Judah wrote:

What's the highest altitude that you ever saw a bird swoop past your plane

I was once returning to my home airport at 4,000 feet msl when ATC called
"traffic ahead, primary target only, slow moving, probably migratory
birds."

We spotted the birds, a flock of Canada geese at what appeared to be 5,500
feet or so. This was in central NY state, USA.

--
Peter