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

Abstract

When selecting a breeding site, individuals can use social information to reduce the uncertainty regarding habitat quality. Individuals of several bird species tend to reuse nests previously occupied by conspecific or heterospecific competitors but the proximate mechanisms underlying this behaviour remain unclear. Reoccupying nests previously used by competitors could result from individuals copying competitors' choices (the 'social information' hypothesis). Alternatively, it could allow individuals to fulfil their need for a soft 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 non-mutually exclusive mechanisms triggered the preference for old conspecific nest material in a secondary cavity-nesting raptor that does not add lining material to its nests, 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 soft substrates (peat moss or old conspecific or European roller, Coracias garrulus, nest material) over coarse mineral substrate, especially when the soft substrate also provided social information about previous nest use by a competitor. Despite the apparent absence of preference when directly comparing settlement patterns in soft 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 to early arriving ones. This hypothesis is supported by a later laying date of young breeders in our population. Our findings suggest that both comfort seeking and social information use explain the preference for previously used nest cavities, and that nest site choices may depend on individual competitive abilities and experience.

The cloacal microbiome of a cavity-nesting raptor, the lesser kestrel (Falco naumanni)

“…The lesser kestrel is a cavity nester that lays its eggs directly on the substrate without lining the nest nor adding specific aromatic plants useful to decrease the nest bacterial loads before egg laying ( Mennerat et al, 2009 ). It also prefers nests containing organic material from previous breeding attempts ( Podofillini et al, 2018 ; Morinay et al, 2021 ). Both males and females spend time in the nest, incubating the eggs and attending the offspring ( Ramellini, 2022 ; Sarasola, Grande & Negro, 2018 ).…”

“…1,000 breeding pairs, La Gioia, Luca & Lorenzo, 2017 ). Here, lesser kestrels breed in holes and crevices of buildings in the old town, but many pairs settle in specially designed nestboxes placed on the terraces of large public buildings ( Podofillini et al, 2018 ; Podofillini et al, 2019 ; Morinay et al, 2021 ). Birds forage in the farmland areas surrounding the city ( Cecere et al, 2018 ), hunting on grasslands and cereal crops ( Morganti et al, 2021 ) and feeding mainly on large invertebrates, lizards and small mammals.…”

Within-colony segregation of foraging areas: from patterns to processes

“…We may thus expect lesser kestrels to use other social cues at the colony, like eavesdropping on departure or return bearing of neighbours (Boyd et al, 2016). Besides, with the observed state-dependent use of social information for breeding site selection (Morinay et al, 2021), we could expect, in our case, that if one sub-colony is more attractive, it may be favoured by some specific phenotypes.…”

“…In particular, they rely on the colony breeding success for settlement decisions (yearlings avoiding the competition while older ones prefer sites with high past reproductive success, Aparicio et al, 2007). Moreover, early breeders tend to favour nest sites containing cues of previous breeding events, while late, usually young, breeders tend to avoid such cues (Morinay et al, 2021). We may thus expect lesser kestrels to use other social cues at the colony, like eavesdropping on departure or return bearing of neighbours (Boyd et al, 2016).…”

“…Given previous evidence in other systems, we tested whether lesser kestrels spatially segregate also at the sub-colony level. Given that lesser kestrels are non-territorial during foraging and have been shown to use social information obtained at the colony in some contexts (nest site selection, Aparicio et al, 2007;Morinay et al, 2021;antipredator vigilance, Campobello et al, 2012), and are known to sometimes forage in groups (typically in patches with ephemeral and high prey density, in fields being ploughed), we might expect them to also use foraging social information obtained at the colony, by eavesdropping on the departure or return bearings of other sub-colony members. We first tested whether members of the two sub-colonies differed in the bearing taken when departing on foraging trips, and then compared the bearings taken by individuals when departing the sub-colonies to those of concomitantly departing and returning individuals.…”