Passage III

Scientists studied how perceived predation (hearing the calls and sounds of predators), in the absence of direct predation, affects the production of offspring by female song sparrows (Melospiza melodia). M. melodia sparrows produce 2 clutches (groups) of eggs in a breeding season.

Study

At the beginning of a breeding season, 24 M. melodia nests without eggs were located in low-growing shrubs. Direct predation was eliminated by surrounding each shrub with an electric fence to protect the nest from ground predators and by covering each shrub with a net to protect the nest from aerial predators.

The nests were evenly divided into 2 groups: a predator treatment (PT) group and a nonpredator treatment (NPT) group. A speaker that broadcast the calls and sounds of predators of M. melodia was set up near each nest in the PT group. A speaker that broadcast the calls and sounds of nonpredators of M. melodia was set up near each nest in the NPT group.

Throughout the breeding season, each speaker broadcast the appropriate calls and sounds every few minutes on a 4-day-on, 4-day-off cycle. (The parents and offspring associated with each nest did not hear sounds emitted by speakers other than their own speaker.) The number of eggs and the number of hatchlings produced in each nest were recorded. The number of hatchlings that successfully left each nest (such hatchlings are called fledglings) was also recorded. The figure shows, for each group, the average number of offspring (as eggs, as hatchlings, and as fledglings) per nest in each clutch.

Figure adapted from Liana Y. Zanette et al. , "Percelved Predation Risk Reduces the Number of Offspring Songbirds Produce per Year." ©2011 by American Association for the Advancement of Science.