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When selecting a breeding site, individuals can use social information to reduce the uncertainty regarding habitat quality. In particular, individuals from several bird species tend to reuse nests previously occupied by competitors. Re-occupying nests previously used by conspecifics or heterospecifics could result from exploiting social information by copying competitors’ choice (the ‘social information’ hypothesis). Alternatively, it could allow fulfilling the needs for a comfortable nest substrate (e.g. by improving thermal insulation or reducing egg breakage risks) at low costs, regardless of previous occupancy (the ‘comfort’ hypothesis). Here, we aimed to determine which of these two mechanisms triggered the preference for old conspecific nest material in a secondary cavity-nesting raptor which does not add lining materials to its nest, the lesser kestrel (Falco naumanni). Using an experimental design forcing settling lesser kestrels to choose between two adjacent nestboxes containing different substrates, we detected a strong preference for comfortable substrates (peat moss, or conspecific or European roller old nest material) over uncomfortable mineral substrate, especially when the comfortable substrate also provided social information about previous nest use by a competitor. Despite the apparent absence of preference when directly comparing settlement patterns in comfortable substrates with and without social information, early-settling individuals favoured the substrate with social information, while late-settling ones favoured the substrate without social information. This could reflect intraspecific competition avoidance by late arriving individuals that may be competitively inferior compared to early arriving ones. This hypothesis is supported by a later laying date of young (up to two years old) breeders compared to older ones in our population. Our findings suggest that both comfort seeking and social information use explain preference for previously used nest cavities, and that nest site choices may depend on individual competitive abilities and experience.

Additional file 1: Table S1. Summary of package nestR functions.

Additional file 4. Sensitivity analysis on buffer size for breeding-site detection.

Additional file 3. Package nestR vignette.

Additional file 2. Documentation for package nestR.

Since Darwin, evolutionary biologists envisaged various hypotheses to explain the advantages for males to exhibit exaggerated secondary sexual traits, also known as ornaments. One of these hypotheses proposes that male ornaments may signal to the females the benefits they can get by mating, and that female preference for ornamented males is one of the main selecting forces explaining the evolution of secondary sexual traits. The evolutionary scenario becomes more complex if we consider that in many species males bear multiple ornaments. Why multiple ornaments should evolve? Why and how do females base their choice among different male traits? Different ornaments could signal different aspects of male quality (=benefits). For example, individual condition at different stages of male’s life may by reflected by the expression of different ornaments, or diverse ornaments could respond differently to a similar stress experienced during a male life-history event due, for example, to a different response to carry-over effects. Condition dependence of male ornaments has been supported by a large number of studies on sexual secondary traits, yet whether and how different stresses at different life-history stages affect the expression of multiple ornaments has been investigated in a more limited number of cases. In my PhD project I used a passerine bird, the European starling (Sturnus vulgaris), as a model species given that males bear multiple secondary sexual traits. In this species it has been shown that females prefer males with long throat feathers that have high UV reflectance, that produce a complex song and, although with less empirical support, with more brightly colours on the beak. During my PhD I attempted to investigate the mechanisms that underlie the expression of male ornaments by experimentally manipulating individual condition during two energy-demanding life-history events, i.e. during nestling development and during the post-juvenile moult. Male attractiveness was directly evaluated in a female choice experiment. The first experimental manipulation aimed at investigating the effect of an immune challenge on growth and physiological responses on nestlings that were naturally infested with ectoparasites (manuscript 1), and to assess the temporal pattern of their first- and second-year moult (manuscript 2). The second experimental manipulation was used to test the effect of removing the stress caused by the nest ectoparasites on parental investment during the incubation (manuscript 3), and on parent-offspring communication during the nestling stage (manuscript 4). I then examined the temporal pattern of the post-juvenile moult in males from ectoparasite-free and naturally infested nests (manuscript 5). A third experimental manipulation consisted of supplementing with or depriving of carotenoids the diet of males in the course of their post-juvenile moult. I further tested the effect of nest ectoparasites and diet i) on the preening activity of males during winter, i.e. from the end of the moult to the start of the breeding period (manuscript 6), ii) on the expression of male ornaments and iii) on female preference (manuscript 7). My results suggest that an early stress, i.e. endotoxin or ectoparasites, does not significantly affect the nestling growth. Nestlings whose immune system was challenged with an endotoxin (LPS) showed similar antioxidant capacity and oxidative damage to those measured on control nestlings, whereas hematocrit was higher in second-brood LPS-nestlings than controls. Nestlings in ectoparasite-free nests produced a more conspicuous postural begging than that produced by nestlings in naturally infested nests. Parents were also sensitive to nest ectoparasites because they increased the time spent at the nest during the incubation period, although the provisioning effort during the nestling period was not different among deparasitized and control broods. Moult was advanced in birds injected with LPS and in those grown without nest ectoparasites, as they were probably in better condition than their counterparts and could therefore anticipate such costly life-history event. Birds from ectoparasite-free nests moulted over a longer period than controls, whereas moult duration of LPS and control birds was similar. The supplement of carotenoids during moult resulted in an increase of time invested in preening their plumage a few months after the end of the moult, and showed an increased yellow colouration of the beak in the following breeding season. As result, males that were grown in ectoparasite-free nests and were supplemented with carotenoids during the following moult were preferred by females in the mate choice experiment. In conclusion, the results of my PhD project provided an experimental evidence of carry-over effects in starlings. Males that experienced different stresses during the early stages of their life were less attractive for the females in their first breeding season. While I found a different colouration of the beak among birds from ectoparasite-free and control nests, the throat feathers were of similar length and colouration. This difference in attractiveness could be due to song characteristics during mate choice tests. Any attempt to individually record male song unfortunately failed and the effect of early stresses on song could not be assessed directly. Despite this, the resulting female preference for males that experienced favourable juvenile conditions suggests that male ornaments are considered as a whole during mate choice, and their expression is probably influenced by carry-over effects. These findings seem therefore in agreement with the redundant signal hypothesis, which suggests that female integrate the information of all male ornaments to assess the mate quality.