Ectoparasite density is associated with mouth colour and size in nestling House Sparrows Passer domesticus

Dependent offspring influence allocation of parental investment using specialized traits thought to contain information about offspring condition. To test this hypothesis, we examined the relationship between one component of avian begging, the phenotype of the nestling mouth, and the density of a haematophagous mite, Pellonyssus reedi, in nestling House Sparrows Passer domesticus. Ectoparasite density in broods was negatively associated with average nestling mass and flange colour intensity and positively associated with mouth width, but was not related to the intensity of blood‐based gape coloration or flange width. These results support the hypothesis that components of offspring solicitations signal high quality, and highlight the diverse selective pressures shaping offspring–parent communication.