|The Barn Swallow Project||
Once we found evidence of hybridization between rustica and gutturalis in Jiuquan, we needed to map the width of the hybrid zone. We set out on our zigzagging plan: after 3 nights of catching birds in Jiuquan, we hopped the high-speed train to Zhangye. Again, we found excellent stretches of old courtyard-style houses, and again, we found birds that looked like both rustica and gutturalis nesting close to each other. The birds in Zhangye were early in their breeding cycle- only a few had begun laying eggs, and most were still building their nests. Luckily they were sleeping on their nests, and we managed a big sample size over 3 nights of work.
Searching for a hybrid zone over a 600-mile stretch of desert could be like finding a needle in haystack. Luckily for us, there aren’t many towns along the Hexi corridor. It’s a barren strip of land, with only a handful of places big enough to have stops on the high-speed train line (yes, there are high speed trains even in the Chinese outback). Since there wouldn’t be any barn swallows in places with no people, we were able to narrow our search down to five towns: Yumen, Jiuquan, Gaotai, Zhangye, and Wuwei. These fell at roughly 100mile intervals on the train line between Dunhuang in the west (our pure rustica population) and Lanzhou (gutturalis) in the east. Once we identified our sampling options, the tricky bit was to figure out how to allocate our time: we wanted to maximize time catching potential hybrids and minimize time catching pure parentals, but we didn’t know how wide or where the hybrid zone was. I decided we’d have to do some zigzagging- we’d start in Jiuquan, about a third of the way from Dunhuang to Lanzhou. If we found both subspecies there, we’d sample for three days and move on to Zhangye. If we found only gutturalis, we’d sample a little bit and then backtrack to Yumen, and if only rustica, we’d sample a little and then keep moving eastward. We’d do the same thing in Zhangye, moving east to Wuwei or back west to Gaotai depending on which birds we found.
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
Well, I’m back in China. It feels like I just left China, but I guess it’s been almost a year since our epic trip last summer. In the intervening months we were busy analyzing data, and were particularly focused on variation in color and body size measurements from our 14 different Chinese barn swallow populations. Our intuitions from the field were soon confirmed with quantitative data (always a good feeling for a biologist): there are three distinct “phenotypic clusters” of swallows in China.