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Clues to Species Decline Buried in Pile of Bird Excrement
17 April 2012 7:01 pm
In 2009, while searching for ways to help endangered birds, research technician Chris Grooms heard that a chimney on his university campus used to host a migratory species known as the chimney swift. When he investigated, he found a pile of bird excrement 2 meters deep. The poop lay at the bottom of a five-story-high chimney and had been deposited over 48 years by the birds, which had roosted there until the top was capped in 1992. Now, Grooms and his colleagues have dug into that pile of guano, revealing new clues about why the chimney swift and other species like it have begun to disappear.
Grooms volunteers for an environmental group in Ontario, Canada, that's trying to conserve local wildlife. He also works in a lab at Queen's University in Kingston that studies sediments in lakes. As dirt and dead things sink to the bottom of these bodies of water, they preserve a record of environmental conditions. Grooms wondered if the same thing had happened with his pile of bird poop. He brought the idea to the head of the lab, ecologist John Smol. Smol was intrigued: "It could be 2 meters of bird poop, or it could be a pretty important story."
The researchers entered the chimney through a little door near the bottom that was only big enough to crawl through. Behind the door was the wall of poop. It took 2 days to dig out enough of the crumbly, dark-gray, dry excrement so that the researchers could stand up. After 20 years, the poop had lost its smell, but the researchers wore respirators just in case some pathogen was hanging around.
With the help of radioisotopes produced by nuclear bomb tests, which linger in sediments and can be used for dating, the researchers worked out that the deposit built up between 1944 and 1992. A team at the University of Ottawa measured levels of DDE, a chemical that comes from the pesticide DDT, to see if DDT affected what insects the birds were eating. Another set of samples went off to Joseph Nocera, a conservation biologist at the Ontario Ministry of Natural Resources in Peterborough, who sorted out insect remains. Most were beetles; the next most common remains were from the Hemiptera, an order known as "true bugs" that includes stink bugs and cicadas.
As DDE increased through the lower layers of the deposit, beetles showed up less often in the birds' diets and true bugs became more common, the researchers report online today in the Proceedings of the Royal Society B. This result agrees with other reports that DDT is hard on beetles, while true bugs can evolve resistance quickly. The change in diet may also help explain why chimney swifts have declined so precipitously, Nocera says.
Canadian surveys have found that the number of chimney swifts dropped 95% between 1968 and 2005. Some researchers have suggested that part of the reason is that chimneys like this one, swifts' preferred habitat, have been capped or redesigned, making it harder for birds to get in. But the new work suggests that the decline may be diet related. Beetles generally contain more calories than do true bugs. Swifts need a ton of energy—they spend a lot of time on the wing, looking for food. A change in their diet, like substituting less-nutritious true bugs, could have a big impact. DDT was banned in the 1970s, but the beetles never seem to have gotten back their original place in the food web, Nocera says.
Nocera thinks DDT and other pesticides may have effects far beyond their well-known impacts on the eggshells of large birds, such as taking away the foods that chimney swifts, barn swallows, flycatchers, and other insect-eating birds relied on. He says he doesn't know of any other studies that have looked at a pile of bird poop on the scale of decades—other studies have looked at older guano. There are probably many more archives like this in the chimney swift's range, he says, and this study shows that it's possible to get useful information out of them.
Çağan Şekercioğlu, a conservation ecologist at the University of Utah in Salt Lake City, agrees with the team's conclusions. Pesticides get more concentrated as they move up the food chain, which means they can be worse for insect-eating birds than for birds that eat fruit or nectar, he says. Still, Şekercioğlu says he would have liked to see more discussion of how the loss of nesting and roosting sites—like the chimney in this study—affected chimney swift populations. But "it's a very good historical data set," he says. "We don't have that opportunity for almost any other bird species. It's a brilliant idea and very well thought out, and the fact that they found this potential link to DDT is fascinating."