Mechanisms underlying parasite infection: influence of host body mass and age on chewing louse distribution among brown-headed cowbirds.

Animal populations exhibit various patterns in ectoparasite distribution across different sexes and age classes, and numerous factors can potentially contribute to ectoparasite abundance and distribution. We examined the influence of host body size and age on the distribution of a chewing louse among brown-headed cowbirds. Differences in louse prevalence (males 62.9 ± 2.8%, females 47.5 ± 4.9%) and intensity (males 15.42 ± 1.51, females 9.04 ± 1.69) were primarily driven by differences in host body mass and not host sex. Larger birds had larger louse infra-populations, which likely translated into a lower risk of local extinction, a possible explanation for higher louse prevalence observed among larger birds. Among males, younger individuals showed higher louse prevalence (70.21 ± 4.72%) compared to older males (59.36 ± 3.59). We speculate that this pattern is likely driven by behavioural difference and not body size, with young males spending relatively more time foraging in large groups, increasing their risk of louse transmission. By examining the mechanisms that underlie the sex- and age-biased infections observed in natural populations, we can better identify the hosts most responsible for parasite transmission.