A murmuration of starlings is truly wonderful to behold. The above video (by Keep Turning Left) shows an incredibly beautiful murmuration, set to the always magnificent Pachelbel’s Canon in D.
But… how many birds would there need to be for gravity to take over and force them into a gargantuan ball of birds? The answer is lots (xkcd). Lots and lots. Not just a sky full of birds, but a whole solar system. Then they’d all collapse into a star.