Cerulean Warblers in Decline

Last month, the National Audubon Society released a report in which they indicated that nearly half of North American bird populations are threatened by climate change. To carry out this study, scientists examined the current geographical ranges of North American species and projected if and how those ranges would change over the next century under current projected climatic changes. (This is, of course, a massive oversimplification of a scientific compilation that was more complex than I could probably ever articulate). As I perused this report, I was entirely unsurprised to find the Cerulean Warbler as one of the 314 species projected to experience significant range shifts.

Doing Science. (Photo: Claire Nemes)

Given my long absence from this blog, you may or may not be aware that I spent spring and summer of 2014 studying breeding Cerulean Warblers in southern Indiana. As part of a collaborative research project on the larger effects of various silvicultural practices, we monitored Cerulean Warblers on nine different study plots, studying abundance, nesting location and outcome, territory extent, and vegetative makeup of nest plots and territories.

Our research was part of an ongoing survey of Cerulean Warblers in Indiana, now in its 14th year. Cerulean Warblers have received a lot of research attention recently because they have one of the fastest declining populations of all North American songbirds. Since the 1960's, it has been estimated that Cerulean Warbler populations have decreased between 2 and 4% every year!

Male Cerulean Warbler.

Female Cerulean Warbler.

Cerulean Warblers are described as "neotropical migrants," meaning that every year, they fly from North America to South America, a migration averaging 4,000 miles, twice a year! Some of these birds actually fly over the Gulf of Mexico, which is believed to require 18-20 hours of continuous flight. And this is coming from an 8 to 10-gram songbird, which simply makes it all the more incredible.

Preferred habitat is the canopy-tops of old-growth forests. The nests we monitored were typically located at least 20 meters into the canopy and almost always in the top third of the chosen tree. It's rare to see a Cerulean Warbler at eye level, unless they are tending to newly fledged offspring, who remain close to the ground for the first few weeks of their lives out of the nest.

Fledgling Cerulean Warbler, perched on a log on the ground.

One of the principle questions in conservation biology is: what makes populations susceptible to declines? There are a number of factors that can make a given species more vulnerable to sudden and significant declines, including having a narrow geographic range, requiring a large area to establish a territory, and requiring a long time to reproduce viable offspring. Species may naturally possess one or more of these characteristics; however, it is also possible for a given species to be "forced" into one of these categories based on environmental changes. It is when these changes are forced upon a species that extinction often becomes a likely possibility.

Which brings us to the Cerulean Warblers. Without considering human interference, they already have a pretty specialized niche (canopy tops of old-growth forests) and annually embark on a dangerous and energy-expensive migration. So their populations are already kind of wobbly. Also, because they are migratory, they rely on not one, but two habitats every year (one old-growth forest in North America for breeding and another old-growth forest in South America for overwintering) not to mention all of the stopover habitat needed during migration.

A clear-cut patch with old-growth forest in the background.
(Photo: Claire Nemes)

Now, throw in the humans, and what do humans love? We love paper and we love wooden furniture and we love coffee. And what do all these things have in common? Of course, it's those old-growth trees that the Cerulean Warblers rely on. And whether through the creation of coffee plantations in Peru or mountaintop removal mining in Appalachia, we have done a great job of destroying tons of ideal Cerulean Warbler habitat. (I plan to unpack each of these issues in future blog posts, which I will write sometime in the next 16 years).

The Audubon report is extremely concerning for Cerulean Warblers. With all the current threats to their habitat, they have managed to hang in there, finding pockets of good habitat to breed and overwinter and survive. However, the climate report suggests that in the next 100 years, the Cerulean Warblers will have to relocate 98% of their range to survive. That's a lot of adaptation that needs to happen and not a lot of time (relatively speaking) in order to do it.

Is it all doom and gloom for the Cerulean Warblers? Of course not. But, as is almost always the case with these conservation issues, humans need to get their act together, and soon, or the Cerulean Warblers could end up being another species that exists only in our memories.