Wordorigins.org

The name for this songbird is from the Old English nehtægale, which appears in the Corpus Glossary manuscript from c.725. A modern spelling of the Old English would be nightgale. It’s a compound of night + galan (to sing). So a nightingale is a bird that sings at night.

The modern form nightingale appears c. 1275 in The Owl and the Nightingale:

An hule and one nigtingale. [One manuscript has it as nyhtegale]

(Source: Oxford English Dictionary, 3rd Edition)