Birds of Prey has a reason to stick around after the credits. Let's get that out of the way straight away. If you want to see absolutely everything the newest DC Comics adaptation has to offer, you'll want to wait right until the final moments.
It's tradition to include a post-credits sequence in superhero movies, now, thanks to Marvel's many efforts in this area. Warner's DC movies have been doing post or mid-credits scenes since Suicide Squad, and followed with similar scenes in Justice League, Aquaman and Shazam. Birds of Prey, then, is the latest to give us something after the movie ends.
So unless you're dying to get out of the cinema before you get a parking ticket, it's worth sticking around and seeing what the deal is. Let's be clear, though: it's not exactly a Nick Fury-style big moment. It's just one more joke at the end of the credits.
Birds of Prey has a brief post-credits Easter Egg...thing
It's really not much, but Birds of Prey has a cheeky Deadpool and Ferris Bueller-style closing gag voiceover at the end of the credits. It's essentially just Harley Quinn pointing out that you, the person waiting in the audience, have wasted your time waiting for a post-credits sequence.
Harley then suggests she'll let you in on a secret, saying "Did you know that Batman-", before it cuts off. It's not a reference to anything, it's not even really a tease for the next Batman movie, it's just a little joke to be enjoyed by you, the moviegoing public.
And that's it. It's basically two lines you wait several minutes to watch.