Northern Waterthrush Winter Habitat
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.
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 scientific paper:
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.