N15 2019 Northeastern Naturalist Notes Vol. 26, No. 2 L.R. Reitsma, C. Burns, and J. Sullivan Poecile atricapillus (Black-capped Chickadee) Feeding Catharus guttatus (Hermit Thrush) Nestlings Leonard R. Reitsma1,*, Christian Burns1, and Jess Sullivan1 Abstract - We describe what we believe to be the first recorded instance of an adult Poecile atricapillus (Black-capped Chickadee) feeding the young of a different species. On 25 June 2017, we observed a Black-capped Chickadee feeding 3 Catharus guttatus (Hermit Thrush) nestlings. We recorded a video of this behavior the following day, in which the adult Black-capped Chickadee delivered food to the Hermit Thrush nestlings 12 times in 105 min of continuous recording. The Black-capped Chickadee also removed fecal sacs from the nest. At the end of this video, 1 of the nestlings fledged. We did not observe an adult Hermit Thrush feeding the nestlings during the entire recording. The considerable differences in body size, nesting strategy, and nestling growth rate between these 2 species comprise a unique instance of interspecific parental feeding that resulted in successful fledging. Instances of interspecific parental care, in which an adult bird regularly feeds the young of a different species, have been recorded across at least 32 bird families (Jiang et al. 2016, McNair and Duyck 1991, Oswald et al. 2013, Shy 1982), 71% of which are in the order Passeriformes. The most commonly proposed cause of this behavior is the proximity of both species’ nests (Jiang et al. 2016, LaBarbera and Spencer 2016, Shy 1982). Other hypotheses include nest failure of the provisioning bird, a mateless bird subject to hormonal imbalance, attraction to begging calls of the provisioned young (Batisteli and Sarmento 2016, Fiss et al. 2016, Shy 1982), brood parasitism (Batisteli and Sarmento 2016), nest takeover (Samplonius and Both 2014), and competition for a nest site (Suzuki and Tsuchiya 2010). In all cases of interspecific parental care, feeding until the young are fledged is unusual (Shy 1982), although there was an instance when care persisted weeks after fledging (Fiss et al. 2016). Herein, we detail observations of a Poecile atricapillus L. (Black-capped Chickadee) feeding 3 Catharus guttatus (Pallas) (Hermit Thrush) nestlings with documentation of at least 1 of the young fledging. We believe this to be the first recorded instance of interspecific feeding by a Black-capped Chickadee. The Black-capped Chickadee is a non-migratory resident songbird ranging across the northern US and most of Canada. The species is a cavity- nester, favoring Betula (birch) and Acer (maple) trees when available (Foote et al. 2010). In the northeastern US, the nesting season is from early May through mid-July (Foote et al. 2010). Typically only 1 brood of 6–8 eggs is laid per season, although a second brood is possible if the initial brood was hatched early in the breeding season (Foote et al. 2010, Odum 1941), and multiple nesting attempts may occur in response to nest failure (Foote et al. 2010, Ramsay and Otter 2007). Black-capped Chickadees have a wide distribution; thus, they co-exist with hundreds of avian species across their range. However, to our knowledge there has not been a documented case of interspecific feeding performed by Black-capped Chickadees until this report. There have been several and varied cases of this behavior within the family Paridea (Foote et al. 2010, Samplonius and Both 2014, Shy 1982), including in 2 other North American species: Poecile carolinensis (Audubon) (Carolina Chickadee) (Murphy 1968) and Poecile 1Department of Biological Sciences, Plymouth State University, Plymouth, NH 03264. *Corresponding author - leonr@plymouth.edu. Manuscript Editor: Jeremy Kirchman Notes of the Northeastern Naturalist, Issue 26/2, 2019 2019 Northeastern Naturalist Notes Vol. 26, No. 2 N16 L.R. Reitsma, C. Burns, and J. Sullivan gambeli Ridgway (Mountain Chickadee) (Russell 1947). In these reports, interspecific parental care was usually the result of mixed-species clutches, often because of nest takeover by another species of chickadee (Murphy 1968) or tit (Cyanistes caeruleus (L.) [Eurasian Blue Tit] and Parus major L.[Great Tit]; Samplonius and Both 2014, Shy 1982). Moreover, the young being provisioned were usually in the same family (e.g., Samplonius and Both 2014), or similarly sized species, such as the Phoenicurus phoenicurus (L.) (Common Redstart) (Mackenzie 1954). In some cases, chickadees attended to the young of considerably larger species (Murphy 1968, Russell 1947), although this behavior is less common. On 11 June 2017, we flushed a Hermit Thrush, presumed to be female, from an open-cup nest with 4 eggs and recorded the nest location using a hand-held GPS receiver. We revisited the nest on 25 June as part of a collaborative effort to photographically document nest-site selection of North American birds. Three nestlings of advanced age were in the nest. After we finished photographing the nest, we observed a Black-capped Chickadee deliver food to the nestlings. They responded normally by opening their mouths and accepting the food. We observed this sequence of events 2 more times in less than 10 mins. The following day (26 June), we set up a video camera (Canon 7d mounted on an A011 Tamron150–600-mm lens fixed at 600 mm, with audio captured by a Rode GO Shotgun microphone) ~10 m from the nest, and recorded from 07:00 to 08:45 in 12 consecutive 10-min intervals. During the 105-min recording, we documented 12 provisioning events by the Black-capped Chickadee. The video footage also shows an adult chickadee removing a fecal sac from the nest. At 08:40, 1 nestling fledged (Sullivan 2017a, 2017b). We do not know whether 1 or 2 adult chickadees were delivering food because Blackcapped Chickadee plumage is sexually monomorphic and the birds were not banded. In both visits on 11 and 25 June, we heard a male Hermit Thrush singing nearby. We also heard the characteristic “jeer” note, but we have no evidence of adult thrushes bringing food to the nestlings on either day despite 105 consecutive minutes of video on 26 June. Among the more extraordinary aspects of these observations is the size difference between the 2 species, the apparent successful growth of the thrush nestlings to a size sufficient for fledging, and the profoundly different nesting habits of each species—one is a tree-cavity nester and the other is an open-cup ground nester. The adult Black-capped Chickadee may have had a failed nest coincident with mortality of the mother Hermit Thrush, a combination of the hypotheses listed above. Even if these 2 events were concurrent, the transfer of parental care by this parid species to a ground-nesting thrush must be a rare phenomenon. While parids have been observed feeding turdid species of comparable size, these have been cavity nesters (Gowaty and Plissney 2015, Shy 1982), like the parids themselves (Shy 1982). Acknowledgments. We thank the Mascoma Watershed Conservation Council for permission to conduct research at the Bear Pond Natural Area. Literature Cited Batisteli, A.F., and H. 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