Combining social information use and comfort-seeking for nest site selection in a cavity-nesting raptor

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.