Homebody or restless spirit? If you don't have a permanent address, you're like to be missed when the census comes around, and the same is true of birds. Some American Redstarts (Setophaga ruticilla) wintering in Jamaica claim a territory and stick to it, just as they would on their breeding grounds, while others remain transient, constantly moving around in search of the best resources. Different methods for counting birds are better at detecting different types of individuals, and new research forthcoming in The Auk: Ornithological Advances by Ashley Peele of Tulane University and her colleagues shows that methods that focus on mapping territories may miss transient birds and don't accurately portray the population as a whole.
Peele and her coauthors spent four years working in different habitats spanning Jamaica's ecosystems--mangrove swamp, dry scrubland, and wet mountain forest--using two different but complementary methods to study redstart populations. In territory mapping, birds are captured and marked and their movements monitored to identify individual territories, while in point counts all the redstarts seen or heard from a specific point within a ten-minute window are recorded. Point counts detected transient individuals that were cumbersome to identify with territory-mapping alone, especially in mangrove swamp habitat, where transient birds were most common. Comparisons with territory-mapping data showed that females and young males were more likely to be transient than older males, which are more socially dominant.
While territory-mapping techniques can provide a greater level of detail about the birds that are captured and tracked, missing transient individuals means missing information about the status of the population. Increasing numbers of transient individuals can be early indicators of a variety of changes in the ecosystem, such as habitat loss or degradation, a changing climate, or an increase in population size. "In the past, it was assumed that the vast majority of individuals did in fact defend a territory in the non-breeding season," explains Peele. "However, our ability to detect and recognize individuals using alternative strategies such as transience has been limited by both survey methods and awareness of their prevalence. Our research demonstrates that there are much higher proportions of transient individuals present in wintering populations of redstarts in Jamaica than anyone realized."
"This study is important because it not only advances our understanding of how to better quantify the abundance of wintering migrants by combining point count methods with intensive mapping of individually marked birds, but it also reveals that a thorough analysis of the demographics of non-breeding birds may require multiple field methods to detect birds because of the important consequences of transient and territorial strategies used by non-breeding migrants," adds Matthew Johnson of Humboldt State University, an expert on the distribution patterns of Jamaica's birds. "By extension, Peele et al.'s work will apply to other systems in which transient individuals comprise a portion of the birds in the community, in either the breeding and non-breeding seasons."
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"Combining survey methods to estimate abundance and transience of migratory birds among tropical non-breeding habitats" will be available September 30, 2015 at http://www.aoucospubs.org/toc/tauk/132/4.
About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists' Union. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.