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.
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.
The two primary habitat types at our sites in Jamaica are black mangrove and second-growth scrub. These habitats differ in 3 important ways:
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.
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.
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?
We predicted that redstarts upgraded from scrub to mangrove would:
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.
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.
Mangrove redstarts left on migration an average of 6 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.
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.
Photos courtesy of Colin Studds
This article summarizes the information in this publication:
Studds, C. E. and Marra, Peter P. 2005. Nonbreeding habitat occupancy and population processes: an upgrade experiment with a migratory bird. Ecology, 86: 2380-2385.
Evidence is accumulating tliat winter liabitats occupied by migratory birds produce differences in individual condition that can carry over into subsequent stages of the annual cycle. Despite strong observational evidence, experimental work is needed to strengthen support for this hypothesis. We experimentally upgraded individual American Redstarts (Setophaga ruticilla) from low-quality second-growth scrub habitat to high-quality mangrove forest habitat by permanently removing behaviorally dominant, primarily adult males from mangrove, allowing females and immature males from scrub to colonize vacated territories. Prior to the manipulation, upgraded and control redstarts had stable-carbon isotope values in their blood indicative of scrub habitat occupancy and were comparable in body mass. Relative to control birds that overwintered exclusively in scrub, upgraded redstarts incorporated mangrove isotopic signatures, maintained body mass from winter to spring, departed earlier on spring migration, and returned at a higher rate in the following winter. Furthermore, insect biomass on upgrade territories was significantly greater than on control territories, suggesting food availability as a proximate mechanism underlying gradients of nonbreeding habitat suitability. Findings here demonstrate that winter habitat occupancy can be an important determinant of individual performance in migratory birds. Restricted access to food-rich winter habitats may limit survival of females and immature males, an outcome that could be an important driver of population structure and dynamics.
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