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:
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:
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 scientific paper:
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.
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