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.
The plan went pretty well. In Jiuquan we found a strip of old courtyard-style houses backed up against an empty lot dotted with garbage, rubble, and goats- perfect barn swallow nesting habitat. Urban development has been hard on barn swallows in China- they prefer older buildings that have dark, sheltered areas where they can build their nests. Traditional houses, which are a single story, have courtyards for easy entry, and elaborate wooden doorways for nests (see the picture), are perfect. Ten-story high-rises with closed windows and slick concrete surfaces? Not so great.
In addition to good places to build their nests, barn swallows need places to forage for flying insects. Agricultural fields, stockyards, garbage dumps, outdoor food markets, anywhere with livestock- these are great places. Shiny banking districts and shopping malls? Again, no so great. Our barn swallow searching strategy in China is thus to find those rapidly disappearing older areas and outdoor markets in each city we visit. Our strip of old courtyard houses in Jiuquan turned out to be a jackpot- a cluster of 20 or 30 nests.
The courtyard-style houses are great for barn swallows, but they’re not so great for biologists trying to catch barn swallows. This is because about half of them are private residences, belonging to people who go to sleep at night and lock their doors when they do so. If we want to catch the barn swallows (which we do at night because their eyesight is too good to be caught in nets during the day), we have to 1) get permission to catch the birds from the homeowner during the day, and 2) arrive after the birds go to sleep (once its dark) but before the people go to sleep (9 or 10pm). This usually gave us about an hour in the evening to catch as many birds as we could.
The courtyard-style houses are great for barn swallows, but they’re not so great for biologists trying to catch barn swallows. This is because about half of them are private residences, belonging to people who go to sleep at night and lock their doors when they do so. If we want to catch the barn swallows (which we do at night because their eyesight is too good to be caught in nets during the day), we have to 1) get permission to catch the birds from the homeowner during the day, and 2) arrive after the birds go to sleep (once its dark) but before the people go to sleep (9 or 10pm). This usually gave us about an hour in the evening to catch as many birds as we could.
The other half of the courtyard- style houses in Jiuquan were rent houses- long courtyards flanked by rows of rooms that were rented out to tenants. The main doors never locked in these places, meaning we could show up later at night, but having many people around often meant it was harder to get permission to catch the birds. Swallows are a sign of good luck in China, so people are sometimes suspicious of us catching their birds. Just one vocal objector at a rent house often tanked our chances of getting that nest.
Despite the difficulties, these courtyard houses were our best bet for catching birds in Jiuquan, and so we set off on the laborious task of knocking on every door, asking if there was an active nest inside, and asking permission to come back at night and catch the birds. We returned about an hour before dark to check that birds were roosting in the nests (sometimes they roost elsewhere, just to mess with us), and, once it was dark and the birds were sleeping, managed to scoop up 13 individuals. A pretty good haul for 1 hour of door-to-door netting!
Despite the difficulties, these courtyard houses were our best bet for catching birds in Jiuquan, and so we set off on the laborious task of knocking on every door, asking if there was an active nest inside, and asking permission to come back at night and catch the birds. We returned about an hour before dark to check that birds were roosting in the nests (sometimes they roost elsewhere, just to mess with us), and, once it was dark and the birds were sleeping, managed to scoop up 13 individuals. A pretty good haul for 1 hour of door-to-door netting!
The real excitement came when we set about measuring and photographing the swallows. The rustica and gutturalis subspecies can be difficult to tell apart, because both have pale bellies (although gutturalis is a bit darker). The main way to differentiate the two is body size- rustica are bigger than gutturalis, and in our 2014 Chinese data their size distributions do not overlap. There wasn’t a lot of time to compare the Jiuquan birds while we were banding- we try to go as quickly as possible at night so we can return the birds to their nests. Still, there seemed to be a lot of variation in body size and color in this set of birds. We finished banding after about 2 hours, and were back in our hotel room a bit before 1am. I immediately grabbed our data sheets from that evening and pulled up the 2014 data on my laptop. Sure enough, the measurements for the birds we caught in those courtyard houses matched both rustica and gutturalis- plus we caught some birds with intermediate measurements, the only ones we’d found in China! This was pretty good evidence that we had both parental subspecies plus hybrids at this site. We had found our hybrid zone.
-Liz
-Liz