Probability of incubation by male Japanese Barn Swallows increases with plasma testosterone level

Empirical studies often support the notion that testosterone inhibits paternal behaviour in animals, although most studies have focused on high, as opposed to low, levels of paternal care. We studied the relationship between plasma testosterone levels and incubation in male Japanese Barn Swallows Hirundo rustica gutturalis, in which some males participate in incubation. Contrary to our prediction, the probability of incubation by males increased with plasma testosterone level. Male incubation behaviour might have emerged as part of nest monitoring behaviour by high-testosterone males in some subspecies that experience high nest predation rates.