Saturday, December 13, 2014

Orioles and elms

A good-sized American Elm, Ulmus americana, stands alone on a field edge. Its whiskbroom shape is apparent even from afar. I tend to notice big elms, and generally pay them more than a passing glance. The American Elm is still a very common tree in Ohio, but big ones are scarce.

In 1928, Dutch Elm Disease was first detected in North America, and it spread like wildfire. It is caused by three species of microfungi, two of which afflict trees on this continent. The fungi is spread by a trio of bark beetles. Once a tree is infected, the tree attempts to thwart the spread of the invader by plugging its xylem channels, which transport various nutrients and water throughout the plant. This ultimately fails, and the elm dies. Trees usually succumb before reaching the size of the specimen in this photo, which I found last Thursday at Killdeer Plains Wildlife Area. Most large elms that I see are isolated like this one is; it may be more difficult for the vector beetles to reach them.

When I slowed to admire the stately tree, I noticed it bore a nest. You can see it in the branches furthest to the right in the photo. This is not the first time that I have seen such a nest in a surviving mature elm. In fact, when I do find such a tree I scan it specifically for this type of nest.

A work of art indeed! The beautifully woven basket of a Baltimore Oriole, suspended from the elm's slender branches. The fact that it is still in the tree, and in good shape at this late date, is a testament to the oriole's basket-weaving skills.

There is no question that Baltimore Orioles have a propensity for siting their nests in the boughs of American Elms. Before Dutch Elm Disease appeared on the continent and laid waste to our elms, this tree was commonly used as a street tree. Avenues and boulevards would be lined with elms, their overarching branches connecting to form a shady arbor over the roadway. Numerous accounts describe the colorful orioles that graced the elms, delighting people with their musical flutelike whistling and brilliant splashes of orange. A paper entitled The Nesting Habits of the Baltimore Oriole appeared in a 1930 issue (vol. 42:4) of the Wilson Bulletin. In it, the author describes nine nests that occurred along a street in Sigourney, Iowa in the summer of 1927 (pre-DED). Eight of the nests were located in American Elms. Such pro-elm favoritism was probably the rule in midwestern towns and cities prior to DED.

Why the orioles' preference for elms? Probably for safety's sake. The spindly, drooping ultimate branches of elms prohibit access to many predators. A raccoon would certainly never make it to a nest such as shown in my photo. Nor would that most effective of avian nest predators, the black ratsnake, I would guess. Fortunately orioles have proven to be adaptive, and have jumped the arboreal ship to other trees for nesting purposes. A favorite is the Eastern Cottonwood, Populus deltoides.

I have watched orioles create their ornate hanging baskets on a number of occasions, and it is a remarkable thing to observe. Here, a female busily weaves her nest in a maple in northern Michigan. Somehow she takes an insensible tangle of grasses and plant fibers, and in a flurry of activity crafts a sturdy, well designed cup from the mess in just a few days. I couldn't do such a thing armed with my ten fingers.

The oriole's intricate cup spawns one of the world's most colorful blackbirds, named for the colors of the coat of arms of Sir George Calvert, who was the first lord of the Baltimore colony. The females, who one might argue do most of the heavy lifting in the oriole's world, are much more muted in plumage than is this showy male.

No dummies, most of our Baltimore Orioles are now in the tropics of Central America where they will ride out the winter. Come late April, their cheery whistles will once again ring from the Ohio treetops, every bit the harbinger-of-spring.


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1 comment:

Anonymous said...

I leave common milkweed stalks standing and the orioles strip the fibers of the stalks for nesting material. This year I also saw Yellow Warblers stripping the fibers too.
Brian