mystery squash

A big volunteer pumpkin/squash plant appeared early this season in the middle of my onion bed. I was thinking it was likely a pumpkin, and since I had brought all my pumpkin seedlings to my parents garden, I thought I'd let it grow. Its always fun to have a mystery plant.

The vine has now overtaken the onion bed (I'm cutting off a few leaves here and there to give the onions sunlight), and is running into the sweet potatoes on one side and potatoes on the other. It has produced many male flowers and finally now has its first female.

I am thinking it doesn't look like  a pumpkin. Maybe a buttercup squash? I can't remember what I put in my compost bin last year, but I have grown some nice varieties of buttercup in the past.