Parasites are major effectors of natural selection and also play a role in sexual selection processes. Haemosporidian blood parasites are common in vertebrates and have been shown to vary in their effects depending on both the parasite and host species, on the host trait investigated as well as on host condition and stage of infection. Here we investigated infection of adult barn swallows (Hirundo rustica) by Plasmodium, Leucocytozoon and Haemoproteus species during the chronic stage of infection and the consequences for host fitness traits. Prevalence was higher than 10% only for Plasmodium. Chronic stage infection by Plasmodium was associated with reduced female breeding success, but did not affect breeding dates. Infection did not affect the expression of male secondary sexual traits (tail length and melanin‐based plumage coloration), but was associated with paler coloration of females. Finally, we found a negative effect of infection by Plasmodium on feather growth rate in older but not in yearling individuals. Because feathers are moulted during wintering in sub‐Saharan Africa where infection of barn swallows by Plasmodium occurs, our results suggest that male secondary sexual traits have little potential to reveal acute‐stage infection whereas plumage coloration of females may advertise their infection status. In addition, these results suggest that infection by Plasmodium can influence the course of plumage moult. Thus, our results add to the observations of negative effects of haemosporidian infection on fitness traits in birds and provides evidence that these effects can vary among traits and in relation to age and sex.

Inter- and intraspecific variation in eggshell colouration has long fascinated evolutionary biologists. Among species, such variation may accomplish different functions, the most obvious of which is camouflage and background matching. Within species, it has been proposed that inter-female variation in eggshell pigmentation patterns can reflect egg, maternal or paternal traits and hence may provide cues to conspecifics about egg, maternal or paternal phenotypic quality. However, the relationship between protoporphyrin-based eggshell pigmentation and egg or maternal/paternal traits appears to be highly variable among species. We investigated patterns of intraspecific variation in Eurasian barn swallow (Hirundo r. rustica) protoporphyrin-based eggshell pigmentation, and analysed its association with egg and clutch characteristics, maternal/paternal phenotypic traits and parental feeding effort. Eggshell pigmentation pattern significantly varied between breeding colonies, was significantly repeatable in clutches laid by the same females in different years (intraclass correlation coefficient ranging between 0.56 and 0.63), but it was not significantly associated with egg traits, such as position in the laying sequence, egg mass, yolk testosterone concentration and antioxidant capacity. It was weakly or non-significantly associated with female and male traits (sexual ornaments), but females laying darker (higher pigment intensity) first clutches had higher hatching success, suggesting that eggshell pigment intensity may predict fitness. Nevertheless, males did not significantly modulate their parental nestling feeding effort accordingly. In addition, females with darker breast plumage colouration (a melanin-based trait related to fitness) laid highly protoporphyrin-covered eggs, suggesting the presence of a previously unappreciated link between protoporphyrin biosynthesis and plumage melanisation. Moreover, the proportion of male offspring increased in clutches originating from highly protoporphyrin-covered eggs, suggesting that parents could acquire visual cues about their future brood sex composition before egg hatching. Our results support the idea that intraspecific signalling via eggshell pigmentation is a species-specific rather than a general feature of avian taxa.

Natural and sexual selection arise when individual fitness varies according to focal traits. Extra-pair paternities (EPPs) can affect the intensity of selection by influencing variance in fitness among individuals. Studies of selection require that individual fitness is estimated using proxies of lifetime reproductive success (LRS). However, estimating LRS is difficult in large, open populations where EPPs cause reallocation of biological paternity. Here we used extensive field sampling to estimate LRS in a population of barn swallows (Hirundo rustica) to estimate selection on lifespan and ornamental traits of males. We found selection on lifespan mediated both by within- and extra-pair fertilization success and selection on tail length mediated by within- but not extra-pair fertilization success. In addition, we found selection on tail white spots via extra-pair fertilization success after controlling for selection on other traits. These results were not confounded by factors that hamper studies of LRS, including non-exhaustive sampling of offspring and biased sampling of males. Hence, natural and sexual selection mediated by LRS operates on lifespan, tail length and size of the tail white spots in barn swallows.

Trade-offs select for optimal allocation of resources among competing functions. Parents are selected to maximize production of viable offspring by balancing between progeny number and “quality.” Telomeres are nucleoproteins, at the ends of eukaryotic chromosomes, that shorten when cells divide. Because shortening below a certain threshold depresses organismal functioning and rate of shortening depends on environmental conditions, telomeres are good candidates as mediators of trade-offs. We altered brood size of barn swallow Hirundo rustica and found that brood enlargement caused a reduction in relative telomere length (RTL). Reliable signals of offspring quality should evolve that mediate adaptive parental care allocation. Because nestlings with darker coloration receive more care, we analyzed the covariation between RTL and coloration and found that RTL increased with plumage darkness, both within and between broods. Hence, we provide unprecedented evidence that signals relevant to parent-offspring communication reflect telomere length and thus offspring reproductive value.

Sex allocation theory predicts that parents are selected to bias their progeny sex ratio (SR) towards the sex that will benefit the most from parental quality. Because parental quality may differentially affect survival of sons and daughters, a pivotal test of the adaptive value of SR adjustment is whether parents overproduce offspring of the sex that accrue larger fitness advantages from high parental quality. However, this crucial test of the long-term fitness consequences of sex allocation decisions has seldom been performed. In this study of the barn swallow (Hirundo rustica), we showed a positive correlation between the proportion of sons and maternal annual survival. We then experimentally demonstrated that this association did not depend on the differential costs of rearing offspring of either sex. Finally, we showed that maternal lifespan positively predicted lifespan of sons but not of daughters. Because in barn swallows lifespan is a strong determinant of lifetime reproductive success, the results suggest that mothers overproduce offspring of the sex that benefits the most from maternal quality. Hence, irrespective of mechanisms causing the SR bias and mother-son covariation in lifespan, we provide strong evidence that sex allocation decisions of mothers can highly impact on their lifetime fitness.