I took a picture of a nest today because I wanted to find out what type of eggs were in it. Just before I reached the nest, a female red winged blackbird was frightened away. So that much I now knew. But upon inspection, one egg was different. I suspected a cow bird had placed the one odd egg with Mrs Blackbird's three. So when I returned to the house I Googled things and discovered I was right.
Here's my photo with the smaller lower right speckled egg being the cowbird's:
Here is a link that might be useful: cow birds