Published Oct. 11, 2006, in the Wyoming Tribune Eagle, “The last to go, warblers put on a late season show.”
2014 Update: The spring migration Cheyenne Big Day Bird Count total warbler species count is now at 31. Click HERE for the Cheyenne Big Day Bird Count Warbler List.
By Barb Gorges
As the leaves turned yellow-green in late summer, you may have noticed them shaking without benefit of a breeze.
Did you see small, greenish yellow warblers picking through the foliage for insects?
Since 1993, Cheyenne-High Plains Audubon Society has documented 29 species of warblers on its Big Day counts held mid-May, at the peak of spring migration.
We haven’t given the same scrutiny to fall migration since warblers trickle through Cheyenne beginning late August and on into October. In the spring, the timing of their migration is more concentrated.
Most of the reports I’ve received this fall are for easily recognized warblers: Townsend’s with its mask, Wilson’s with its black cap and beady black eyes and yellow-rumpeds with their yellow rump in contrast to blue-gray back and wings.
The yellow-rumped warbler stands out in many ways. First, it’s just about the most common wood warbler species, which is probably why it has a well-known nickname.
Can’t you just hear the ornithologist tracking the quick-flitting unknown bird deep in the bushes and finally exclaiming, “It’s just another butter butt!”
The yellow-rumped comes in two forms that were previously two separate species. One, the myrtle warbler, has a white throat, and is considered the eastern form. The other, Audubon’s warbler, has a yellow throat. It breeds in the Rocky Mountains and winters in southwestern U.S. and Mexico.
We see both forms in Cheyenne so it is always worthwhile to scrutinize this common bird.
The yellow-rumped has odd habits for a warbler. Last month friends and I hiked up to Emerald Lake in Rocky Mountain National Park, elevation 10,000 feet. Although there was fresh snow on the peaks and aspen in full color lower down, we found yellow-rumpeds busy catching flying insects in a most unwarbler way.
They perched on a picturesque dead tree at the lake’s edge, then flew out over the water after their prey and circled back to their perches. This activity is called hawking and flycatchers are the group of birds that use it most often. According to my books, it’s a recognized feeding behavior for yellow-rumpeds too, but not for most warbler species.
What also sets the yellow-rumped apart is its wide range of gastronomic preferences. Other warblers have to head south when it is too cold to find live insects, but the yellow-rumped starts picking berries. That’s how the myrtle got its name—it likes to eat wax myrtle berries.
Apparently, yellow-rumpeds have a digestive system that can deal with the berries’ waxy coating. I don’t think around here we have any myrtle, or bayberry, its other favorite food.
But both Audubon’s and Myrtle forms stick around Cheyenne quite late, eating other kinds of berries and seeds. Robert and Jane Dorn list records as late as the first week in December.
Other warbler species’ latest dates are in mid-October.
Pete Dunne’s Essential Field Guide Companion calls yellow-rumpeds “The Swarm Warbler.” I have seen this phenomenon myself, in Lions Park.
Warblers migrate at night. By morning they are ready to come to earth and refuel.
As I walked the dog one spring morning, between the new community house and the lake, yellow-rumped warblers tumbled across the path at my feet like wind blown leaves.
While they may swarm during migration, yellow-rumpeds prefer to spread out for breeding in the coniferous forests of the mountains and the north. Little is known about this part of their lives compared to that of other more gregarious songbirds.
David Flaspohler, one of the authors of the extensive account in Birds of North America, made a lot of observations while completing his dissertation on metapopulation dynamics and reproductive ecology of northern forest songbirds in the upper Great Lakes.
Yellow-rumpeds are considered to be monogamous. The female is usually the sole nest builder though the male may sing and keep her company while she works.
Flashpohler was able to watch at least eight nests in northern Wisconsin in 1996 and documented that incubation is almost entirely done by the female.
“Male often sings in vicinity of nest during incubation,” he wrote.
When it’s time to feed the young, the male helps, in between bouts of singing. In other species, parent birds are very quiet near the nest because they don’t want to attract predators.
Many other sections of the account, however, state “No information.” It looks like aspects of butter butt life history could provide many more topics for theses and dissertations.
For instance, the last time yellow-rumpeds were tested for the effects of spruce budworm pesticides was 1987, in only one place and for only one kind of pesticide.
Or, why was the yellow-rumped the most abundant warbler found in collisions with towers in Florida, but rarely in Pennsylvania?
Yellow-rumped populations are said to be stable or increasing, but standard avian demographic data is lacking.
Another species, the greater sage-grouse, is suffering from rampant oil and gas development in Wyoming today and is finally attracting lots of research funds.
Had more research been done earlier, wildlife biologists may have been able to make better recommendations sooner to stave off the disastrous situation we have now. Then again, sound biological recommendations need to fall on willing Administration ears to have any effect.
Meanwhile, enjoy warbler watching. Consider posting your bird observations on eBird.com. Every little bit helps us figure out the puzzle that is life on Earth.