Friday, April 27, 2012

Chimney Swift decline and possible answers

The Chimney Swift is declining throughout its range in North America, as are many of our aerial insectivores, for reasons that are not entirely clear. Certainly, habitat loss in terms of breeding and migration enters into the equation in many cases, but this is not the complete answer for why the drop has been so precipitous. This article discusses a fascinating find that may help answer some of the reason why this particular species is in trouble:

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.

This was actually a treasure trove of information! All of that waste is a pile of insect remains that could be shifted through to see what the birds had eaten over time, possibly providing clues as to what may have changed to accelerate their significant decline.

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.

That is a lot to consider and takes a couple liberties, but it makes a great deal of sense. While more research is definitely needed in this area perhaps there will be more discoveries of such, well, chimney piles that can yield similar results. At some point, a large effort for bird conservation may end up involving the restoration of the natural chain of insect life on the continent, a monumental task. We may have stopped putting DDT and some similar chemicals into the environment (certainly not all like we should) but remediation may be necessary to undo all of the damage it caused.

Scott Kruitbosch
Conservation Technician

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