easy question this ,what is a birdcage corsair?

Birdcage refers to the canopy. The birdcage models have a different canopy that has metal ribbing on it, unlike the regular single piece bubble style canopy.

The word "birdcage" refers to the canopy framing on the early corsairs. It resembled a cage and thus earned the nickname "birdcage".

Here's a birdcage:

Here's the later bubble version:

In the beginning . . . . there was the F4U-1 with its framed, flat-topped canopy, and designer Rex Beisel and the Bureau of Aeronautics pronounced it good . . . .


Seriously, the F4U-1 was the first operational Corsair, and it and the derivative F4U-2 nightfighter had a canopy framing much like a P-40, with a sliding, heavily-framed canopy and small rear-quarter vision panels aft of the cockpit. That's a birdcage Corsair.