P.S. edit: Looks like you might not see this since I'm on kbin. Maybe someone on my side of the Fediverse finds this tidbit of wildlife knowledge!
I've had some experience with seabird and waterfowl nesting research so this caught my attention. I haven't come across anything like this personally, but I've only ever worked with larger birds with higher nest building costs and greater territorial behavior across seasons.
I managed to find a specific explanation for American Robins (excerpt below). I can't guarantee the validity of this author's 'supernormal stimuli' explanation, but it's a compelling one:
A. This is a question we hadn't been asked before, so we wrote to Len Eiserer, the author of The American Robin: A Backyard Institution. Len answered:
"Building multiple nests simultaneously happens every now and again with 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. Because some human structures provide repetitive sites with strong 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 provided by Mother Nature (tree limbs). Animals have a hard time resisting supernormal stimuli. There are many 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 won't lay two complete sets of eggs and try to incubate both of them at the same time."