Which bird has the most species?

I don't understand this question. Aren't birds a species of animals? Need a zoologist on this one.

A passerine is a bird of the giant order Passeriformes. More than half of all species of bird are passerines. Sometimes known as perching birds or, less accurately, as songbirds, the passerines are one of the most spectacularly successful vertebrate orders: with around 5,400 species, they are roughly twice as diverse as the largest of the mammal orders, the Rodentia.
https://en.wikipedia.org/wiki/Passerine#Taxonomy_of_passerines

No, birds aren't a species. There is a Phyllum in the Animal Kingdom which is Birds. This is split into Orders, then Families, then Genuses and finally you get to the species. Here's one of the best descriptions I've seen of how it works. As gdec says, the order Passeriformes has the most species.
https://www.birds.com/species/classifications/