In the column “Since You Asked” in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here is a question from our May/June 2020 issue.
Q: I have a robin’s nest on my property and noticed the female sits in it and guards it aggressively, but it is empty. Why would a robin sit in and protect a totally empty nest? — Trish Cain, via email
A: For decades, a wide variety of birds have been reported sitting on empty nests — owls, hawks, corvids, woodpeckers, and a variety of songbirds, including European Blackbird, a relative of the American Robin. It’s not unusual for this to happen a day or so before females begin to lay eggs, but it is more unusual for a female to occupy and defend an empty nest for days or weeks at a time. The phenomenon is difficult to explain due to the challenge of locating enough of these females and then determining why they were unable to lay eggs.
In Europe, it has been noted that this situation has been reported more frequently in recent decades in some songbirds, such as the well-studied Great Tit, a relative of our chickadees. One study of the species found that more females were sitting on eggless nests near a copper smelter than farther away, but examination of some of these birds found that physical impairments did not fully explain why they did not lay eggs. Every study I read indicated we need more information in order to understand this rare but widespread behavior.