Icel. tittr, Norw. tite, tita “a little bird” (Klein, 1971. Comprehensive etymological dictionary of the English language. Elsevier. Norw. titta is teat, “akin to tit” (Webster’s).
Engl. dial. and slang. 1. tit “a small horse”. 2. “A very small person or thing; a chit; a morsel, bit”. 4. “small faggots for kindling”. 5. a minute hole in a piece of cloth. tit. “the wren”. titing “the titlark”; titling “the Meadow Pipit”; titty “small, tiny, little”; the wren; tittynope “a small quantit of anything”; tiddy, tiddly “very small”; tiddler “a small trout”; tiddy-wren “the wren”; tiddy-white-throat Sylvia cinerea" (Joseph Wright, 1970, The English dialect dictionary, vol. 7. Oxf. Univ. Press).
titty, tidley Essex the Wren (Swainson, C. 1885. Provincial names and folk lore of British birds).
Also for the Wren:
tit e.York., Ire.: Coleraine, Ess., titty Ess., w.Cy., Glo., Oxf., Suff., Wil., Som., Som., Dev., tintie Not., tittywren Glo., Suff., Wilt., titter-wren Glo., Suff., tit-o-wren litt., tittereen Suff., tiddy-wren Ess., w.Cy., tittywran Ire.: Coleraine, tidly, tiddly w.Som., Dev., tidley Ess., w.Cy., Som., Dev., tiddley-wren Som., tidley-creeper (-), tiddle, tiddle-tope, tiddlee, tiddlee-tope Devon, titty-todger Devon, Som.
And chit “small” (Oxf. Engl. Dict.). Names for the Wren: chit, chit-wren Ire.: Down, jitty Ches., chitty York., Lanc., Cumb., Westm., Kent, n.Lanc., Ire.: Lurgan, Antr., Down, Midl., Ballymena, Hillsborough, Islandmagee, Cookstown, chitter Kent, chitty-wran Scot., chitty-wran Ulster, Antr., Down, Donegal, Dunadry, chitty-wren Som., Antrim, chatter-hen Cumb., chitter-wren Westm., chittywer-wren Lanc., chittywi-wren w.York., cf. chit “a child or young person” (Wright, Swainson etc.)
I have also consulted the following (Scandinavian) works:
Hantsch, Bernhardt, 1905. Beitrag zur Kenntnis der Vogelwelt Islands. Friedländer & Sohn, Berlin.
Hortling, Ivar, 1944. Svenska fågelnamn. Försök till tydning av deras ijnnebord. Bok förlaget Natur och Kultur. Helsingfors.
Kjærbølling, Niels, 1875-1877. Skandinaviens Fugle. Helsingfors.
Wetterwik, Sinar, 1962. Europeiska fågelnamns forekomst i släkt-och apoteksanamn jämte etymologiska notiser. Vår Fågelvärld, 28: 211-226.
Haftorn, Svein, 1971. Norges Fugler. Universitetsforlaget, Oslo.
Brøndegaard, V.J. 1985. Folk og fauna. Vol. 2. Rosenkilde of Bagger, Købnhavn.
Tyrberg, Tommy, 1996. Svenska fåglars namn. Ursprung, historia och innebörd. Vår Fågelvärld, suppl. No. 24, 82 p.
Cf. also:
Scand. tit small
Icelandic tittr any small bird
Norwegian tita any small bird
English Scotland titling smallest in a brood
Norwegian tite and var. Titmouse
Czech titêra trifle
Czech tít’ák Wren, 406
Latvian tita chick
Spanish tita (Basque id.) hen
Catalan: Baleares tit chicken, young of an animal
French: Liège (childish) titi hen
Anc.Greek titthós small; child
Italian south titu small