I have one Bluebird box that always hosts the first nesting of the season. The pair fledged there five little ones two weeks ago. Last week saw them building a second nest, or at least I thought I did. Upon inspection yesterday, noticed that eggs were WHITE, not blue as they had been for first nest. This means a different pair is now using the box. If original pair was using box, she would be laying blue eggs, not white. Yes I have observed coming and goings, and pair are definitely Bluebirds. Approx 5% of female Bluebirds lay white eggs