The reactions of Killdeers in the breeding season toward intruders of their own and other species, including man, result from drives to escape, attack, or, more frequently, a combination of both. Different combinations bring about different behaviours, since the escape drive urges the bird to flee and protect itself, whilst the aggressive drive, which becomes prominent in the breeding season, motivates the bird to stay and defend its territory, eggs or young. Intraspecific encounters produced many behaviours. Behaviour towards other birds, and man, was similar to that in intraspecific encounters, but crouch-runs, mobbing flights and injury-feigning were caused mainly by the presence of human intruders. Males were more aggressive than females in intraspecific encounters, but in the presence of a human intruder females were more aggressive, more vociferus, and kept closer to the intruder. Aggressiveness reached a peak in the middle of the breeding season. Crouching as a reaction to intruders was not displacement-brooding, but contained a strong escape elemen% that may be related to the habit of "freezing" by young Killdeers. Injury-feigning, like many other intruder reactions, canbe best explained as a conflict between escape and aggression, resulting in a mixture of the intention-movements of both drives. Variations in injury-feigning resulted from differences in the degree of conflict between the drives. At its simplest, a bird merely crouched in a depression, or behind an object, with its back towards the intruder. Weak displays of injury-feigning occurred before egg-laying and the strongest were usually at hatching. Injury-feigning largely ceased soon after hatching, but catching the young, even when they were well developed, sometimes evoked injury-feigning by parents which had otherwise ceased to display. When eggs were lost, the intensity of injury-feigning declined immediately. During the first nesting of each pair, the male performed injury-feigning more often than the female. However with second nests and broods the aggressive drive of males had declined, and they injury-feigned only occasionally. At this time the strong aggressive drive of females had declined to a level at which it no longer dominated the escape drive, so that females frequently performed injury-feigning. Injury-feigning may have originated from the locomotory intention-movements of escape and attack. These have become ritualised and adapted to elicit hunting by predators, or to startle or threaten them.