Some motifs show up oddly often in tessellation art. Why?
And, what rules should guide you in choosing an easy motif?
When you choose a motif for your tessellation, choose something exotic with a shape that's got vagueness, variety, and flexibility.
Why are birds a popular theme? They're not quite as flexible as fish and gym socks. However, they can be posed with no wing, one wing, or both extended; the feet can be down or folded against their body. Their necks are sometimes as long and snake-like as an eel. Their wings can arc forward or back, or be held out straight.
OK, so they're posable. That's not the only reason, or even the best reason. I can think of two other reasons.
It's a simple body plan. There are no finicky rigid parts that stick out like the spines on a porcupine or the antlers on a jackalope.
The body plan is vague and varied. Unless the artist says "this is a yellow-crested sand diving beach warbler", the artist has a lot of freedom to mix and match body parts from several species. Unless you were a bird expert, you probably wouldn't notice the wrong beak on an eagle, or a long round tail that should be short and square.
Think about it: you know what a parrot looks like, but can you remember whether its tail is square or round? Are its wings long or short? I don't remember either. A drawing of a parrot would have a lot of flexibility, because our vague memory makes so many variations acceptable.
However, looking at a mammal, we'd spot a mix-and-match right away. Antlers on an elephant? A trunk and tusks on a lion? A horse face on a housecat? Bunny ears on a cow? Riiiiiidiculous!
So, why is a bird a good choice for a tessellation theme? Well, it's fairly flexible; its shape is simple; it's fairly posable; and our concept of an acceptable bird is very vague, so you can change its shape without people saying "thaaaat's noooot riiiight!"