I have had it happen 2 years ago,But not with bluebirds.It was Tree Swallows and House Wrens,They switched nest and began incubating each others eggs,The House wren was on 3 TRES eggs ,2 hatched.The Tres was on House wren eggs and a couple of her own,She did hatch 2 house wrens and 1 of her own.It is a long story.But in the end,1 TRES with the Wren died,the other was taken to a rehaber.With the TRES,1 house wren died,the other had a handicapped wing,it also went to the rehaber.The one remaining TRES with the TRES went on to fledge fine.I did get some video of the House Wren feeding the 2 TRES(see the link below),The smaller one did die a few days later.You can see they are panting alot,it was very hot that day.

Here is a link that might be useful: House Wren feeding TRES nestlings.

Keep very good documentation on this and report it to Cornell. Keep us informed too, very interesting!

That was a fascinating video. How did they end up with the wrong chicks? Did they switch boxes halfway through laying?

The boxes were paired about 10 feet apart,they each began building nest and the both began laying eggs ,Then All of the sudden,The TRES decided they wanted the other box.They moved in and began laying more eggs with the Wren eggs,even carefully adding nesting material under the the Wren eggs.I noticed the Wren was going in the other box where the TRES used to be.I thought for sure they would throw out the TRES eggs and rebuild another nest,But they didn't,she began incubating the TRES eggs.The start of incubation was a week apart,so I couldn't just switch everything.I was surprised some of the eggs hatched.I did contact a rehaber as soon as I noticed what was going on.Since TRES feed in the air,there was no way each Species would have survived once fledging.Here is the surviving TRES from the video,He/She was pulled from the nest just before it was due to fledge.It was released a few weeks later by the rehaber.

That is the most amazing story. Thank you so much for taking the time to tell what happened. It is a curiosity why they would switch boxes, and why the wren didn't toss or puncture the TRES eggs. What an adorable baby TRES.

When I was a graduate student working in New Mexico, I documented a Say's Phoebe and Barn Swallow that shared a nest. There were three Barn Swallow eggs and two phoebe eggs. Both species successfully fledged young. I documented this in the journal Auk. In that article is a pic of an adult phoebe that just fed a swallow chick a grasshopper. Interspecific cooperative nesting is rare among birds.

Below is a link to the paper.

BN

Here is a link that might be useful: Kozma and Matthews 1995

Wow, who knows, if when my pair of titmice was fighting with my pair of bluebirds, it certainly could have ended up with two females doing something weird like mentioned above (ie. switching nests, using the same nest, etc). The nest started as a titmouse nest and the Mama Blue added pine needles on top.

Seems that there's not much predictable with our birds, eh?

Bluebird blessings, Linda...
NW GA

BN, Thanks so much for posting that article. That was fascinating reading, and with pictures too. I'm amazed the parent birds tolerated each other as well as they did.

Update: I checked my bluebird nest today and discovered the 4 bluebird eggs have hatched today but the 2 chickadee eggs still remain. Today was day 15 of incubation so these eggs may not likely hatch.