Ateji spelling for the bird name; rarely used in modern Japanese.
Alternate kanji spelling using bird-related characters; also rare.
Katakana spelling often used for animal names in field guides or casual writing.
The origin of the reading えっさい is uncertain. The kanji 悦哉 are ateji (phonetic equivalents) and do not contribute to the meaning. The spelling 雀𪀚 uses bird-related characters but is also rare.