When they are in the tropics, wintering migrant warblers often find a home range and remain faithful to it. Throughout their lives, they return to the same spots each winter. One of the first studies to document this behavior did so with northern waterthrush back in 1960. But more recent studies have found that in some places, wintering waterthrush are not so faithful to a single spot.
We set out to discover whether this discrepancy had something to do with the habitats the waterthrush were using. Like many other migratory birds, waterthrush can be found in a variety of habitats in winter. At field sites in Puerto Rico, we studied them in 4 habitats: 3 different types of saltwater mangrove forest (red, white, and black mangrove) and dry forest, which grows on the dry land adjacent to mangrove swamps along the coast.
By tracking the movements of individual birds, we found that waterthrush had 2 basic movement strategies. They either stay put in one small area; or stay in one small area for a period of days or weeks, and then suddenly move to a new place (usually no more than half a mile away).
We wondered why all the birds didn't just stay put? Did it have anything to do with the habitat they were using?
The answer: yes. The majority of the birds that moved came from two habitats—dry forest and black mangrove. Almost all of the birds using the other two mangrove habitats (red and white mangrove) never budged. So, how did these habitats differ?
All habitats at our study sites in Puerto Rico undergo seasonal changes in the late winter, when there is very little rainfall over a three-month period. This pattern is so consistent that plants have adapted to the dry season by losing leaves to conserve water.
This is most pronounced in dry forest. Mangroves keep more of their leaves, but in late winter the standing pools of water that usually cover the swamp floor begin to dry up. Our measurements showed that black mangrove and dry forests are drier during the dry season than are white and red mangrove, which retain more standing water and lose fewer leaves. Why does moisture matter to these birds?
We thought that wetter places would have more food for waterthrush (they eat insects and other invertebrates that live in the leaf litter and mud). As we expected, the invertebrate prey of waterthrush declined in the drier habitats and remained stable in the wetter habitats.
Northern waterthrush in the drier habitats might have moved to new locations because food was declining in their original locations. We looked for evidence that these birds were searching for better places to forage, so we compared the birds' old locations to their new ones.
We found that, yes, the places they moved to were both wetter and had better food resources than the places they left. We also found that the birds that never moved had the wettest, most food-rich spots of all.
This study shows that the 4 habitats used by wintering northern waterthrush are affected by the dry season in different ways, and these differences drive the decisions of waterthrush to stay at or go from a particular location. A key finding is that birds living in drier habitats which decline in food availability during the winter can compensate for this by moving on to other places.
But is this compensation enough for a waterthrush to survive and thrive throughout the winter? We'll seek to answer this question in a subsequent study.
This article summarizes the information in this scientific paper:
Joseph A. M. Smith, Leonard R. Reitsma, and Peter P. Marra. (2011) Multiple Space-Use Strategies and Their Divergent Consequences in a Nonbreeding Migratory Bird (Parkesia noveboracensis) The Auk 128:1, 53-60
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