Nest-Building Effort is Not An Indicator of Male Quality In Bonelli’s Eagles, Aquila fasciata

Abstract Raptors often use a variety of materials to build their nests (natural, such as branches, but also non-natural objects), presumably due to their insulating properties, their suitability to advertise occupancy of the nest, and to decrease pathogen and parasite loads. The amount of branches used in a nest is an indicator of parental quality and is often associated with increased breeding success. However, in raptors where both sexes collaborate in nest construction, it is unclear whether the effort expended by males (taking the amount of material carried to the nest as the potential predictor) could constitute an honest signal of parental quality to female conspecifics. We examined data on sex, type of material brought to the nest, breeding experience, timing, and nest-building investment prior to egg-laying from 32 identifiable Bonelli’s Eagles ( Aquila fasciata ) during the pre-laying period to investigate the relative contribution of the sexes to the amount of nest material gathered. We asked: (1) whether the nest-building investment of males could provide information to the females about their quality; and (2) whether the amount of material delivered to the nest by the male was related to breeding success. Despite the considerable investment of males in nest-building during the pre-laying period, our results indicate that this effort is not a consistent indicator of male quality to the female. Therefore, male nest-building behaviour and investment by Bonelli’s Eagles cannot be considered as an extended expression of their phenotype (an extended phenotypic signal). Nest-building behaviour by males in the early and late stages of nest-building, and the fact that males were not significantly more active builders, are discussed in the contexts of signaling nest occupancy to conspecifics and competitors, the decrease of parasite loads, and the strengthening of the pair-bond during the pre-laying period.