Nestbox use is linked to increased paternity loss in infected blue tits, but not great tits

Irene Di Lecce, Joanna Sudyka, Marta Szulkin, Anna Dubiec

Host-parasite interactions and reproductive strategies are critical aspects of avian life history, yet knowledge of their interplay in the context of nesting site microhabitat, in particular between natural and artificial nesting sites, is lacking. The choice of nesting site is critical for successful breeding, by ensuring protection from predators and a suitable environment for incubation and chick development. Furthermore, an optimal nesting site could mitigate the adverse effects of infection and other stressors on the resulting mating success and fitness of breeding birds. In this study, we investigated the prevalence of haemosporidian parasites and their relationship with paternity loss in blue tits (Cyanistes caeruleus) and great tits (Parus major) breeding in natural cavities and nestboxes. High parasite prevalence was observed in both species, with 72% of blue tits and 67% of great tits infected, mostly with Plasmodium spp. In blue tits, paternity loss by a social father, defined as the proportion of extra-pair offspring per nest, was influenced by an interaction between infection status and cavity type. Infected blue tit males breeding in nestboxes experienced higher paternity loss compared to those in natural cavities, suggesting that properties of nesting sites might alleviate or exacerbate the negative effects of infection on mating success. This effect was species-specific, as it was not observed in great tits. These results highlight how microhabitat, individual health, and reproductive strategies interact, and showcase the potential evolutionary implications of breeding in nesting sites provided by humans.