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We put up a blue bird nest box. We were delighted when a couple of tree swallows started moving in. They have laid their eggs... there were three or four little tree swallow eggs in the box when I checked last week and now there is a larger light blue egg in the box too. Hmmm... should I remove it? I am guessing it is a bluebird egg. Please advise. Thank you!
odd. Well, I guess if it was me, I'd remove it. Or, I might let it play out and see what happens.
I am a bluebirder and also have Tree Swallows and Purple Martins nesting on my property, so I have lots of experience monitoring cavity nesters, but I can honestly say I have never seen this before. I would guess that the bluebird may have lost her nest cavity due to competition from a House Sparrow or starling, or due to predation from a snake, raccoon, etc. It is also possible that a female without a nest has engaged in "egg dumping," a behavior which is known to happen in bluebirds, and got the wrong species' nest.
I would suggest leaving the egg alone. First of all, it may be infertile and might not even hatch. If it does hatch, it would be an interesting nature study to see if the swallows would raise the bluebird. Finally, it is technically against the law to interfere with the nests and eggs of native birds protected by the Migratory Bird Act.
If you decide to leave the egg, I would be very interested in the rest of the story.
A friend of mine had a bluebird egg in a tree swallow nest. The egg hatch and mama tree swallow fed the bluebird natchling. A bluebird can't be raised by tree swallows after fleding, though. The foraging tactics vary too much so the tree swallow parents wouldn't be able to teach the bluebird fledgling how to forage. Tree swallows fly through the air with their mouths open. Bluebirds catch crawling insects on the ground and eat berries in the winter. Therefore, a bluebird hatched in a tree swallow nest need to be fostered into another bluebird nest with nestlings about the same age.
Fostering of bluebirds does work. I know some people who have done it many times with success. It is a good idea to network with other bluebirders so if you have a bluebird that needs to be fostered, someone in your network can find a suitable nest and vice versa. Consider joining the Ohio Bluebird Society (if you haven't already) and meet other bluebirders in your area.