Hybridization in Terns: A Review

Hybridization—the interbreeding of different species—is a relatively common phenomenon in birds, but its study is biased towards certain taxa. In this review, we focus on a bird group that has received less attention: terns (Laridae, Sterninae). Based on an extensive literature search, we found records for 16 hybrids between tern species, with varying levels of supporting evidence: ten cases had strong support, one had medium support, one had low support, and four cases were deemed unreliable (two were refuted and two could not be evaluated). The strongly supported hybrid records involved 14 different tern species (out of 41 species, 34%). The underlying mechanisms of tern hybridization remain to be studied in more detail. The most likely processes that can result in the breakdown of reproductive isolation are (1) a scarcity of conspecifics, (2) interspecific extra-pair copulations, and (3) mis-imprinting after brood parasitism. Based on the available evidence, a scarcity of conspecifics appears to be the mechanism that most frequently results in the formation of mixed pairs and the production of hybrid offspring in terns. Our literature search revealed that population-level genetic analyses of tern hybridization are currently lacking. Given the occurrence of multispecies hybridization, it is possible that some tern species act as “genetic bridges” within a network of tern hybrids. All in all, the interbreeding of tern species provides exciting opportunities for further research that may generate novel insights into avian hybridization and speciation.