Here are some options, depending on the specific type of bird and the desired tone:
General:
* A loveliness of birds (emphasizes beauty)
* A congregation of birds (formal and neutral)
* A flock of birds (most common)
* A company of birds (slightly formal)
* A flight of birds (suggests movement)
More playful options:
* A radiance of birds
* A dazzle of birds
* A spectacle of birds
* A shimmer of birds
* A kaleidoscope of birds
You could also use a collective noun that describes the specific type of bird, like a parliament of owls or a murder of crows.
Ultimately, the best collective noun for a group of beautiful birds depends on your personal preference and the context.