Why do you say tuna fish but not cow mammal or chicken bird?

That's a great observation! It's a quirk of English language usage. Here's why we say "tuna fish" but not "cow mammal" or "chicken bird":

* Redundancy: "Tuna" already implies "fish." The word "fish" in "tuna fish" is considered redundant.

* Historical Usage: "Tuna fish" has been a common phrase for a long time, perhaps stemming from an older time when "fish" was used more broadly to refer to any seafood.

* Emphasis: While the "fish" in "tuna fish" is redundant, it can sometimes be used for emphasis, like when you want to differentiate tuna from other types of seafood.

Why we don't say "cow mammal" or "chicken bird":

* Specificity: "Cow" and "chicken" are already specific animal names, and "mammal" and "bird" are broader categories. Adding these broader categories feels unnecessary and redundant.

* Clarity: Saying "cow mammal" or "chicken bird" could be confusing, especially if you're talking to someone who doesn't know much about animals. It's clearer and simpler to just say "cow" or "chicken."

Ultimately, language is constantly evolving, and these are just some of the reasons why we use certain phrases and not others.