"And it drove forth (va-yagoz) quail from the sea" (Numbers 11:31) — meaning, it made them fly, as it says, "for it is swiftly cut off and we fly away" (Psalms 90:10). "And about two cubits above the face of the earth" — for they would fly no more than two cubits high, so that the people would not be troubled to take them.
Why the Quail Flew Only Two Cubits Above the Ground
Midrash Aggadah, Numbers 11:31
ויגז שלוים מן הים. הפריח, כמה דהוא אמר כי גז חיש ונעופה (תהלים צ י): וכאמתים על פני הארץ. שלא היו פורחים כי אם אמתים למעלה, כדי שלא יצטערו ללקחם: