I am using the wcanadian-insane wordlist under Linux, and it is also in there.
First I thought it would be related to ladybird, which is another common name for the ladybug. But after a few pages of internet meme crap, I discovered manbird is the same as birdman - an aviator.
When I think of birdlady, I can't help thinking of Kids In The Hall https://www.youtube.com/watch?v=0YwnAdT9I_o