eWestern Birds

Low Incidence of Cowbird Parasitism on Swainson’s Thrushes in Central Coastal California

Jennifer D. White and Thomas Gardali

ABSTRACT: We found a low incidence of Brown-headed Cowbird (Molothrus ater) parasitism (4%; 9/224 nests) of Swainson’s Thrush (Catharus ustulatus) in central coastal California despite the high rates (33%) reported here for Wilson’s Warbler (Wilsonia pusilla). Both species nested primarily in blackberry shrubs and ferns in similar proportions and at similar heights. Thrush nests were significantly better concealed than warbler nests. In both the thrush and warbler, however, concealment of parasitized and unparasitized nests did not differ. The number of potential cowbird perches was not significantly greater at warbler nests or at parasitized nests of either species. No cowbirds fledged from thrush nests; in thrush nests cowbird nestlings observed were last seen at ages of 4 to 7 days, whereas thrush nestlings fledged. At our study sites Swainson’s Thrushes fed nestlings a variety of fruit; this partially frugivorous diet may not be suitable for cowbird nestlings, leading to their death. Parasitism significantly reduced the thrushes’ clutch size, number of nestlings, and number of fledglings. In four thrush nests experimentally parasitized with real cowbird eggs we observed no rejection response, suggesting that ejection of cowbird eggs is not responsible for the low rate of parasitism observed at our sites. It is unclear why Swainson’s Thrushes are parasitized infrequently at our sites, but we suggest that nest concealment may be partly responsible. We did not study behavioral differences between Swainson’s Thrush and Wilson’s Warbler, but such differences may contribute to the observed difference in parasitism rates.

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