One Word Substitution: One who specializes in the study of birds?

Ornithologist

Zoologist

Biologist

Naturalist

'Ornithologist' is a person "who specializes in the study of birds".
'Biologist' is a person who studies biology.
'Naturalist' is a person who studies and knows a lot about plants and animals.
'Zoologist' is a person who scientifically studies animals.