White-throated sparrows can perceive pitch change using the frequency ratio independent of the frequency difference. Weary, D.M., Lemon, R.E. and Perreault, S. 1992. Song repertoires do not hinder neighbor-stranger discrimination. Read, A.F. and Weary, D.M. 1992. Weary, D.M, and Krebs, J.R. 1992.
Great tits classify songs by individual voice characteristics.
Lemon, R.E., Weary, D.M, and Norris, K. J. 1992. Temporal relationships in white-throated sparrow song. Butynski, T.M., Chapman, C.A., Chapman, L.J. and Weary, D.M. 1992. Weary, D.M. and Mountjoy, D.J. 1992. On designs for testing the effect of song repertoire size.