Nestling sex and behaviour determine the host preference of insect vectors in avian nests

Sexual differences in pathogen prevalence in wildlife often arise from varying susceptibility influenced by factors such as sex hormones and exposure to pathogens. In the case of vector‐borne pathogens, host selection by insect vectors determines the exposure of hosts to infections, largely affecting the transmission of these infectious diseases. We identify the blood‐feeding patterns of insect vectors in Blue Tit (Cyanistes caeruleus) nestlings in a 3‐year study. Blood from both nestlings and insect vectors (Culicoides spp. and Simuliidae) captured inside nest‐boxes were used to molecularly determine the sex of the host. We then compared the sex‐ratios of the nestlings that had been bitten and those of the complete brood in each nest. We found that males were bitten more frequently than females in 2021, when males weighed less in comparison to other years. Additionally, we molecularly identified bitten nestlings individually by genotyping the DNA of blood obtained from both, the vector's abdomen and nestlings of each brood in 2022. Nestlings more frequently bitten by vectors were males, weighed less and were closest to the nest entrance. To our knowledge this is the first study identifying the nestling selection by insect vectors in bird nests under natural conditions. These results contribute to understanding the mechanisms of host selection by insect vectors, shedding light on pathogen transmission and offering insights into the observed sex‐biased infections in wildlife populations.