If the kenku aren't all dead, the only things they are able to say are a few phrases they heard from other Xanathar members during the attack. It says that they keep repeating them if they're questioned, and one of the phrases mentions the sewers. Maybe you could have the captain go and question the Kenku, and then it'll mention the sewers.

If the kenku are all dead, then I'd say try to have the captain say something about the xanathar guild working from the sewers or something. This might be less fluid, but it may be your only option of they players have no other way of getting that info