The two weeks since our last post have been extremely harrowing, and we have now successfully completed the transect across the subspecies contact zone. Apart from a few unfortunate cases like the one described above (for some reason several of our interesting sites with both barn swallow subspecies are owned by total jerks), we've been extremely successful. We found a few barns and dairies with loads of barn swallows in a few places to get our sample sizes up, and the rest we filled in with Georgy and Yulia's door-to-door salesmanship. We also found that there is a very steep transition from extremely pale Hirundo rustica rustica to the very rusty Hirundo rustica tytleri over a stretch of about 100km. We sampled very intensively across this area, and found that there were some intermediate-looking birds, which might be hybrids or backcrosses, but for the most part, the swallows seem to mate assortatively. This is not entirely surprising, as these two subspecies are over 1% divergent in their DNA sequences and have probably evolved in relative isolation over thousands of years. But this is also interesting because bird species which are diverged by even several millions of years can often produce fertile offspring. The fact that these closely related subspecies don't seem to hybridize much suggests that differences in the mating preferences which have shaped these populations in different directions seem to also be keeping these birds from hybridizing when they come into contact. It will be very interesting to see at the genetic level to what degree this holds up. If DNA sequence data shows very low levels of hybridization across our sampling transect, this could provide some of the strongest evidence in the scientific literature for the importance of sexual selection in the process of speciation. Very exciting stuff for we nerdly types!