Hard to tell without feeling it, but it looks like a bird bone to me. Is it really light? Like lighter than it should be for it's size? Birds have honeycomb-like bones to keep them light but strong. The two indentations at the top show that it articulates with two other bones, one on each side... so a rib would be my guess. And the hole at the top does look like it was drilled.
But I'm not a bird guy, so I might be full of S#*T
Chris