Can only fly with a light load?

Ki Ryn said:

I've heard that a winged creature can't fly if it is carrying a medium or heavy load but I can't find the rules pertaining to this. Could someone give me a pointer to the relevant text?
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Movement Modes: Creatures may have modes of movement other than walking and running. These are natural, not magical, unless specifically noted in a monster description.

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Fly: A creature with a fly speed can move through the air at the indicated speed if carrying no more than a light load. (Note that medium armor does not necessarily constitute a medium load.) All fly speeds include a parenthetical note indicating maneuverability, as follows: ...

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