Just a small update here (Version 1.8, available by clicking the Source Files link top right).
I wanted the birds to spawn properly in their correct places, so I added a little routine for this. The birds Update method is getting a little messy. It might be time to start breaking it into smaller subroutines.
I also wanted to see what happens if you get loads of enemies, so I added a spawner routine which creates birds every few seconds. My old laptop can handle lots of birds, it seems.
Also, it turns out I'm quite pleased with the current flight pattern of the enemies. They seem to spread themselves around the screen quite nicely, so I'm not going to bother with the different flight algorithm for now.