Astral streakers are friendly avian creatures the size of sparrows common in the Astral Plane (though it's thought that they originally came from the Beastlands - certainly, nothing can breed in the Astral, except in the vicinity of a few dead gods of time).
They are swifter than most other Astral creatures, and are often used as messengers.
Sources:
In the Cage: A Guide to Sigil
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