>Spring loaded flap with a solenoid latch comes to mind.

I would be tempted to make the doors of cloth on a U shaped wire frame. If
you have two doors, each half the width of the opening then you have less
risk of doing permanent damage to the bird if it is quick enough in reaction
to mix it with the doors. The cloth should be a dark colour, and I doubt the
bird will try and get through it.

This will also mean the doors will have less mass, and so will close faster.
It may even be possible to punch some holes in the cloth to allow air to
escape through the material so it is less of a "sail" action for the spring
to drive.

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