Yellow-rumped warbler flock at our backyard pond

The tiny birds shivered up from our backyard pond when I got a little closer to the window, one seemingly taking the place of the other. Then they fled into the neighbor’s yard. They were smaller than the house sparrows that gluttonously hog our bird feeder. They were after water, and hopped down the sandstone rocks along the pond edge to get at it. The rising and falling of the birds from the dark water’s edge almost reminded me of a goldfinch’s undulating flight, but these were not goldfinches. I saw the white in their outer tail feathers first and then the bright yellow at the base of their tails. These were yellow-rumped warblers, previously, called myrtle warblers, Dendroica coronata. I bet they get split off as their own species from the Western Audubon’s warbler sometime soon. These are our most common winter warblers. Their loose flocks often contain other species too, though this one of a half dozen or so birds had only these butter-butts, as my Katy Prairie Christmas Count pals call them.

Cin-Ty, a famous geologist and premier birder, artist, and conservationist, always says first to look at what the birds are doing, their jizz, not at their marks for identification. The soft, nervous movement of these light jewels identified them as warblers. I love our winter birds, and hope to get out more to see them. According to the range map in Sibley, these birds winter here, and down into Mexico, and don’t breed here. So, they’ll be gone by summer.

It is a constant surprise to me how little-studied our most common birds are. If you enter Dendroica coronata, the scientific name, into Web of Science, a great tool for getting nearly all the scientific papers on a species, you get 86 entries. That may seem like a lot, but hardly any of them are social behavior, what I love. The first paper was from 1927 and was on the curiosity that the birds were in New Jersey in January, along with some tree swallows, and feasting on bayberries, something the author, Leon Augustus Hausman, speculated kept them from migrating away from the freezing conditions. This paper was published in The American Naturalist, a journal that today would never publish such a note. Leon was at Rutgers University, and I wonder how he fared through the depression. I see from Amazon that he wrote a lot of books on bird identification. I could get a used copy of his Encyclopedia of American Birds from 1947 for $4.75. So all through the war, I imagine, Leon looked at birds.

According to Web of Science, the most recent paper on this common bird is from this year and is on the impact of logging on nesting success. There are a lot of papers on ecology, how the bird lives in different habitats, and on systematics, work that ultimately lumped the myrtle and audubon’s warbler into the yellow-rumped warbler. There’s also some on toxins, heavy metals in birds, that kind of stuff.

Is this sounding kind of boring? I thought so. What I really want to know is what’s known about its behavior. Do the males take care of their babies? Do the females sneak off and mate with other males, the way so many bird females do? What can I find? OK, nothing in these articles, nothing in this most sacred primary literature, that passed by peer review.

But this is kind of cool. In the Mexican highlands of Chiapas, in the far South, in the winter, these little jewel-butts defend territories with scale insects in them. Mostly it is Townsend’s warblers they defend against, and I’m sure also their own species. Each yellow-rumped warbler has a territory of 3 to 7 oak trees infected with homopterans that excrete sweet honeydew. The honeydew flows out along some pale thread-like things, and that’s where the birds capture and eat it. The scale insects make these threads, not to help the birds, but to get rid of the honeydew so they don’t drown in this sticky mess.

Birds eating honeydew is unusual, only previously reported in southwestern Australia. These authors think this is possible at their study site because it is so high and cool, at 2500 meters (over 8200 feet) elevation, that there are few ants, and none on this honeydew. Normally ants would win this sweet prize, and even care for and protect the insects that produce it.

Why do scale insects excrete sugar? Why don’t they use it themselves? Well, they also need protein in their diet, and it is lacking in the sap they feed on, so they just conduit through the sugars until they get enough protein. The birds can get their protein elsewhere, including eating these very insects. Actually, the scale insects don’t process all that sap to get what they need nutritionally on their own. They have enlisted the help of an even smaller partner, a bacterium. They have special cells that make cozy homes for bacteria, and these bacteria produce vitamins, amino acids and other nutrients for the scale insects. It is a fascinating partnership. Nancy Moran has done some excellent work on this topic. I first met her when she took Animal Behavior and I was the teaching assistant back in the 1970s at the University of Texas. This is what I love about biology. Everything is connected. And you get to go to cool places if you plan well.

There are more details on the butter-butt honeydew story in the original paper by Russell Greenberg, Claudia Macias Caballero, and Peter Bichier, and it was published in Oikos in 1993. I’d love to do field work at that site near San Cristóbal de las Casas, but I’d probably be looking for wasps, or collecting handfuls of soil for social amoebae, if I had a permit to do so!

Next time I get out, I’ll be looking for more butter-butts, and I’m sure I’ll find them, and maybe some orange-crowned warblers too. I’ll wonder if the birds I see are staying here, or headed south for some sugary honeydew.

By the way, these just might winter as far north as St. Louis. I guess I’ll get to find out. But there will be many other Houston birds I’ll miss, like the monk parakeets that screeched overhead as I biked to work today. No wonder we have parakeets – biking home at 6 pm it was 82 degrees according to the West University Little League Field sign.

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This photo is similar to the birds I saw. It is taken by Alan D. Wilson, http://www.naturespicsonline.com, in Port Aransas, Texas in January 2006.This photo is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license. I sure do wish more people would put their photos under this kind of copyright.

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About Joan E. Strassmann

Evolutionary biologist, studies social behavior in insects & microbes, interested in education, travel, birds, tropics, nature, food; biology professor at Washington University in St. Louis
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