Male orange-crowned warblers in Alaska and Catalina Island, California have very different levels of the sex hormone testosterone throughout the breeding season. This hormone is important to male birds as it helps them:

• sing more
• court females
• defend territories
• guard their mates
• chase after other females

The evidence suggests that high testosterone males sire more young but are also poorer fathers.

The orange-crowned warblers in Alaska belong to the subspecies Vermivora celata celata and range across the continent from Alaska to Labrador. The Catalina birds belong to the subspecies Vermivora celata sordida and only nest on the Channel Islands off California.

The Alaskan birds show the typical pattern of seasonal testosterone levels: high levels when courting females and then lower levels when the female is incubating eggs and when feeding nestlings. But Catalina birds maintain high testosterone levels throughout the breeding season.

The different patterns of hormone levels are likely due to the differences in life style between them. As compared to Catalina birds, Alaska birds:

• have a shorter breeding season
• don't live as long
• lay more eggs in a nest

So Alaskan birds will try to pair up as quickly as possible with a female. His high testosterone levels aid him in this. But once she is sitting on eggs, he needs to switch into father mode quickly. He'll likely only get one chance to nest per season and he needs to get busy catching bugs for a lot of nestlings.

In contrast, Catalina males can easily recover if a nest fails. They can just start a new nest. By keeping his hormone levels high he can be ready to sire more young during the long breeding season.

This study is one of the few that show different hormone levels within one species of bird.

This article summarizes the information in this publication:

Seasonal and population variation in male testosterone levels in breeding orange-crowned warblers (Vermivora celata). Horton, Brent, Yoon, Jongmin, Ghalambor, Cameron K., Moore, Ignacio T., Sillett, Terence Scott. 2010. General and Comparative Endocrinology, 168(3): 333-339.

View abstract

Comparative hormone studies can reveal how physiology underlies life history variation. Here, we examined seasonal variation in plasma testosterone concentration between populations of male orangecrowned warblers (Vermivora celata) breeding in Fairbanks, Alaska (V. c. celata) and on Santa Catalina Island, California (V. c. sordida). These populations face different ecological constraints and exhibit different life histories. Alaska birds have a short breeding season, low annual adult survival, and high reproductive rates. In contrast, Catalina Island birds exhibit high adult survival and low reproductive rates despite having a long breeding season. We examined seasonal variation in male testosterone concentrations as a potential mechanism underlying differences in male reproductive strategies between populations. From 2006 to 2008, we sampled males during the pre-incubation, incubation, and nestling stages. Alaska males exhibited a seasonal testosterone pattern typical of northern passerines: testosterone levels were high during pre-incubation and declined during incubation to low levels during nestling provisioning. Testosterone concentrations in Catalina Island males, however, did not vary consistently with breeding stage, remained elevated throughout the breeding season, and were higher than in Alaska males during the nestling stage. We hypothesize that in Alaska, where short seasons and high adult mortality limit breeding opportunities, the seasonal testosterone pattern facilitates high mating effort prior to incubation, but high parental investment during the nestling stage. On Catalina Island, elevated testosterone levels may reflect the extended mating opportunities and high population density facing males in this population. Our results suggest that population variation in seasonal testosterone patterns in orange-crowned warblers may be a function of differences in life history strategy and the social environment.

Download pdf

Related research:

Learn about our orange-crowned warbler research project