Need some advice

Okay I know some of Jenn's bird friends frequent this blog, if you can check out a few posts back some wierd stuff happened with my cardinal's nest. She had three eggs, then one, I found two broken near the nest, she came back and then there where three again, I was looking at her nest and I think that one is bigger than the rest, I have recently had some brown headed cowbirds at the feeders and I wonder if she has been parasitized (sp?). Here is a pic of her current nest:

Here are some pics of brown headed cowbird eggs in other nests:

What should I do? Should I remove the bigger egg? From the Peterson Field Guides " Easter Birds' Nests" book: "Cowbird lays egg at dawn; removes egg of host day before or day after parsitic egg laid" Could a cowbird be the reason the eggs were removed from the nest? Advice please......