Hmm when you say flying in formation do you mean in like a cloud or a specific shape? If so could be starlings as they have a very varied repertoire of songs and can mimic other birds (which might explain why Merlin can't identify them)

When you say 'jagged wings' do you mean you can see the individual feather tips (like fingers)? Could be fish crows? The juvenile sound could be a little seagull-y?

Grackles are usually fully black apart from having lighter eyes. Some species flock together, and like starlings they also minic other birds.

Edit: one thing to note is Merlin doesn't always work well if you've turned GPS off on the phone. That might also interfere with identification

Where are you based approximately? Europe/Asia/US/etc? Is the bird fully black? Same size as a crow? Any distinguishing information might help!

If anyone is wondering, that's a red-backed fairywren. Here's some other great fairywrens (literally, it's in their names):

Lovely fairywren

Superb fairywren

Splendid fairywren

Should...should we tell Squizzy?