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.
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.
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.