Winter habitat of the northern waterthush, these mangove trees grow in mud along coastlines and have above ground roots.
Tiny, lightweight radio transmitter mounted on the back of a waterthrush, the antenna runs down the tail.
Joseph Smith holding an antenna that will allow him to track waterthrushes to their roost sites.
While conducting research in Puerto Rican mangrove forests on the winter ecology of the northern waterthrush, Joseph Smith, a scientist with the Smithsonian Migratory Bird Center, noticed an unusual phenomenon.
Every evening, his study subjects flew away, far out of sight, abandoning the small patch of mud and mangrove roots where they had been feeding—and vigorously defending their patch from other waterthrushes—all day long.
Birds of many other species travel to specific roost sites to spend the night. But this behavior is rather unusual for its many warbler relatives and for most songbirds, and had never been reported in waterthrushes.
How far did they go to roost? Where did they go? And why? With his scientific curiosity aroused, Joseph set out to answer these questions.
It was impossible for Joseph to follow the waterthrushes at dusk on foot. The forests these birds inhabit are extremely difficult to walk through. In addition to thick mud at low tide and pool of water at high tide, the aboveground roots of the mangrove create fence-like barriers.
Fortunately, recent technological advances in radio-tracking proved to be the solution. Small, lightweight transmitters were fastened to the birds after catching them with mist nets. Signals from the transmitters allowed Smith to narrow down the location of the birds' nighttime roost sites to a 10-square meter area (about 12 square yards).
The waterthrushes crepuscular (dawn and dusk) migrations averaged about 600 meters (1/3 mile). Surprisingly, the vast majority—87 percent—of these birds roosted in red mangroves, a tree that occupies only 2 percent of the available habitat!
Although relatively uncommon, red mangroves do grow in areas with deeper water than the more common black and white mangroves do. This additional water depth may be a barrier to some predators, and the reason waterthrushes choose red mangroves as roost sites.
Joseph also wondered if the birds roosted communally, as some birds do, or alone. Radio-tracking had allowed him to narrow the location of the roost site to a small area, but not to locate individual birds. To accomplish this, another modern technical advance was needed.
Joseph used a new technique called thermal imaging, which detects heat sources. In this case, a bird's warm body could be located among the cooler surrounding vegetation.
After locating the exact roost site, Joseph found that the birds tended to roost alone, although they were sometimes within the communal roosts of birds of other species, such as gray kingbirds.
Joseph repeated the study the following winter, and discovered that 4 of the 5 birds he found in both years returned to the exact same roost site.
The availability of winter roost sites may be an important limiting factor for some bird species. Roosting ecology needs to be considered in conservation decision-making.
In the map of Puerto Rico below, daytime foraging areas (D), and their nighttime roosts (N), for individual birds are shown.
This article summarizes the information in this scientific paper:
Smith, Joseph A. M., Reitsma, L. R., Rockwood, L. L. and Marra, Peter P. 2008. Roosting behavior of a Neotropical migrant songbird, the northern waterthrush Seiurus noveboracensis, during the non-breeding season. Journal of Avian Biology, 39(4): 460-465.
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