Northern Waterthrush Space-use Strategies
The space-use strategies of northern waterthrush are influenced by the wintering habitats they choose. But how do they make these choices? Do they compete for the most favorable sites? To answer these questions, we closely examined the lives of waterthrush wintering in coastal areas of Puerto Rico.
Wintering migratory birds, like northern waterthrush, that are faithful to the same location year after year are often territorial. This means that they defend their home area from others of the same species. A method often used to determine whether individual birds are territorial and to gauge their aggressiveness is the playback experiment.
In this case, we sneaked into the home range of a northern waterthrush and used a tape player and speaker to "play back" the bird's call note—and waited for the bird's response.
The results were fascinating. Some birds behaved aggressively, popping out of the undergrowth in vigorous pursuit of the apparent intruder. But others did not behave in this way. Some laid low, remaining discreet and giving no sign that they were present. And still others responded by actually leaving the area.
Although the aggressiveness we documented suggested that some waterthrush were territorial, we went a step further to prove the territoriality. We mapped the locations of birds that were part of the playback experiment, along with the locations of their waterthrush neighbors. This allowed us to tell if waterthrush had true territories that excluded others of the same species.
We found that the most aggressive birds were sedentary and had home ranges that excluded their neighbors. They were territorial. As for the birds that were not aggressive when they heard our playback, their home ranges did overlap with neighboring birds and they were not as sedentary, sometimes moving to new areas during the study period. They were not territorial.
The aggressive birds were more often than not male. Male northern waterthrush are bigger than females and this may give them a competitive advantage when establishing territories.
The key question that remained is whether there are benefits to being territorial. To measure these benefits, we monitored the habitat conditions in waterthrush home ranges and evaluated the physical condition of these birds during the late winter, just prior to their northward migration.
We found that territorial waterthrush lived in places that had more food than the places occupied by non-territorial birds. Physical condition followed the same pattern. Territorial birds gained weight during the study period, while non-territorial birds tended to lose or maintain their weight.
These results suggest that there may not be enough high-quality winter habitat available for northern waterthrush where we worked in Puerto Rico. There appears to be stiff competition for the best spots. The birds that don't get the best spots must live in poorer quality sites that require them to sometimes pursue a more itinerant pattern of space use. This extra work leaves them in poorer condition at a critical time, just prior to migration.
Working from a deficit to fatten up for migration can delay their departure for the breeding grounds. Findings like these emphasize the importance of protecting high quality habitat for waterthrush and the many other migratory birds that winter in the tropics.
This article summarizes the information in this scientific paper:
Joseph A. M. Smith, Leonard R. Reitsma, and Peter P. Marra. (2011) Influence of Moisture and Food Supply on the Movement Dynamics of a Nonbreeding Migratory Bird (Parkesia Noveboracensis) in a Seasonal Landscape The Auk 128:1, 43-52
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