Winter habitat quality and the battle of the sexes
Adult male American redstart.There is growing evidence that events in the annual cycle of birds that migrate between the Neotropics and the Nearctic interact strongly to influence population dynamics. While all of the studies showing this have been extremely rigorous, they have also all been observational.
In this study, research scientist Colin Studds took an experimental approach. He moved individual American redstarts overwintering in Jamaica from low- to high-quality habitat and then followed these birds throughout the winter to determine the effects of this manipulation.
Short Primer on Seasonal Interactions
Differences in the quality of winter habitat can lead to differences in physical condition of the birds. This, in turn, alters the timing of the birds' departure for spring migration, resulting in variable arrival schedules and, subsequently, substantial variation in reproductive success. Let's go back to the beginning of this cycle and take look at what this habitat-quality gradient looks like on the ground.
Winter habitats of American redstarts
Lush, green habitat on left is mangrove; drier, more open habitat on right is scrub.
Winter Habitat Quality
The two primary habitat types at our sites in Jamaica are black mangrove and second-growth scrub. These habitats differ in three important ways:
- Mangrove habitat has standing water present throughout much of the season whereas scrub habitat rarely has standing water.
- Mangrove habitat has a closed canopy of tree leaves; scrub habitat plants progressively shed their leaves in winter.
- Because of the closed canopy, mangrove habitats are cooler than scrub habitats.
Consequences of Habitat Choice
There are distinctly different consequences for redstarts that occupy these two habitats. Redstarts in mangroves maintain their body weight through the winter, depart early on spring migration, and return at a high rate the following winter. In contrast, birds overwintering in scrub decline in physical condition, depart late on spring migration, and return at a low rate the following year.
Males Are from Mangroves...
This young male American redstart looks very similar
to female redstarts. Adult male redstarts banish females
and young males to inferior habitats.Redstarts also show an important behavioral response to winter habitat quality called sexual habitat segregation. Older male redstarts predominate in mangrove habitat. Overall, males make up about 70 percent of birds in this habitat. Through behavioral dominance, older males exclude females and younger males, forcing them to occupy second-growth scrub habitat. Females make up about 70 percent of the birds in this habitat.
These sex differences are extremely important because they mean that females suffer disproportionately from the negative consequences of wintering in scrub habitat. Population dynamics are very sensitive to the number of females in a population. By upgrading redstarts—primarily females—from scrub to mangrove, we were able to make direct inferences about how winter habitat quality acts on population dynamics.
Research Question
Can we provide experimental evidence that winter habitat quality produces differences in individual condition that can carry over into later phases of the annual cycle?
Predictions
We predicted that redstarts upgraded from scrub to mangrove would:
- maintain weight overwinter;
- depart early on spring migration;
- return at a high rate in the next winter.
Experimental Removals
We permanently removed male redstarts from a mangrove habitat. This created territorial vacancies that scrub birds were able to colonize, effectively making them into mangrove birds.
Was the Experiment Successful?
Birds that moved from the scrub habitat to the vacant mangrove habitat maintained their weight through the winter. However, birds that remained in the scrub habitat lost up to eight percent of their weight. This difference is extremely significant to a small migratory bird. The first place we see this is when we look at the timing of departure for spring migration.
The Early Bird
Mangrove redstarts left on migration an average of six days earlier than birds remaining in scrub. This suggests that the primary benefit of occupying a high-quality winter territory and remaining in peak physical condition is early departure on spring migration.
Favorite Vacation Spot
Birds that had moved to the mangroves were significantly more likely than birds remaining in the scrub to return the following year. Overall, 59 percent of mangrove upgrades were sighted the following year, compared to only 33 percent of scrub birds.
Implications
- These results strengthen support for the hypothesis that winter habitat affects individual condition in ways that can carry over to later phases of the year.
- Delayed departure date may be important if it delays arrival to breeding areas and limits reproductive success.
- These results also provide a potential explanation for the male-biased sex ratios seen in many species of migratory birds.
SMBC Investigators:
Related Resources:
Photo Credits: courtesy of Colin Studds
