The rule for stix appears to be that animals are drawn as naturalistically as possible.
I have always liked the size and shape and glossiness of starlings, and the rapid bouncing way they move along the ground. When I was a kid I got a book about them out of the library, and that is when I learned that their beaks change color from black in the winter to yellow in the spring. I checked and sure enough, it was true! It was amazing that someone could observe that and write it down in a book and that I could read that book and confirm the observation, in some totally different place, with totally different starlings. Because the world operates according to observable rules and language lets us all share our observations! Thanks, language! Thanks, starlings!