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Groups of domestic chicks were preexposed to moving objects on Days 1-5 after hatching. Only one object was visible at a time, and the time between the appearance of one stimulus and the immediately succeeding appearance of the other was called the "interonset time". Chicks which had been preexposed to both discriminanda in rapid alternation (interonset time of V4 min.) learned a subsequent discrimination task more slowly than chicks familiar with neither stimulus; chicks preexposed to both discriminanda at separate times (interonset time of 30 min.) learned the discrimination faster than chicks familiar with neither stimulus. The evidence suggests that the proportions of slow-learning and fast-learning chicks in a group are changed by varying interonset time.1 This research was submitted as partial fulfillment of the requirements for the PhD degree at the University of Cambridge while the author held a Medical Research Council studentship. The author is indebted to T. Weis-Fogh and H. W. Lissmann for the facilities provided for this work, and he would also like to express his gratitude to P. P. G. Bateson, who acted as research supervisor, for his invaluable advice and criticism.2 Requests for reprints should be sent to D.

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The response was observed of territory‐holding male European Robins ETithacus rubecula to red— and brown‐breasted model Robins which were presented either simultaneously with tape‐recorded Robin song, or with no song. Robins displayed and sang at the silent red‐breasted model, but sang and displayed much more at the models that were presented with song. It is suggested that, while the red breast is one important feature of Robins eliciting aggressive behaviour, song is also important. The relationship of these findings to studies of stimulus features eliciting behaviour in other species is discussed.

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