Robin Dual Nesting
Location: Outside U.S.
Date: Summer 2012
We have a robin's nest on the pergula against the wall of the house underneath the buildout. The nest sits on the top of the flat part of the frame but what made us very curious is that they build the second nest two feet away from the first one. The first nest is build high and strong but the second one is flater and doesn't look completed. Robins use both nests and lay in them periodicaly. I haven't heard of this behaviour.
Please help us with this mystery.
More detailed observation might help answer this question. How many robins use the two nests? You might be able to find some characteristic that will let you identify individual birds. Are there ever robins on both nests at the same time? When you say "lay in them" do you mean lay eggs or that you see them resting there?
If eggs are being laid in both nests my guess is that it is two different females. Nests of two pairs that close together would seem to be quite unusual, but robins are famous for building in a variety of circumstances. An old, but still quite useful, reference is the Life Histories of North American Thrushes, Kinglets and Their Allies, by Arthur Cleveland Bent, originally published by the Smithsonian in 1949. It includes details of a variety of unusual robin nests, including this: "Several cases have been reported in the literature where robins have built a series of nests, placed on a row along a beam." It does not say if those nests were built by a single pair or more than one pair.
I wonder if the robins have any identifying characteristics. By this, I mean I wonder if the same birds always visit the same "nest".
Nest construction is typically a constant per breed, whether in the tree or on the ground, or inside a nesting box. Perhaps one is a nest for brood, and the other is a simpler structure for resting, not "nest"ing.
As further observation, see if you can figure out which birds use which "nest", and then see, as I predict, if eggs only occur in the normal-appearing nest structure.
Thanks for using NEWTON!
Unaware of this behavior myself, I did some research. This is how the Journey North American Robin site answered this question.
A. This is a question we hadn't seen before, so we wrote to Len Eiserer, the author of THE AMERICAN ROBIN: A BACKYARD INSTITUTION. He answers, "Building multiple nests simultaneously happens every now and again in robins. One started 26 different nests on roof rafters of a garage under construction; another built 8 on successive steps of a fire escape.
Support from underneath is the primary site selection factor for the female robin -- it's more important than concealment, e.g. Since some human structures provide repetitive sites with terrific support, the female can get "seduced" into building multiple nests.
This is an example of "supernormal stimuli" -- artificial stimuli that are even more effective than those provide by Mother Nature (tree limbs in this case). Animals have a hard time resisting supernormal stimuli. There are many other examples. Your robin will probably settle on one site and just lay eggs in that nest, or else just incubate eggs in that nest after laying, say, one egg in one nest and two in the other. She almost definitely will NOT lay two complete sets of eggs and try to incubate both of them.
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Update: November 2011