Words for bird and (sky)lark in Celtic languages.

Proto-Celtic	*ɸetnos = bird
Gaulish	Etnosus = bird(?)
Old Irish (Goídelc)	én [eːn] = bird
Irish (Gaeilge)	éan [eːnˠ] = bird, young bird, chick
Scottish Gaelic (Gàidhlig)	eun [ian] = bird, fowl, chicken
Manx (Gaelg)	eean [jiːən] = chick, fowl, young (of birds); juvenile, cheeky child, brat
Proto-Brythonic	edn = bird
Old Welsh	atar = birds
Middle Welsh (Kymraec)	edn [ˈɛdn] = bird
Welsh (Cymraeg)	edn [ˈɛdn] = bird, fowl, fly, bee adar [ˈadar] = birds aderyn [aˈdɛrɨ̞n / aˈdeːrɪn] = bird
Old Cornish	hethen = bird
Cornish (Kernewek)	edhen = bird
Middle Breton	ezn = bird
Breton (Brezhoneg)	evn = bird

Etymology: from the Proto-Indo-European *péth₂r̥ (wind, feather), which is also the root of the English word feather [source].

Breton (Brezhoneg)	labous = bird

Etymology: unknown

Old Irish (Goídelc)	uiseóc = lark
Middle Irish (Gaoidhealg)	(f)uiseóg = lark, skylark
Irish (Gaeilge)	fuiseog = lark, skylark
Scottish Gaelic (Gàidhlig)	uiseag [ɯʃag] = lark, skylark
Manx (Gaelg)	ushag [ˈoʒaɡ] = bird, shuttlecock

Etymology: unknown

Proto-Celtic	*alaudā = bird
Gaulish	*alauda [ˈa.lau.daː] = skylark

Etymology: unknown. Also the root of the Latin alauda (lark, skylark), and words for (sky)lark in Romance languages, such as the French alouette, the Spanish alondra and the Italian allodola [source].

Welsh (Cymraeg)	ehedydd [ɛˈhɛdɨ̞ð / ɛˈheːdɪð] = lark, skylark; flier, one that flies, bird or fowl
Old Cornish	evidit = lark, skylark
Middle Breton	ehuedez = lark, skylark

Etymology (Welsh): from ehedeg (to fly) and -ydd (-er) [source].

Cornish (Kernewek)	ahwesydh = lark, skylark
Breton (Brezhoneg)	alc’hweder = lark, skylark

Etymology: unknown

Words marked with a * are reconstructions.

Sources: Wiktionary, Am Faclair Beag, MacBain’s Dictionary, In Dúil Bélrai English – Old-Irish Glossary, teanglann.ie, On-Line Manx Dictionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau