Brown-headed Cowbird
Molothrus ater
Notes:

A part of me wants to protect the phoebes from cowbird eggs each year. But cowbirds are native and protected by law. The Nestwatch link below provides options for deterring them.

Links:
Photos:
Till now man has been up against Nature; from now on he will be up against his own nature.
Dennis Gabor
Inventing the Future, 1964