowbird, Molothrus ater, is a brood parasite known to have laid in nests of 220 host species (Friedmann et al. 1977, Friedmann and Kiff 1985). The Brown-headed Cowbird is a generalist that tends to parasitize passerines building open nests, although relatively rare cases of parasitism of hole-nesting species such as the House Wren, Troglodytes aedon, have been reported (Friedmann et al. 1977). The purpose of this note is to describe the first case of cowbird parasitism of the Marsh Wren, Cistothorus palustris, and hence add this passerine to the list of the cowbird hosts. The Marsh Wren is a small (about 12 g) passerine breeding in marshes across temperate North America. Male wrens build many domed nests, each with a small entrance (about 3 cm in diameter) near the top, concentrated in the sites (e.g., Verner 1965). The female usually chooses one of these courtship nests for breeding, lines it, and then lays a clutch that she alone incubates (Verner 1965). From 1977 through 1982 I collected data on the breeding ecology of Marsh Wrens in a brackish water marsh on Westham Island, Delta, B.C., Canada, and obtained information on over 1,200 breeding nests. On 26 June 1981 I found a Marsh Wren nest that contained two broken wren eggs and one cold, undamaged cowbird egg. The nest was not damaged; i.e., the entrance near the top of this domed structure was not enlarged. Because the entrances into Marsh Wren nests are presumably too small to allow entry by cowbirds, the female cowbird had to lay an egg by perching on the nest top, placing her cloaca near the entrance, and depositing an egg. My data on Marsh Wrens suggest that cowbird parasitism of this host is extremely rare. The probability that some parasitized nests escaped detection is very low, because contents of all nests were checked twice a week