There are already examples of this (not flying turtles though! haha), there are ‘Ligers’, ‘Tigons’, ‘Zonkeys’ which are all hybrids caused by 2 different animals mating! See this link:
The 2 animals in question though are always closely related in terms of species, I dont think you could cross a turtle with a bird as the DNA they possess is too different! Plus, how would they even mate?? Weird …
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