Tern hybrid produces backcross offspring Common Tern Sterna hirundo and Arctic Tern S. paradisaea hybridization produces fertile offspring Carolyn S. Mostello 1* , Derek LaFlamme 1,2 and Patricia Szczys 3 * Corresponding author. Email:
[email protected] 1 Massachusetts Division of Fisheries & Wildlife, 1 Rabbit Hill Road, Westborough, Massachusetts 01581, USA; 2 Central Connecticut State University, 1615 Stanley Street, New Britain, Connecticut 06053, USA; 3 Eastern Connecticut State University, 83 Windham Street, Willimantic, Connecticut 06226, USA. Abstract 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.