Help! Imposter laying eggs in Redbird nest?

Hi all, I really know nothing about birds, but I've had two redbirds build nests this year very low to the ground, so I've been keeping an eye on them. The first one did quite well and the hatchlings have all left, but there's a new nest (constructed last week) in one of my Passiflora cages and there's an egg that doesn't look like the other ones that are in there; picture is below, the white one is the egg in question lol. If it is a foreign egg, what should be done? This may all be really dumb, but like I said, not a lot of bird knowledge here.

Very cool pic indigo--I don't know much about birds either except we do have redbirds and they put their nests entirely too low to the ground I would think. They go for holly bushes up against chain link fences around here. I'd leave it be--maybe there is variation among eggshell patterns. Good research question!
Debbie

Brown Headed Cowbirds lay their eggs in other nests, but their eggs are not white, but brown spotted similar to cardinal eggs. The baby birds will compete with the other fledglings for food, but many times it does not hurt them. Here is an interesting website about Nest Parasite birds. http://www.learner.org/jnorth/tm/spring/Cowbird.html

Oh, I just found something. Bronzed Cowbirds http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Bronzed_Cowbird_dtl.html are in south Texas and have light blue green eggs with no markings. Does this egg have a blue green tint?

I'd remove the egg:

The intruder, likely a bigger variety of bird, will gobble up the groceries & the baby cardinals will starve.

I just read that baby cuckoos have extra broad "shoulders", which they use to shove the cardinals' (or other birds') babies out of the nest, so that the cuckoos can have all the food.

Thanks for the info everyone. I didn't have a lot of time to observe the egg, but it looks to be just an off white color, but I did look up cardinal eggs and they are supposed to be the brown spotted ones, not white.

I think I'll remove it as soon as I see the mother off to the pond to drink tomorrow. I don't really like the idea of them wasting time rearing an ugly cowbird baby. Now that I have a suspect in mind, I do recall a blackish bird of some kind sitting on a trellis observing the area a few times during the nest building, and that was probably the culprit.

I thought I might add this picture... this was the first nest which was in a rosebush about waist high no less.

aww, how cute! why didn't that bird get busy and build his/her own nest for their own baby?

What nice bird pictures!!! They let you get so close!

I had a nest on my front porch once, in a four foot planter. There were five babies about a week old, when a neighbors cat attacked the mother bird one night, the babies were too young to save.

I would take the egg out, to make room for the rest of her eggs..and hope that the mother will not abandon her nest

Just to let everyone know, the foreign egg is no more. It was indeed a bluish color under closer inspection, so it more than likely was a cowbird. The mother didn't seem to notice anything, and is still sitting on the remaining two eggs.