Female incubation attendance and nest vigilance reflect social signaling capacity: a field experiment

Due to the reduced conspicuousness of female signals, their evolution has traditionally been interpreted as a by-product of sexual or natural selection in males. Recent studies have argued that they may be the result of sexual or social selection acting on females. Here, we explored the role of the white wing patch during the incubation period in female-female competition contests in a migratory cavity-nesting songbird, the pied flycatcher Ficedula hypoleuca. At this stage, female investment is crucial for offspring survival, while competition among females for nest cavities is still operating. We experimentally performed an extreme reduction of signaling capacity by covering the wing patch with dark paint in a group of females and compared their incubation attendance and social interaction patterns (vigilance and aggression at the nest as defense variables) during simulated territorial intrusion tests with female decoys, with those of an unmanipulated group of females. Tests were performed both before and after the manipulation. We hypothesized that these patches constitute signals of social dominance in female-female interactions that allow them to maintain high levels of incubation attendance by reducing the need for vigilance. We found a marked decrease in incubation attendance in experimental females after manipulation, a change that was not found in the control group. Moreover, vigilance decreased in the control group after the manipulation, a change not detected in manipulated females. No effects on aggressive nest defense were found. Female wing ornamental traits would act as a badge of status in social interactions allowing more intense incubation and reduced vigilance. Implications of social interactions on incubation patterns should be incorporated in future studies. Significance statement: Animals can transmit information to the others through different traits. Ornamental signals have been proposed as status indicators in social interactions in birds in many studies. Although some of them concern their function and evolution in females, only a few have been focused on their role during incubation. Our experimental study suggests that the female’s white wing patch in pied flycatchers acts as a badge of dominance status in social encounters. Thus, dominant females may enforce respect from competitors through signaling, thereby being able to reduce vigilance at the nest with positive effects on incubation efficiency. Implications of social status for incubation patterns should be considered in future studies as an additional factor modulating this crucial parental activity.
This study was financed by project CGL2013-48193-C3-3-P to JM from Spanish Ministerio de Economía y Competitividad. MP and AC were supported by FPI and FPU grants from Ministerio de Economía y Competitividad and Ministerio de Educación, respectively.