On May 24, 2017, Jason Estep noticed something unusual. A male Eastern Bluebird was inspecting an old robin’s nest above a light fixture in an open-air facility in Franklin County, Ohio. He had seen a bluebird pair in the area on previous occasions, but did not think much of it until the male appeared to start working on this old robin’s nest. Jason then monitored the nest for eight consecutive days and found, each time, a female bluebird sitting low in the nest. She appeared to be incubating!
For the next 23 days, Jason had to be away and could not monitor the nest (sometimes life gets in the way of data collection). Unfortunately, we will never know if the nest fledged or failed, or how many eggs were in it, but we do know that it was empty on the final nest check on July 11. At 25 feet high and under a roof overhang, the nest was challenging to monitor. Jason’s notes indicate that the female would step away from the nest periodically and return to it throughout the day, ruling out the explanation that she was simply using it as an overnight roosting location. However, we cannot rule out the possibility that the pair “adopted” a nest full of blue eggs (the same color as their own eggs would be) after happening upon it within their territory. Although we have heard of birds incubating the eggs or feeding the young of other species, fostering is rare. Might this have been caused by a coincidence of timing, similarity of egg color, and/or loss of their own nest?
We think that Jason probably did see Eastern Bluebirds using a salvaged nest to try to raise their own young, and there are two main reasons. (1) It is the simplest explanation, and (2) Eastern Bluebirds have been documented to occasionally use cup nests. Captain S. G. Reid, writing in 1884 in The Birds of Bermuda, states about the Eastern Bluebird that “It occasionally drives the Red Bird (Cardinalis virginianus) from its nest, even after eggs have been laid, and uses it as a foundation for its own.” Reid also noted finding nests “on the branches of trees” in Bermuda. Mr. A. Sprunt, Jr. (1946) records an instance of an Eastern Bluebird nest in South Carolina which was “saddled on the horizontal limb of an oak tree.”
We asked Dr. Patricia Gowaty, a Distinguished Professor of Ecology and Evolutionary Biology at UCLA who has studied bluebirds in the field for 30 years, how unusual this sighting is. According to Gowaty, the frequency of open-nesting in bluebirds is incredibly hard to know. “Almost nobody studies Eastern Bluebirds in natural nests. Focusing on nest boxes means there are a lot of variations in life history that we don’t see.” Gowaty explained that we tend to canonize what we expect from bluebirds by focusing so closely on nest box results. In the book Longleaf, Far As The Eye Can See, one can find a photo of an Eastern Bluebird nest situated within a vertical burn scar on a tree trunk, on a short projection of bark. Gowaty points out that this type of nest may have been more common in the ancestral pine forests of the southeastern United States.
Don’t Box Them In
So was Jason’s observation actually rare, or has our close study of nest boxes colored our perceptions of “normal” bluebird behavior? Jason told us that the small woodlot behind the facility was clear cut last summer. It’s possible that the pair returned to their territory that otherwise had food and resources, but now lacks sufficient nesting cavities. Perhaps they exhibited the flexibility of their ancestors, before nest boxes were backyard staples.
At the time of writing, NestWatch holds 77,729 digital records of Eastern Bluebird nests. Of these, we could only find one that was similar to Jason’s. Barbara Starr had Eastern Bluebirds nesting under the deck of her Victor, New York home in June of this year. Barbara reports that it was possible to peek between the floorboards to check the nest. Although the below-deck nest was successful, the bluebirds did not attempt a second nest in this spot (there are two nest boxes on the property, which are also used).
Have you ever seen bluebirds doing something that they “weren’t supposed to be doing”? If so, tell us about it in the comments.