Recent studies have demonstrated that preen oil acts to reduce or eliminate feather-associated bacteria. The mechanisms underlying this antibacterial activity, however, are incompletely understood. In addition to the activity of alcohols (i.e. 3,7-dimethyloctan-1-ol), recent research has suggested that antimicrobial peptides may underlie the antibacterial activity of preen oil. Here, we document the presence of innate and adaptive immune proteins, lysozyme and immunoglobulin Y (IgY), in the preen oil of house sparrows (Passer domesticus). We suggest lysozyme functions as an antimicrobial agent, with potentially important impacts against Gram-positive feather degrading bacteria. Furthermore, both lysozyme and IgY likely act in local immune defence of the preen gland, and may also play a role in regulating the local microbiome, with potentially important consequences for chemical communication and signalling. Our findings suggest that the preen gland and its secretions should be considered an integral part of the body’s first line of defence against invading infections.

Preen gland secretions were collected from male house sparrows and measures of lysozyme and IgY were obtained through laboratory assays. Morphology data was collected from males before they were released.

Individual ID is based on ring number of each male.

Capture time and Sampling time are calculated as minutes since 00.00 on the day of sampling. Time_diff_minutes is the amount of time between capture and sampling.

Preen gland length, width, and depth are based on the mean of three measurements taken for each dimension.

Lysozyme concentration is measured in μg ml−1. IgY concentration is measured using an ELISA assay and the resulting optical density values (absorbance units) reflect concentration.

Body condition was calculated as the scaled mass index following Peig and Green 2009, using tarsus length as the morphology measure of body size.