On 14 and 15 April 2004 I documented an adult male California Gnatcatcher (Polioptila californica) repeatedly feeding the nestlings of a pair of Bewick’s Wrens (Thryomanes bewickii) that had built a nest inside the remnant of a rodent burrow in western Riverside County, California.
Among the many published accounts of interspecific feeding of young in birds (e.g., Skutch 1960, 1961, 1987; Shy 1982; Welty and Baptista 1988), only one involves a gnatcatcher feeding the young of a species other than the parasitic Brown-headed Cowbird (Molothrus ater): Erickson (1998) described a male California Gnatcatcher feeding fledgling Bushtits (Psaltriparus minimus). True cooperative breeding has not been reported for the California Gnatcatcher (Atwood and Bontrager 2001). An adult male thought to have recently lost its mate “helped” a gnatcatcher pair feed their single fledgling, then later displaced both members of the pair from the territory (Atwood and Bontrager 2001). The gnatcatcher nests in low shrubs, whereas Bewick’s Wren is a cavity nester. Both species are typically intolerant of birds of any other species approaching their nests, and there are no published references of a gnatcatcher repeatedly entering the nest cavity of another species, much less an underground cavity nest.