Morpho-sedimentology of coastal natural sites colonized by Sand Martin (Riparia riparia)

Morpho-sedimentology of coastal natural sites colonized by Sand Martin (Riparia riparia). — The Sand Martin or Bank Swallow Riparia riparia returns annually to a same site to establish a colony. These sites are natural or anthropogenic origin but are still subject to permanent changes. The species is characterized by its ability to dig tunnels in which it carries out its reproductive cycle. Several authors have attempted to identify the criteria used by the swallows to choose a favourable site to the installation of a colony. Among the many parameters that can help explain the location of colonies, morpho-sedimentary settings have been poorly documented. Among abiotic factors, the composition of the soil particles seems to play an important role in the depth of the tunnels which is directly proportional to the reproductive success. The aim of this article is to contribute to explore the morpho-sedimentary parameters of two colonies localized in coastal area. We investigated the supposed effect of morpho-sedimentary parameters on the presence and layout of tunnels, their depth and if the colonized sites have different characteristics from nearby favourable but not colonized sites. We took into consideration several morphological parameters of the colonies and sampled sediments, which were analysed and compared using statistical tests and linear regression. We show that, in the Bay of Mont Saint-Michel, Bank Swallows do not choose their nesting site depending on the nature of the sediment unlike what was proposed in a previous study on a continental site. For each site, we were only able to find tenuous differences between colonized and no colonized sites. The swallows have all chosen the same sedimentary strata regardless of the site. The choice of this stratum is linked mainly to the limitation of the risk of flooding but also to the fact that the uppermost stratum that supports vegetation is more difficult to be excavated due to the presence of roots and rhizomes. The characteristics of sedimentary material are however structuring in the depth of the tunnels.