system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do birds and fish come in such a spectacular variety of colors and shapes compared to other animals?
gpt
Well, two things:

1. Their colors only stand out compared to other reptiles and mammals. Amphibians and nearly every kind of invertebrate come in just as many spectacular colors. The better question is "why don't mammals have such a spectacular variety of colors compared to other animals?" 

2. Their variety in shape doesn't really stand out amongst anyone; obviously invertebrates all have enormous variation, but even boring old mammals have everything from tiny/round mice to lanky/springy deer to weird long ferrets to giant stocky rhinos. 

It's also worth noting that fish in particular may be so widely varied because they're miscategorized; there's a push among some biologists to split up "fish" into several differently groups because there's so much more variation among "fish" than among equivalent groups. 

Mammals don't have as much color variety because our color comes almost exclusively from melanin, which can only do shades of tan/brown/black. Most other animals can synthesize more pigments than that. 

But birds in particular do have some other tricks up their sleeves:

Some birds, famously the flamingo but also plenty others, can absorb pigments from their food that their body can't make by itself. 

Most green and blue birds actually don't use specc pigments, but their feathers structurally create that color (which is why they're so iridescent). 

I don't know if either of those apply to fish.