This is the 6174th most frequent Japanese word.
カワセミ
Kingfisher (a type of bird).
In this sentence, 'カワセミ' refers to the bird species known as the kingfisher. It is used as the subject of the sentence to introduce the bird.
カワセミは美しい川の鳥です。
The kingfisher is a beautiful river bird.
Here, 'カワセミ' is used as a direct object to describe what the speaker observed.
私は昨日、川でカワセミを見ました。
Yesterday, I saw a kingfisher at the river.
In this case, 'カワセミ' is in a genitive construction, specifying the owner of the cry being described as unique.