Why blackbirds sit on empty nests

In the “Since You Asked” section of each issue of BirdWatching, editor Julie Craves answers readers’ questions about birds and their behavior. Here’s a question from our May/June 2020 issue.

Q: I have a robin’s nest on my property and have noticed the female sitting in it and aggressively guarding it, but it is empty. Why would a robin sit down 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 the European robin, a relative of the American robin. It’s not unusual for this to happen about a day before the females start laying eggs, but it’s 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 difficulty of locating enough of these females and then determining why they were unable to spawn.

In Europe, it has been noted that this situation has been reported more frequently in recent decades in some songbirds, such as the large Chickadee, a relative of our chickadees. A study of the species found that more females sat on eggless nests near a copper smelter than farther away, but examination of some of these birds revealed that physical deficiencies did not fully explain why they failed to lay eggs. Every study I read indicates that we need more information to understand this rare but widespread behavior.

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