Helpers are associated with increased nest attentiveness and more constant egg temperatures in chestnut-crowned babblers

The impacts that helpers have on offspring in cooperative breeding systems depend on parental responses to help. However, the vast majority of investigations into parental responses to the presence of helpers have considered investment responses during a single phase of breeding—offspring provisioning. Whether or not responses during other phases of breeding complement those during offspring provisioning is therefore unclear. Here, we use temperature probes in model eggs of chestnut-crowned babblers ( Pomatostomus ruficeps ) to determine: (a) maternal responses to helpers, which (like partners) allofeed incubating females but do not share incubation; and (b) the implications of these responses for egg temperatures. We found that during incubation, increased helper number was associated with longer bouts of incubation interspersed with fewer foraging bouts, which resulted in greater nest attentiveness overall. Further, although helpers had limited effects on average egg temperatures, they were associated with reduced variation in egg temperatures. This reduced variation arose because helpers were associated with higher minimum egg temperatures during off-bouts, presumably because of the negative association between helper number and off-bout number. Together, these results suggest that helpers have additive effects on maternal incubation schedules, which contrasts markedly from the dramatic declines in offspring provisioning shown by mothers with helpers during the nestling phase. Further studies are required to assess the generality of these contrasting patterns and their implications for the quantification of helper effects in cooperative breeders. Significance statement Maternal responses to the presence of helpers are thought to be species-specific, but most tests are only conducted during offspring provisioning. Using temperature probes in model eggs of cooperative chestnut-crowned babblers ( Pomatostomus ruficeps ), we show that mothers increase contributions to incubation in the presence of helpers, resulting in more constant egg temperatures. This response contrasts markedly with maternal responses to helper presence during nestling provisioning in this system, suggesting that maternal response rules differ across distinct phases of breeding.