A widespread breeding bird in boreal forests across Canada and the northern United States, the northern waterthrush (Parkesia noveboracensis) migrates to the tropics to spend the winter.

The northern waterthrush's favorite winter habitat is mangrove forests that grow along the coastlines. Here the birds forage for insects and other invertebrates on muddy ground between the stiltlike roots of mangroves.

Joseph Smith, a graduate student in Zoo scientist Peter Marra's lab, just completed intensive studies of the winter ecology of the northern waterthrush in Puerto Rico. He found that waterthrushes use 4 basic habitats within the mangrove system: black mangrove, white mangrove, red mangrove, and scrub.

Sexual Segregation

Male waterthrushes, the larger and more aggressive sex, were most common in white mangroves while females dominated the other habitats.

Smith wondered if there was a difference in habitat quality between white mangroves and the other habitats that might explain this.

The Dry Season

The dry season in Puerto Rico extends from January to March. The effects of reduced precipitation were most noticeable in the scrub and black mangrove habitats—standing water dried up, leaves fell from trees, and insect prey became less abundant.

Many waterthrushes compensated either by having larger territories in these marginal habitats or by moving to red or white mangrove habitats. In contrast to scrub and black mangrove habitats, red and white mangrove habitats remained wet and lush through the winter, and food supplies were stable. In fact, insect prey abundance actually increased through the winter in white mangroves.

What did the differences between these habitat mean to waterthrushes? Waterthrushes that wintered in white mangroves, where insect prey increased, gained weight throughout the dry season, and before spring migration in late March and early April, they had larger stores of fat than waterthrushes that wintered in other habitats. In black and red mangrove habitats, the birds maintained their weight throughout the winter, while in the scrub habitat, birds lost weight throughout the dry season.

The Race Is On

Waterthrushes wintering in white mangroves likely had a head start on spring migration. Well-fed and with good fat stores, they could head north earlier and were better prepared for the rigors of migration. They would likely arrive on the breeding grounds first and thus could lay claim to the best territories.

This would likely result in these birds being able to produce more offspring than other waterthrushes. So, where a waterthrush chooses to spend the winter can have profound implications for the ability of a bird's lineage to survive.

So why do male waterthrushes select the best habitat—white mangroves—more often than females do? Male waterthrushes migrate north before females do. Regardless of which habitat females winter in, they have more time before they migrate to fatten up at the beginning of the wet season (April), when insects become more abundant, than males do.

This article summarizes the information in this publication:

Smith, Joseph A. M., Reitsma, Leonard R. and Marra, Peter P. 2010. Moisture as a determinant of habitat quality for a nonbreeding Neotropical migratory songbird. Ecology, 91(10): 2874-2882.

View abstract

Identifying the determinants of habitat quality for a species is essential for understanding how populations are limited and regulated. Spatiotemporal variation in moisture and its influence on food availability may drive patterns of habitat occupancy and demographic outcomes. Nonbreeding migratory birds in the neotropics occupy a range of habitat types that vary with respect to moisture. Using carbon isotopes and a satellite-derived measure of habitat moisture, we identified a moisture gradient across home ranges of radiotracked Northern Waterthrush (Seiurus noveboracensis). We used this gradient to classify habitat types and to examine whether habitat moisture correlates with overwinter mass change and spring departure schedules of Northern Waterthrush over the late-winter dry season in the tropics. The two independent indicators of moisture revealed similar gradients that were directly proportional to body mass change as the dry season progressed. Birds occupying drier habitats declined in body mass over the study period, while those occupying wetter habitats increased in body mass. Regardless of habitat, birds lost an average of 7.6% of their mass at night, and mass recovery during the day trended lower in dry compared with wet habitats. This suggests that daily incremental shortfalls in mass recovery can lead to considerable season-long declines in body mass. These patterns resulted in consequences for the premigratory period, with birds occupying drier habitats having a delayed rate of fat deposition compared with those in wet habitats. Taken together with the finding that males, which are significantly larger than females, are also in better condition than females regardless of habitat suggests that high-quality habitats may be limited and that there may be competition for them. The habitat-linked variation in performance we observed suggests that habitat limitation could impact individual and population-level processes both during and in subsequent periods of the annual cycle. The linkage between moisture and habitat quality for a migratory bird indicates that the availability of high-quality habitats is dynamic due to variation in precipitation among seasons and years. Understanding this link is critical for ascertaining the impact of future climate change, particularly in the Caribbean basin, where a much drier future is predicted.

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