La fonction des caroténoïdes chez les hirondelles de cheminée (Hirundo rustica)

The function of carotenoids in the barn swallow (Hirundo rustica) A signal will evolve and spread only if costly, since its costs will prevent cheating, thereby making the signal reliable. In this thesis I investigate carotenoid-based signals that affect fitness in the barn swallow. Carotenoids are coloured molecules synthesised by photosynthetic organisms, and animals therefore have to ingest carotenoids with their diet. Apart from being important in reproduction, they act as free radical scavengers and have several physiological functions related to immune defence. Rarity of carotenoids in the environment leads to a trade-off between carotenoid use in reproduction, allocation to signal coloration and maintenance of health. Simply stated, only high quality individuals are hypothesised to acquire a sufficient amount of carotenoids to invest in coloured signals while maintaining prime health status. Firstly, I focused on carotenoid availability in the diet of barn swallows, to investigate whether they are a limited resource. My results indicate that while carotenoids are present in their diet, the carotenoid content of prey that barn swallows obtain are on average lower than that found in randomly collected samples within their foraging range. Greater variance in carotenoids in prey among than within nests indicates that individuals vary in their ability to obtain carotenoid-rich food. Secondly, I focused on the arrival date of barn swallow at their breeding grounds. Arrival date has previously been shown to affect fitness in this species. Early arrival after migration is a costly activity and the high metabolism associated with migration is likely to increase oxidative stress due to free radical production. Concentration and variation in carotenoids in blood of recently arrived adult males suggest a role of carotenoids in determining arrival date. If low quality individuals suffer more from peroxidation due to higher production of free radicals than high quality individuals, they cannot advance their arrival, because of greater need for carotenoids and other antioxidants. In addition, I analysed amount and variation of carotenoids in blood of adult females during reproduction. Carotenoids are invested both in eggs and maintenance of female health during reproduction. I report a decrease in carotenoid concentration in blood of females during reproduction and a significant difference between sexes during reproduction. During laying, females had a significant lower concentration of circulating carotenoids than males indicating difference in use of carotenoids between the sexes. Finally I investigated whether sexual dichromatism exists in colour of carotenoid-based red facial feathers of adult barn swallows. Males developed a more saturated colour, with a higher absorbance in the lutein band, and a hue more shifted to the red than females. Pigments also covered a relatively greater part of each feather in males than in females. My results show that males with a higher absorbance in the lutein band have a larger probability of obtaining a mate, indicating that the carotenoid-based red facial coloration could have evolved through female choice for more conspicuous male coloration. Together, these results suggest that carotenoids play a central role in the evolution of reliable signals in the barn swallow.