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This is actually somewhat mind blowing; https://en.wikipedia.org/wiki/List_of_birds_by_flight_heights
Whooper swan Cygnus cygnus Anatidae 8,200 metres (27,000 feet) This height was attained by a flock of whooper swans flying over Northern Ireland, and recorded by radar.
This allegedly happened in the 70s on a flight from Tenerife so I don't know what sort of plane he was on and what altitude it was flying at but apparently geese have been spotted by pilots all the way up to 25k+ feet.