Social ordering of roosting by cooperative breeding buff-throated partridges Tetraophasis szechenyii

Abstract

The benefits of communal roosting are unlikely to be acquired equally by all roosting birds, and it has been suggested that individual ordering during roost formation can be determined by factors such as social status. However, to date there have been no published data on this issue among precocial, cooperative breeders. We examined social ordering of roosting by buff-throated partridges Tetraophasis szechenyii, a typical cooperative breeding Galliform species newly found where group members roost in a linear huddle. We revealed that the breeding male was normally the first to enter roosting trees and a helper entered last, regardless of whether groups had young or not (n = 50 observations from seven groups). In a linear huddle, a helper frequently occupied the outermost position, especially in groups with young, and young birds were often squeezed into the center (n = 48 observations from seven groups). For buff-throated partridges, we suggest that while structuring of communal roosts confers the greatest energy benefits and protection from predators to young birds, helpers, rather than the breeding male, may face higher costs in terms of lookout and thermoregulation. To further understand social ordering of roosting by buff-throated partridges, future studies need to examine quantitatively the relative value of predation risk and/or thermoregulatory costs among roosting members placed in certain ordering, and determine the relatedness indices among group members.