Expert: Roger Lederer - 6/10/2011

Question
I live in NW Iowa and each year I have purple finches build a nest(s) in my hanging flower baskets. I have noticed there are sometimes two different types of eggs in the nest; the green-blue with brown speckles-purple finch, and a slightly smaller pale blue only. Most often three of each are present. What bird may be invading and using the finch as a host to their young? Should I do something about the intruder's eggs?

Answer
There is only one bird that lays its eggs in other birds' nests as a nest parasite and that is the Brown-headed Cowbird. But its eggs are not blue; they are white and bigger than the Purple Finches'. So the other two possibilities are that this finch laid a normal clutch of six eggs but three happened to vary a bit in size and coloration - entirely possible. Or another bird with blue eggs laid her eggs in the nest - very unlikely. So do nothing about the intruder's eggs as they are not likely to be harmful.