The Function of Stones in Nest Building: The Case of Black Wheatear (Oenanthe Leucura) revisited

Abstract

Black Wheatears ( Oenanthe leucura) transport to breeding territories, including the nest, a large number of stones. Currently, there is negative evidence that stones serve for nest support, thermoregulation, weather protection or anti-predation, but in males, this behaviour appears to represent a form of post-mating display. This ‘sexual selection’ hypothesis, however, does not explain why females also carry stones, and why many stones are associated with the nest. We examined stone distribution and morphology in 277 abandoned nests of Black Wheatears from an arid area of southeastern Spain. Nests were built on the ground (14.4%), or in cracks (7.2%), cavities (54.6%) and ledges (23.8%) above the ground. Over 93% of nests had stones supporting and/or surrounding them, and > 90% of nests with additional space also had stones in contact with the nest forming a ‘rampart’. In 55.6% of elevated nests, stone piles were also found on the ground. Nest stones were significantly smaller than ground and rampart stones; size also differed between the latter two types. The extensive occurrence of nest and rampart stones, with a different morphology, suggests intentional transport and selection of stones to nest sites. Behavioural observations from other wheatear species suggest that female Black Wheatears may play a significant role in these tasks. We also show several lines of evidence that nest and rampart stones play a role supporting the nest. As an example, in 40 nests found on ledges in human buildings, the degree of nest support significantly increased on narrower ledges. Stone-carrying behaviour of Black Wheatears is probably multifunctional, having co-opted several exaptive effects, some of which are yet to be discovered.