Communal roosting shows dynamics predicted by direct and indirect nepotism in chestnut-crowned babblers

Abstract

Family living is a stepping stone to higher-order social structures including cooperatively breeding groups, but understanding why offspring remain with parents by delaying dispersal remains a challenge. One rarely studied aspect of family living is communal huddle roosting, where individuals group together overnight for thermoregulatory and/or anti-predator benefits. Here, we use a PIT-tag detection system to investigate the potential for direct and indirect nepotism in roosting huddles of cooperatively breeding chestnut-crowned babblers (Pomatostomus ruficeps) living in extended families with female immigrants. This species habitually roosts communally in single-chambered domed nests; an average of 8.5 (± 4.4 SD) individuals taking ~6 min to enter roost nests roosted together in this study. We found that the order in which individuals settled in the roost was negatively associated with their exit order, and those settling late were more likely to be detected at the nest entrance during the night, suggesting that settlement order influences individual roosting positions. Juveniles entered the roost in mid-rank positions, and their entries were associated with increased activity levels of carers, suggesting direct nepotistic assistance. In addition, immigrant females more often settled in the roosts later than others, suggesting indirect nepotism through exclusion of immigrants from more central roosting positions. This exclusion was not obviously confounded by age or sex. As those settling later are expected to experience reduced thermoregulatory benefits and increased predation risk, our results suggest that direct and indirect nepotistic benefits during communal roosting might offer part of the explanation for family living through delayed dispersal in this species. Nepotism—preferential treatment towards kin or against non-kin—may lead to individuals benefiting from staying in their natal group rather than dispersing, and so increasing the propensity for family living and the subsequent evolution of cooperative breeding. One possible route through which delayed dispersers might gain nepotistic benefits is in communal roosting huddles, as individuals roosting more centrally are likely to receive greater thermoregulatory and/or predator avoidance benefits. To our knowledge, studies have yet to discover an advantage of natal individuals beyond care received as young during communal roosting. We show that immigrant females settle in the communal roosts later than nutritionally independent natal females and may occupy peripheral positions in roosts of the cooperatively breeding chestnut-crowned babbler (Pomatostomus ruficeps) using a remote PIT-tag system. This apparent immigrant disadvantage might be significant in promoting delayed dispersal.