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Interspecific egg-dumping by a violet-green Swallow in an active Western Bluebird nest.

Article Excerpt
Observations of birds laying eggs in nests of other species are of interest because this behavior, when timed appropriately and directed at nests where parents fail to recognize foreign eggs and offspring, may be the starting point for evolution of interspecific brood parasitism. Other explanations for this behavior are nest usurpation and egg-dumping, which is the deposition of an egg into another species' nest without any specific adaptive function. We collected video footage of a Violet-green Swallow (Tachycineta thalassina) laying an egg in an active Western Bluebird (Sialia mexicana) nest containing nine-day-old nestlings at Hastings Natural History Reservation, Carmel Valley, California (36[degrees] 22' N, 121[degrees] 34' W). Nest boxes have been monitored at Hastings and nearby ranches as part of a long-term study of Western Bluebirds since 1983 (Dickinson et al. 1996). Western Bluebirds are the primary box-nesting species at this study site, but other secondary cavity nesters use nest boxes at lower frequencies, including Ash-throated Flycatchers (Myiarchus cinerascens), Oak Titmice (Baeolophus inornatus), House Wrens (Troglodytes aedon), Bewick's Wren (Thryomanes bewickii) and Violet-green Swallows (Table 1).

We conducted a study of parental feeding behaviors during spring 2005 using video to identify patterns of resource allocation within the nest. The day before filming we replaced the natal nest box and nest material with a box of the same dimensions containing a Western Bluebird nest, but with a Plexiglas side to accommodate the camera. A video camera...