Downy woodpecker pair bond maintenance in winter: Proximate and ultimate mechanisms

James Scott Kellam, Purdue University

Abstract

In most birds, the pair bond relationship during the reproductive period is ubiquitous and often necessary for reproductive success. However, trade-offs between the costs and benefits of pair bond maintenance (PBM) make this relationship more variable in the non-breeding season. I determined some of the proximate and ultimate factors that could contribute to inter- and intra-specific variation observed in avian PBM by investigating the trade-off between pair bond benefits and costs experienced by downy woodpeckers (Picoides pubescens) during winter. I evaluated whether individuals' time budgets varied while in the mate's presence. My data showed that, depending on social environment, both competitive foraging costs and anti-predation benefits were important factors that affected the foraging efficiency of downy woodpeckers. Woodpeckers foraging in pairs or in small mixed-species flocks with the mate reduced the amount of time spent vigilant relative to woodpeckers not with the mate. Woodpeckers in large flocks with the mate showed no change in vigilance, and instead spent less time pecking and more time traveling between pecking sites relative to woodpeckers not with the mate. This suggests that individuals might adjust the frequency at which they associate with mate and flock on a short-term basis to minimize both predation risk and long-term loss in foraging efficiency. Testosterone facilitates pair bond behavior during the breeding season, but the function of testosterone in facilitating winter pair bond behavior is not well understood. I used testosterone implants to raise winter testosterone concentrations in experimental males (T-males) to levels near the breeding season maximum. T-males had more widely dispersed activity patterns within their home range compared to control males, and unlike control males, they increased their frequency of mate association when neighboring male intrusions were frequent. These findings indicate that testosterone in winter promotes male mate guarding behavior, which could increase reproductive success. However, female mates of T-males had reduced foraging efficiency relative to other females. It is possible that natural selection favors relatively low male testosterone levels in winter to minimize the impact of testosterone on existing pair bond relationships. My observational and experimental studies have demonstrated how a variety of factors, from predation risk to intrapair competition to hormone levels, affect the strength and character of the non-breeding season avian pair bond. These findings are important because PBM in the non-breeding season can have a significant impact on the reproductive success of individuals in spring.

Degree

Ph.D.

Advisors

Lucas, Purdue University.

Subject Area

Ecology|Zoology

COinS