House Finch nest with Cowbird egg

Answer 1

It is very possible that the House Finch parents will be able to handle the situation on their own.  Here is an account of a similar situation where the parents detected the problem, and ejected the Cowbird before their own chicks hatched.  

http://www.dfwurbanwildlife.com/detailPage.php?OBSERVATION_ID=BD0000700007002&CLASSIFICATION=Birds&COMMON_NAME=Finch,^House&SCIENTIFIC_NAME=Carpodacus^mexicanus&SORT_CRITERIA=observationId

Regardless of the outcome, it is probably best to leave well enough alone.