Our observations showed that Bonelli’s Eagles selected their nest-site and began bringing hard and fresh material up to four months prior to egg-laying52. The onset of eagle visits to the nest, in October, and the start of nest-building using larger quantities of fine and coarse dry branches, might be related to the need to create a larger nest structure, including size, thickness, mass, and cup volume, which can influence the nest’s thermal properties23,22. This buffers the impact of adverse environmental conditions on the development of embryos and nestlings82 and signals nest occupancy by increasing the visibility of the nest-site to conspecifics and competitors (e.g., Golden Eagles, Aquila chrysaetos), even at large distances83,21.
Our results showed temporal changes in the types of material supplied to the nest: the supply of hard material occurred in the early visits to the nest and during the start of nest-building, whereas the supply of green material tended to increase as the laying date approached (Fig. 3a and b). Previous studies have shown that green material brought to the nest can regulate the nest temperature and may help to decrease ectoparasite and pathogen loads84,85,50 and improve breeding success29. Although fresh nest-material was gathered from vegetation rich in resins, including nine plant species, the most abundant plant delivered to the nests was greenery from pines (Table 1). Pines are characterized by a high level of aromatic compounds, highly repellent to insects29,28.
Previous studies have highlighted the important pair-bond existing between territorial Bonelli’s Eagles throughout the year86,87,88. During the pre-laying period, while copulation activity occurred principally in the evening, with a progressive increase from the afternoon onwards58, both sexes showed the strongest nest-building activity during the morning, with a strong peak between 09:00–11:00 h, showing that birds worked together on nest construction activities (Fig. 4).
Behavioural studies exploring the function of nest-building behaviour in biparental species are scarce47. In raptors, both partners invest in nest construction89,66,90. Thus, if male investment in nest construction is a consistent trait of any particular male, it is likely that females will use male nest-building effort to assess male parental quality and that the amount of branches and sticks that males deliver to the nest would become a sexually selected trait20,33. Previous studies describing biparental care in Bonelli’s Eagles have shown a sex-biased specialization in parental duties51,91. Females invested significantly more effort than males in gathering nest-material during the breeding season51. In contrast, our findings show that during the pre-laying period, the construction activities of males were important, and although the males provided slightly more material they were not more active builders than the females. In fact, our model does not demonstrate that sex is important in explaining the amount of material supplied to nests. Therefore, neither sex appears to use the nest-building process as a signal of their mate’s quality. Our results, therefore, do not lend weight to Hypothesis 1. Neither were any inter-sexual differences observed with regard to the type of nest-material gathered (hard and fresh). This finding is consistent with the results of66, who found that in Pyrenean Bearded Vultures (Gypaetus barbatus), there were no observed inter-sexual differences concerning the amount supplied of either of the two common nest-materials used (branches and wool). Therefore, our results do not support Hypotheses 2 or 3.
However, breeding experience could be a decisive factor determining the investment effort of males83,92. If the nest-building abilities of males are not wholly genetically determined, breeding experience could influence decisions relating to nest material choice36,40 and the amount of material gathered for nest construction37. In this way, younger eagles, with less experience of nest construction, should supply lower amounts of sticks and branches than older, more experienced eagles37. However, our model did not demonstrate any importance of this factor in explaining the effort of nest-building by males, thus rejecting Hypothesis 4.
In some bird species where both partners put similar effort into nest construction, the pair may reduce their effort as the laying date approaches93,94. In other taxa, the parents carry material to the nest throughout the entire breeding season (until after the chicks fledge9). In Bearded Vultures, for example, males invest more effort in nest-building than females, showing a peak of construction activity between 4–2 weeks prior to egg-laying66. These studies showed a progressive increase in material delivered to the nest throughout the pre-laying period. However, our results indicated a bimodal response of males to nest-building (Figure 3a), rejecting Hypothesis 4. This pattern could be explained by the fulfilment of two adaptive functions: (1) signaling occupancy of the nest against conspecifics and competitors (e.g., Golden Eagles) at the beginning of courtship; and (2) accumulation of the maximum amount of green material as the laying date approaches, with the aim of reducing the parasite and pathogen loads in order to maximize the viability of the eggs and the chicks.
Previous researches have shown that males may signal their condition, health, or parental quality to mates by building large or elaborate nests or by intense nest-building activity20,33,47. At first glance, one might expect that male investment in nest-building could act as an honest signal informing females about their parental quality. Male nest-building behaviour could therefore be considered as a sexually selected trait: females might pair with males which are active builders, regardless of their construction ability (good or bad), and females would adjust their reproductive investment based on the nest-building investment exhibited by their mates. Nevertheless, our analysis did not find a relationship between male nest-building investment and consequent reproductive performance. In this respect, our results are not consistent with the DAH33, thus rejecting our sixth hypothesis.