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Abstract
In bi-parentally built nests, there is evidence to suggest that nests are extended phenotypic signals that accurately indicate the quality of the building parent/s. 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. However, in raptors where both sexes collaborate in nest construction, it is unclear whether nest building (taking the amount of material carried to the nest as the potential predictor) is an indicator of parental quality, and whether the effort expended by both sexes could constitute an honest signal of parental quality to their partners. Between 2011 and 2016, we monitored 16 nests of Bonelli’s Eagles (Aquila fasciata), and we examined data on sex, type of material brought to the nest, breeding experience, nest quality, timing, and nest-building investment prior to egg-laying from 32 identifiable Bonelli’s Eagles during the pre-laying period to investigate the relative contribution of the sexes to the amount of nest material gathered. Our results indicate that sex is not a determining factor in nest-building effort, and that females did not increase their parental effort in response to the male’s contribution, and supply of materials did not increase during the pre-laying period. In contrast, our models showed that: (1) the type of material supplied to the nest by both sexes varied significantly throughout the pre-laying period and (2) nest-building effort was determined by individual experience and nest quality. Therefore, our study suggests that male nest-building behaviour and investment by Bonelli’s Eagles cannot be considered as an extended phenotypic signal. The differential use of hard and green material by both sexes in the early and late stages of nest-building period, and the fact that the more experienced individuals contributed a larger amount of material on low quality nests, are discussed in the contexts of signaling nest occupancy to conspecifics and competitors and the decrease of ectoparasite loads during the pre-laying period.
Highlights
In bi-parentally built nests, there is evidence to suggest that nests are extended phenotypic signals that accurately indicate the quality of the building parent/s
Nest-building behaviour has been associated with courtship and pair formation because both sexes can use nest sites and nest material to attract a partner[13]
The use of feathers, fresh fragments of aromatic plants, or even cigarette butts as nest materials can play a key role reducing the adverse effects of pathogenic bacteria and parasites on eggshells[21] and n estlings[22,23], so improving the growth and condition of chicks at fledging[24] to increase parental breeding success[12,25]
Summary
In bi-parentally built nests, there is evidence to suggest that nests are extended phenotypic signals that accurately indicate the quality of the building parent/s. Unlike animals that do not build their own nests, instead using the abandoned nests of heterospecifics to lay eggs and rear their young[6,7], many species expend considerable time and energy in the construction of one or multiple elaborate nests for breeding[8,9], with varying degrees of differential parental investment (i.e., exclusively maternal, paternal, or biparental; see[10]). The use of feathers, fresh fragments of aromatic plants, or even cigarette butts as nest materials can play a key role reducing the adverse effects of pathogenic bacteria and parasites on eggshells[21] and n estlings[22,23], so improving the growth and condition of chicks at fledging[24] to increase parental breeding success[12,25] These direct benefits alone often make it beneficial to choose mates most able to build well-constructed nests[4,26,27]
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