Do you see that panel opening that slides up, in the corner, to the right, just above the steps?
That slides up.
This little fellow is going the wrong way, but he'll catch on in no time.
Now, note the panel he is about to pull up on the crate below. That is the side of the crate the birds walk into.
Outside the loft, on the other side of the opening, the crate awaits the birds, with panel pulled up for the birds to walk through.
And they're in! Ready to go.