Has anyone heard of this – had a gal just call me and said that they found a bluebird nest and 1 blue egg inside one of their martin houses (they have two big ones & about 26 nests). They also have another active bluebird nest which has 4 babies in it, hatched the same day as mine, last Saturday, so I guess she knows what a BB nest/egg looks like. Other than robins (which as not cavity nesters), what bird would lay blue eggs? Is this common or really a very odd occurrence?