This study provides the first molecular evidence for hybridization between Arctic Terns Sterna paradisaea and Common Terns S. hirundo. We studied a mixed pair (P1; male Arctic Tern and female Common Tern) and its offspring on Penikese Island, Massachusetts, USA in 2007–2015. The pair maintained a long-term pair bond (8 years); its reproductive performance was comparable to that of Common Terns and higher than that of Arctic Terns at the site. Molecular analyses confirmed that all young raised by the pair (at least 5 males and 4 females) were biological offspring. We describe hybrid young and the adult hybrid in detail to facilitate field identification. Although F1 hybrid young were intermediate between the parent species in certain characteristics, dark feathers extending below the eye, in particular, gave them a distinctly Arctic Tern-like appearance; however, all had secondary feathers that were darker than the wing-coverts, like Common Terns. We detected one male F1 hybrid that returned to breed; it retained some features intermediate between the parent species, but we qualitatively judged it to be more Common Tern-like. It mated with a Common Tern and produced three confirmed backcross hybrid young that closely resembled Common Terns. We speculate that rarity of Arctic Terns, especially females, at and near the study site and age/inexperience of the Common Tern parent were proximate mechanisms for the formation of the P1 pair. Further, because mixed pairs are occasionally observed and hybrids are cryptic, we suggest that Arctic/Common Tern hybridization may occur more commonly than is currently realized.

Common Terns Sterna hirundo and Arctic Terns S. paradisaea have broad breeding distributions in North America and Eurasia, but their area of overlap is not extensive, as the Common Tern breeding range is largely south of that of the Arctic Tern (Cramp 1985; Hatch 2002; Nisbet 2002a). On the northwest Atlantic coast of the USA, where our study occurred, the species’ breeding ranges overlap between 41°N and 52°N (Hatch 2002). Because morphological and molecular evidence suggests that Arctic and Common Terns are very closely related (Bridge et al. 2005), it follows that hybridization could occur. Mixed Arctic/Common tern pairs, including multi-year pairings, have indeed been reported in the wild (Kullenberg 1946; Panov 1989; Debout & Debout 1989 in McCarthy 2006) as have presumed hybrid young (Degland & Gerbe 1867, Suchetet 1896). Molecular confirmation is, however, absent.

In June 2007, CSM first observed a Common Tern and an Arctic Tern attending two chicks on Penikese Island (41°27’N, 70°55’W) in Buzzards Bay, Massachusetts, USA. Observations over the next three weeks provided further support that these birds were a mated pair. We located this pair in most years from 2007 to 2014 and here we present behavioural and molecular evidence for a long-term pair bond and hybridization of these birds that resulted in production of fertile offspring. We report on: analysis of mitochondrial and nuclear DNA; characteristics of nests, eggs, and young (F1 hybrids); and reproductive performance of the mixed pair in comparison to the parent species. We describe an adult F1 hybrid and its offspring (B1 hybrids) that resulted from backcrossing to a Common Tern and discuss factors that may have contributed to the interspecific pairing at this site.

We dedicate this paper to Jeremy J. Hatch, who had an affinity for Arctic Terns. We thank the New Bedford Harbor Trustee Council, the MassWildlife-Natural Heritage & Endangered Species Program, Eastern Connecticut State University, CSU-AAUP Faculty Research Grant, and the Island Foundation for support; the Penikese Island School for transportation; the Woods Hole Oceanographic Institution for logistical support; J. Gahagan for photographing the hybrid adult; H. Goyert for bringing the grey tern chick on Bird I. to our attention; M. Bromberg and G. Vecchio for translations of papers; and D. Hayward, I. C. T. Nisbet, L. Welch, S. Williams, and the Gulf of Maine Seabird Working Group for information. M. Collinson and an anonymous reviewer improved the manuscript. We especially thank many seasonal biologists and volunteers, including: W. Houghton, K. Blake, L. Mostello-Wetherbee, J. Ebel, L. Flieger, C. Challion, J. Cunningham, W. van Dijk, A. Eibin, T. Maikath, N. French, E. Lencer, K. Scantlebury, S. Woodward, J. Hatt, S. Schulwitz, A. Crabbe, M. Servison, K. Justham, R. Herman, K. DeMoranville, J. Correia, P. Cunningham, E. LaPlante, A. Anderssen, D. Dombkowski, A. Smith, E. Berge, and C. Bates.

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