Tonic Immobility Duration Predicts Antipredator Behavior in Juvenile Western Capercaillies (Tetrao urogallus, Aves)

Abstract Grouse reintroduction programs are often hindered by the rapid loss of released birds to predators. The ability to predict how release candidates will respond to danger is an important step toward selective-release programs that increase the effectiveness of population reinforcement. We studied captive-bred three-week-old western capercaillie chicks. The chicks were placed in a novel environment and sequentially exposed to a moving bird silhouette in its “goose” and “hawk” orientations. We tested whether their individual responses could be predicted from the tonic immobility test, a standard method for assessing individual stress responses. The tonic immobility test was administered twice to each chick: duration was measured at two weeks of age (TI2) and at three weeks of age (TI3), one day before the two model presentations. Chicks with longer TI3 either ignored the “hawk” or resumed exploring or feeding after briefly freezing. Chicks with shorter TI3 responded more strongly, either by freezing for a prolonged period or by escaping. The response to the “goose,” which was always presented before the “hawk,” was unrelated to TI3. Tonic immobility duration was repeatable during the third week of life, but TI2 did not predict defensive responses in three-week-old chicks. Defensive responses showed significant temporal consistency during the fourth week of life and significant contextual consistency at three, but not four, weeks of age.