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 December 2016 issue:
I observed two American Robins incubating eggs in one nest. I couldn’t tell if they were both males or both females. Is this common? — Sam Taylor, Colorado Springs, Colorado
First, it’s likely that both birds were females. In most songbirds, only the female develops a highly vascularized, featherless area on the breast called a brood patch. During incubation, the patch facilitates heat transfer from the body to the eggs or young. Male robins do not develop a brood patch and do not incubate.
Nest sharing is not a common behavior for American Robins, but it has been reported previously. The behavior has also been observed in the closely related Common or Eurasian Blackbird, which is in the same genus, Turdus.
It is generally rare for two females of typically territorial species (especially those that have young requiring a lot of parental care) to share a nest. There are several usual explanations. A female may lose her nest to a predator or other incident during egg laying and then seek a new location to lay the rest of her clutch. Males of monogamous species often copulate with more than one female. If one of those females is unmated herself, she may lay her eggs in another nest. And sometimes suitable nest sites are lacking, so females lay in established nests, but this situation is more common with cavity-nesting species.
The success of such nests depends on the harmony, efficiency, and dedication of the two females (and male mates) and similarity in the age of the eggs and subsequent chicks.
Update: Sam later informed me that five eggs hatched and were being fed by at least two adults. — Julie Craves
About Julie Craves
Julie is supervisor of avian research at the Rouge River Bird Observatory at the University of Michigan Dearborn and a research associate at the university’s Environmental Interpretive Center. She writes about her research on the blog Net Results, and she maintains the website Coffee & Conservation, a thorough resource on where coffee comes from and its impact on wild birds.
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