Injury feigning in the Savanna Nightjar: a test of the vulnerability and brood value hypotheses

Nest predation is a major threat to the survival of young birds and is thus a crucial selective pressure influencing the evolution of life history traits. Injury feigning is a nest-defense mechanism in which the parents attract the attention of predators to secure their offspring. Because parents may incur a cost from injury feigning, they should adjust their injury-feigning behavior to different situations to maximize fitness. In this study, we used the Savanna Nightjar, Caprimulgus affinis , as a model organism to test the vulnerability and brood value hypotheses for predicting the occurrence of injury-feigning behavior. A field study was performed between 2006 and 2012, and observations were recorded from 123 nests. Both nestling movement and injury feigning increased after hatching, reached their peak at a nestling age of 8–10 days, and then declined afterward. In addition, the frequency of injury feigning by the female differed between habitats, being higher in a low-plant-cover habitat than in a high-plant-cover habitat. Both findings are consistent with the vulnerability hypothesis. By contrast, the intensity of injury-feigning behavior did not linearly increase with nestling age and brood size; thus, the brood value hypothesis was not supported.