Search

Blood parasites such as malaria and related haemosporidians commonly infect vertebrate species including birds. Understanding age-specific patterns of parasite infections is crucial for quantifying the fitness consequences of parasitism for hosts and for understanding parasite transmission dynamics. We analyzed longitudinal and cross-sectional infection data in house martins Delichon urbica, a migratory bird suffering from intense haemosporidian infection. We separated within- from among-individual effects of age on prevalence. Our results showed that the probability of blood parasite infection increased as individual house martins aged. We also showed that the prevalence of infection decreased with age at last reproduction when controlling for age, showing a selective disappearance of infected birds from the population (i.e. selection). The estimated effect of age on prevalence was underestimated two- to three-fold if not accounting for such selection. This study highlights the importance of taking among-individual heterogeneity in the capacity to fight a disease into account because such heterogeneity can mask age-related patterns of infection. These findings emphasize the relevance of considering within- and among-individual patterns of infection in order to understand parasite-induced mortality and the potential for parasite transmission. [ABSTRACT FROM AUTHOR]

Body condition is an important trait that reflects the capacity of individuals to acquire food or resist disease, eventually allowing successful reproduction and survival. We first quantified the effects of condition on life history and a secondary sexual trait using long-term lifetime data on condition in a migratory bird species, the barn swallow, Hirundo rustica. Second, we quantified sources of individual variation in condition by separating the within-individual from the between-individual components of variation of environmental condition and age on body condition. Therefore, we were able to partition variation as a result of selection and phenotypic plasticity. Within-individual variation in body condition increased in early life until middle age (i.e. 3-4 years of age) in the two sexes followed by only a slight decrease in body condition during senescence in males but not in females. After accounting for age-dependent variation, condition could be partitioned into a within-individual plastic response to environmental conditions during migration and a nonplastic response (i.e. a between-individual difference) to environmental conditions experienced in the African winter quarters. Specifically, there was a within-individual increase in body condition as environmental conditions during migration improved in both males and females, independent of age. There was a between-individual effect of condition found in the winter quarter in body condition of females, but not in males, which was attributed to the disappearance of females in poor body condition from the study population as a result of the higher natal dispersal of low-quality females compared to high-quality ones during years with favourable environmental conditions in the African winter quarters. Males and females also tended to be in better body condition during the warmer springs upon arrival at the breeding grounds. There was a temporal decline in female body condition during 1991-2007, whereas no significant trend was detected in males. Therefore, both intrinsic (e.g. age and sex) and extrinsic factors (e.g. climate) affected body condition. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 420-434. [ABSTRACT FROM AUTHOR]