I saw a couple of great work-arounds using the round punch by Anthony Warwood on facebook; this is his blog, I'm sure he'd send you instructions if you get in touch! But I don't have a round punch (yet), so I thought I'd try with the extra large oval and bird builder punch instead. Here's what I came up with:
First, cut, fold and mark your oval:
Here's how you line up the folded oval in the bird punch. Line the mark on the fold up with the beak of the bird, and the other mark with its chest and punch. Repeat on the other side:
And the blue is what you end up with, shown against the original Round Tab punched piece:
That'll do for me, hope this helps someone else too!