The role of testosterone in male downy woodpeckers in winter home range use, mate interactions and female foraging behaviour.

Studies of the role of testosterone (T) in birds have typically focused on sexual or aggressive behaviours of males during the breeding period, but males of nonmigratory species may invest in mate and territory long before breeding, and the influence of T in facilitating nonbreeding-season behaviours is poorly understood. We gave free-living male downy woodpeckers, Picoides pubescens, T-implants during the winter to determine whether elevated levels of T increased a male's ability to exclusively occupy territory-based resources, and whether elevated T strengthened a male's investment in an existing pair bond relationship. We also explored how a female's foraging efficiency might be affected by her mate's behaviour if he had elevated T. We found little difference between control and T-implanted males with regard to home range exclusivity. Surprisingly, male-male display rates were significantly lower in T-implanted males than in controls. Regarding male-female interactions, T-implanted males that experienced high incursion rates from other males maintained more frequent spatial association with their mate, suggesting that T facilitates male behaviours that could restrict the mate's access to other male birds. Female mates of T-males showed reduced foraging rates, but because male-female aggression was similar between treatment groups, the cause for this reduction is unknown. The results indicate that exogenous T during winter affects a variety of behaviours in male woodpeckers, and proximate influences on pair bond maintenance in winter may be a fruitful avenue for future research.