Tweets,

It would seem that the most common hybrids are between near-allopatric species of the large northern complex which interbreed freely where there ranges overlap. For example, Western x Glaucous-winged, Glaucous-winged x Herring, Glaucous-winged Slaty-backed, Glaucous-winged x Glaucous, Herring x Glaucous, Thayer's x Kumlien's. It is interesting that species of this complex that are widely sympatric appear to interbreed less frequently, e.g., Herring x Thayer's, Herring x Kumlien's, Lesser Black-backed x Herring, Iceland x Glaucous. Note that Great Black-backed and Herring are widely sympatric, not to mention more different in size and other characteristics than most pairs known to hybridize frequently. (Great Black-backed and Glaucous breeding ranges barely overlap.) Thus it may be that more effective barriers to gene flow have evolved between these two species than between the other species pairs. Note that California, Ring-billed, and Glaucous-winged are sympatric but not known to interbreed. Likewise, Mew and Glaucous-winged and Mew and Herring. So the likelihood of interbreeding is not the same for all large gull species pairs. It would be interesting if actual mixed pairs of Herring and Great Black-backed had been observed and the success of the matings noted.

Gene Hunn.