End of the Yellow-rumped Warbler?

A few years ago I was at Strawbridge Lake during spring migration. I was newer to birding so my identification skills were pretty limited; I was still very reliant on my field guide. Near a small stream was a tiny black and yellow bird. I’ve never seen anything like it before. The most distinctive feature was a yellow patch of tail feathers. It was a Yellow-rumped Warbler, sometimes referred to as “butter butts” by birders. The Yellow-rumped Warbler was the first warbler that I learned to identify, so it holds a special place in my heart. It opened a whole new world of warblers for me. I’m always excited to add a new warbler to my life list.

There have been a lot of articles floating around recently about Yellow-rumped Warblers. They are widespread throughout North America and have two main subspecies: the Audubon’s of the West and the Myrtles of the East. Whether you see a Myrtle or Audubon’s, on your life list, the bird would be considered under the name “Yellow-rumped Warbler”. It hasn’t always been this way though.

“Myrtle” (Image by Kelly Colgan Azar via allaboutbirds.org)

Audubon’s and Myrtle Warblers were once considered 2 different species on people’s life lists. It wasn’t until 1973 that American Ornithologists’ Union decided to “lump” (as Kenn Kaufman, author and ornithologist refers to it as) many species together after new scientific research emerged. In 1969 ornithologist John Hubbard published a paper about how the two species hybridized where their breeding ranges crossed in western Canada. It seemed like people couldn’t tell the difference between the two in this range, and they ended up becoming the “Yellow-rumpled Warbler”. It was a disappointment to some people who “lost” entries on their life lists, but it became widely accepted.

“Audubon’s” (Image by Gregg Thompson via birdnote.org)

But things may be changing for the Yellow-rumped Warbler. Ornithology has grown immensely since the 1970s, especially in the field of genetics. The Yellow-rumped Warbler is one of the many bird species to have their genetics carefully studied by scientists. A paper recently released by David Toews and others found that although they may hybridize in that Canadian region, the Myrtles and Audubon’s are actually very genetically different. Its believed that the Myrtles and Audubon’s separated over the last million years due to ice sheets. The region that they hybridize in is only about 80 miles. Toew and the other researchers believe that there must be some genetic weakness in these warblers to keep them from spreading outside of this area.

But that’s not all. Yellow-rumped Warblers may be split into 4 different categories, not just 2. The “Goldman’s” Warbler of Guatemala is also genetically distinct. The “Black-fronted” Warbler, which lives in the mountains of northern Mexico, is still being studied. “Black-fronted” are much darker in plumage. Scientists have not been able to agree if they should be considered a full species or subspecies, so further research is needed.

(Image by Borja Mila)

Nothing is official yet though. The American Ornithlogists’ Union Checklist Committee has to approve the change first. Changes are usually released in July, so the earliest the split could occur would be July 2017. In the meantime, here are a few quick facts about Yellow-rumped Warblers.

• Yellow-rumped Warblers breed farther north than any other North American Warbler.
• Myrtles have a white throat while Audubon’s have a yellow throat.
• During breeding season, Yellow-rumped Warblers mainly eat insects. In the colder months they switch to mainly fruit. The reason why they can winter farther north than other warblers is because of their unique ability to digest the waxes of wax myrtles and bayberries.

(Image by Borja Mila)

If you want to learn more about the Yellow-rumped Warbler’s species status, you can check out the following articles.

“Genomic variation across the Yellow-rumped Warblers species complex” by David Toew, published The Auk, October 2016

“Goodbye, Yellow-rump” by Hugh Powell, Cornell Lab of Ornithology

“The Yellow-rumped Warbler Will Probably be Split Into Different Species Again” by Kenn Kaufman, editor at Audubon