Habitat degradation relates to reduced immune function in nestlings, but not adults, of a tropical forest bird.

Human-induced habitat loss, fragmentation and degradation are among the leading causes of species decline worldwide. This is particularly true in tropical forests, where unique, often highly specialized fauna is under threat. By altering resource availability, anthropogenic habitat change can impose resource constraints on animals and may influence their allocation of resources to competing life history traits. In this study, we investigated whether nestlings and adults differently invest in self-maintenance depending on habitat degradation in the placid greenbul (Phyllastrephus cabanisi placidus), a cooperative breeder native to the cloud forests of Eastern Africa. We quantified investment in self-maintenance by measuring innate immune function using bacteria killing assays (BKAs) in adult breeders and their nestlings along a gradient of fragmented and degraded forests of the Kenyan Taita Hills. While innate immune function is an important defence against pathogens, resources needed to maintain it may come at a cost to other processes such as nestling development. We show that while forest degradation did not affect adult innate immune function, nestlings bear the cost of growing up in degraded habitats, as their ability to clear bacteria from blood plasma was lower in areas with degraded vegetation. These findings highlight the importance of studying the effects of habitat fragmentation and degradation in the tropics, where most of the global biodiversity occurs, and where long-lived species may respond differently from short-lived temperate ones, for example by prioritising self-preservation over reproduction.