Blue Bird Behavior

Can someone help explain this behavior.
Last week a friend and I were checking out some blue bird boxes that I have been monitoring for a number of years. We saw the this behavior. The tree swallow would sit on the box and the Male blue bird would swoop out of no were to attack it. We watch this pattern for over and over for half an hour. I know the female BB was in the box sitting on the eggs. What puzzled us the most was when the female Bb was not on the nest I looked inside and the eggs where white Never saw white BB eggs before. Do you suppose the BB can rear tree swallows like some birds do with cow birds?
Thanks

I've seen some white or nearly white Bluebird eggs. Here's an article about white Bluebird eggs:

http://www.sialis.org/whiteeggs.htm

The scene: A winter evening in 1958. Down in the bowels of the old house that housed the bird skin and egg collections of the Department of Zoology at Penn State University.

A casual conversation arose among a few undergrad (incuding me) and graduate students concerning the observation of pallid (almost white) eggs in bluebird boxes. Someone came up with the observation that a tree swallow had been suspected of laying its eggs in a bluebird box. Professor Merrill Wood, the guru of Pennsylvania ornithology in those days, opined that the shapes of the eggs of the two species were distinctly different and proceeded to illustrate same with specimens from the egg collection. Sure enough, they were different; the eggs of one species had a clearly more "pointy" end than did the eggs of the other species. Here's where it gets hazy; I can't remember now which was which,or if it had ever been established as fact that tree swallows could be brood parasites after the fashion of cowbirds and European cuckoos. But the eggs of the two species were, indeed, differently shaped.

Got some pictures?

Dave Graham, Estelline, SD

Alans link is a good one, explaining that BBs do indeed lay white eggs.