Gansu is the gateway to China’s wild west: a narrow province that skirts the northern edge of the Tibetan Plateau, connecting the cosmopolitan eastern cities of Lanzhou and Xi’an to the true outback of Xinjiang via the Yellow River valley. The thin strip of land along the river, sandwiched between the mountains of Tibet to the south and the Gobi Desert to the north, is called the Hexi Corridor (“the throat”). As one of the only consistent sources of water in an inhospitable region, the corridor was a critical stretch of the Northern Silk Road, functioning as a caravan route connecting eastern China to central Asia, Africa, and the Middle East for over 1500 years. We came to Gansu in a last-ditch effort to find a hybrid zone between the rustica and gutturalis subspecies. Finding evidence for hybridization between this pair of subspecies would help us understand the mechanisms of species divergence in barn swallows. But we had no idea whether such a hybrid zone actually existed
I knew from our sampling in 2014 that European rustica could be found in Dunhuang, a Silk Road city in western Gansu, and that gutturalis were present in Lanzhou, in easternmost Gansu. We had searched for barn swallows in between these two cities last year, taking a long, dusty bus across the Tibetan Plateau. Although we found a few rustica clinging to life in Golmud, a wind-whipped industrial city perched at over 10,000 feet, we found no evidence for swallows breeding in the lower but more mountainous cities of Xining and Tongren. This suggested that Tibet may represent a true geographic barrier to gene flow, keeping rustica and gutturalis from ever coming in to contact with each other- a very effective speciation mechanism. I also looked for contact between the two subspecies in southern Mongolia, but again found no birds breeding in the small, windy villages of the Gobi Desert. Remember, barn swallows nest exclusively in human structures, so any rustica-gutturalis hybrid zone had to be somewhere that also had people. If there was anywhere on the planet where these two forms- one large with white ventral plumage, one small with peachy-colored feathers- came in to contact, it had to be along that river valley in the Hexi Corridor. This region has sustained small communities of people for thousands of years, and it seemed likely that it could be sustaining barn swallows as well. And so off we went, in search of hybrids in the desert.
I’ll spare you the suspense and cut to the good stuff: we found our hybrid zone! But it took some doing. The difficult thing about hybrid zones is that if you want to learn anything about them, you can’t just collect a sample from one point. You need to find out how wide they are- that is, the distance over which there is a switch from one pure “parental” form to the other. We knew there were pure rustica in Dunhuang and pure gutturalis in Lanzhou, but there were over 600 miles of sparsely populated desert in between those two cities. The hybrid zone could span that entire range, or it could be only 20 miles wide- or it could not exist at all. The challenge for me was figuring out a sampling plan that would allow us to cover than entire 600-mile region- and hopefully catch over 100 birds- in 2 weeks…
-Liz
-Liz