The Collaborative International Dictionary of English v.0.48:

Titmouse \Tit"mouse`\, n.; pl. Titmice. [OE. titemose,
   titmase; tit small, or a small bird + AS. m[=a]se a kind of
   small bird; akin to D. mees a titmouse, G. meise, OHG. meisa,
   Icel. meisingr. The English form has been influenced by the
   unrelated word mouse. Cf. Tit a small bird.] (Zool.)
   Any one of numerous species of small insectivorous singing
   birds belonging to Parus and allied genera; -- called also
   tit, and tomtit.
   [1913 Webster]

   Note: The blue titmouse (Parus coeruleus), the marsh
         titmouse (Parus palustris), the crested titmouse
         (Parus cristatus), the great titmouse (Parus
         major), and the long tailed titmouse (Aegithalos
         caudatus), are the best-known European species. See
         Chickadee.
         [1913 Webster]

WordNet (r) 3.0 (2006):

titmouse
    n 1: small insectivorous birds [syn: titmouse, tit]