What you need for this tutorial is:
In Adobe After Effects you need to create a new composition with your clean plate at the very bottom and the rotoscoped actor on top. Then import the rendered crows. Ensure that you right click imported crows footage in the project window and go to 'Interpret Footage'. Double check that the frame rate matches the frame rate of your footage.
Now place the rendered crows on top of your 2 layers. Since we timed them to match our scene when we set up everything up in 3dsMax, the crows should be spawned with the correct timing.
The first issue you may encounter is the crows not fitting into the shot very well in terms of their colour. This is not usually that big a deal although it can save you some time to make sure the colour matches when exporting from 3dsMax. Since my crows came out rather blue, I applied a simple Hue/Saturation effect to the crows layer and drained a lot of the saturation and lightness to make the crows look nice and black.
Since in 3dsMax I only set up a deflector for the ground plane of the pier and not for the sides, I noticed a few crows penetrate the sides of my geometry. To fix this, I applied a number of animated subtractive masks to cover up the crows for those moments.There might be other cleanups you have to do depending on your footage and the effect you are going for. Most of them you should be able to solve with simple techniques in After Effects.
I cut off the rotoscoped layer of Celina spinning at the moment when all crows have spawned. This causes her to disappear when the crows emerge - but she disappears rather suddenly.