Some passerine songs and calls have fairly specific functions (Kroodsma 1981, Catchpole 1982, Merilä and Sorjonen 1994). Other songs or calls may have more than one function, with the meaning varying with the context in which the song or call is given and received (Lein 1981, Catchpole 1982, Howes-Jones 1985, Nelson and Croner 1991, Slagsvold et al. 1994).