Early development of aggression in chickens: The relative importance of pecking and leaping.

A longitudinal study (employing a multiple separation technique) of chicks' social behavior over the first postnatal month indicated systematic changes in their mode of physical interactions. During the first week or so, intersubject pecking in same-sex pairs was frequent, and was more likely in birds with a home cage advantage (i.e., a prior resident effect). However, there were no sex differences in any of the forms of social pecking (head, body, or feet of the opponent), and all types showed significant decreases over the period of the study. On the contrary, there was a strong effect for sex on rates of aggressive leaping, with males exhibiting more leaps than females. Further, there was a clear increase in leaping over the first weeks of testing, and something of a decline thereafter. These data suggest that in this species the definitive expression of aggressive or dominance behavior may be leaping. Early social pecking (during a period in which this behavior was at its maximum rate) did not predict the subsequent leaping behavior of individuals, but later "leap orders" were highly associated with later "peck orders".