Is nest predation an important selective pressure determining fecal sac removal? The effect of olfactory cues

Abstract

The removal of nestling feces by adults is a common parental behavior in birds. However, this behavior is still poorly understood despite being an important component of parental care, especially in altricial bird species. The threat of nest predation is a major factor that influences many parental activities at the nest and, therefore, it could also be an important selective pressure determining fecal sac removal. To date, this ‘Nest Predation Hypothesis’ has not been tested despite being proposed more than a century ago. Furthermore, it is important to determine whether it is the olfactory and/or visual components of fecal sacs that attract predators. In this study, we have manipulated the presence of real droppings of Common Blackbird (Turdus merula) nestlings in active nests by hiding fresh fecal sacs (experimental), mud (control) or nothing (manipulation control) in a commercial Canary nest attached below natural nests. Our results showed that the presence of fecal sacs did not reduce the daily survival rate of experimental nests in comparison with the other two treatments. It would therefore appear that predation risk is not a selection pressure that maintains feces removal in nests of the Common Blackbird, at least in relation to the olfactory component of the feces. To date, all evidence suggests that this aspect of parental behavior could be affected by other selective pressures (i.e. parasitism) rather than nest predation. Nevertheless, the effect of the visual cues produced by fecal sacs should be further investigated before the Nest Predation Hypothesis is completely discarded.