The Wilson Journal of Ornithology 121(4):830–834, 2009 Adoption by a Territorial Passerine Russell A. Ligon, 1,2 Mark Liu, 1 and Geoffrey E. Hill 1 ABSTRACT.—We observed and video-taped active- ly nesting adult Eastern Bluebirds (Sialia sialis) repeatedly feeding abandoned conspecific juveniles in a nearby nest. We used molecular techniques to confirm these nestlings were unrelated to the adults that fed them. The most likely explanation for the observed behavior is that it was a mistake resulting from parental response to the begging vocalizations in the nearby nest. Received 24 November 2008. Accepted 1 May 2009. Adoption is the phenomenon wherein individ- uals provide sole parental care to foreign young (Riedman 1982). This seemingly maladaptive behavior has been documented in a wide range of animals including fish (McKaye and McKaye 1977), mammals (Bridges 1990), and birds (Pierotti 1988). There are two main pathways in birds to alloparental care and adoption. First, mobile young may abandon their original nest and seek additional care from unrelated adults (Pier- otti and Murphy 1987, Redondo et al. 1995, Roulin 1999, Bize et al. 2002). Nestlings of colonial species (e.g., Ring-billed Gulls [Larus delawarensis], Brown et al. 1995) have more opportunities to receive care from foster parents, and adoption is especially common when adults are unable to discriminate between related and unrelated young (Graves and Whiten 1980, Pierotti 1988, Morris et al. 1991, Brown et al. 1995, Jouventin et al. 1995). Second, adults may adopt unrelated young following territorial take- over events (Rowher 1986). Individuals in these instances may increase the likelihood of main- taining newly formed pair bonds and territorial control by providing care to unrelated young (Rowher 1986). Our objectives are to: (1) report adoption of Eastern Bluebird (Sialia sialis) nestlings by a pair of concurrently nesting, unrelated adults from a neighboring territory; and (2) present a third pathway to alloparental care. We confirmed field observations with evidence from videotaped parental behaviors and molecular genotyping. METHODS Study Species.—Eastern Bluebirds are socially monogamous, territorial, obligate cavity-nesting passerines that frequently breed in artificial nest boxes (Gowaty and Plissner 1998). Eastern Bluebirds, like some other socially monogamous passerines (e.g., Great Tits [Parus major] and Eurasian Blue Tits [Cyanistes caeruleus]) (Gull- berg et al. 1992), experience high levels of extra- pair paternity in some populations (Gowaty and Plissner 1998). Male and female bluebirds defend all-purpose territories around their nest boxes during the breeding season, and breeding pairs are aggressive to other bluebirds. Male bluebirds intensively guard their partners during the egg- laying period of the nesting cycle, and females aggressively defend their nests from other females to reduce conspecific brood parasitism (Gowaty and Plissner 1998). Field Procedures.—We conducted this study on a banded population of Eastern Bluebirds in Lee County, Alabama, USA (32u 359 N, 82u 289 W). We collected feathers and blood samples from adults by trapping males and females at their nest boxes when nestlings were 2 days of age. 1 Department of Biological Sciences, 331 Funchess Hall, Auburn University, Auburn, AL 36849, USA. 2 Corresponding author; e-mail: russell.ligon@gmail.com 830 THE WILSON JOURNAL OF ORNITHOLOGY N Vol. 121, No. 4, December 2009