image: The radio transmitter this wood thrush is wearing enables researchers to track it through the forest at night. view more
Credit: V. Jirinec
We know surprisingly little about what songbirds do after the sun goes down, but past studies have provided tantalizing hints that many forest birds roost for the night in different habitat from where they spend the day. For a study forthcoming in The Auk: Ornithological Advances, Vitek Jirinec of the College of William and Mary and his colleagues captured and radio-tagged Wood Thrushes (Hylocichla mustelina) to track their movements during both day and night. Their results, the first broad description of roosting ecology for a migratory North American songbird during breeding season, show that the birds often move out of their daytime ranges to sleep, seeking dense areas of vegetation where they're safe from predators.
Jirinec and his colleagues tracked 47 birds on the coastal plain of southeastern Virginia during the breeding seasons of 2013 and 2014, including 37 males and 10 of their mates. They found that males were not faithful to particular roosting spots, regularly moving from night to night, and overall one third of the roosting locations were completely outside the birds' daytime ranges. Remote sensing data showed that those nighttime roosts were located in areas with higher-than-average canopy density, and the researchers suggest that the birds could have been making these commutes in order to roost in safer sleeping spots, where they would be well-hidden from predators.
Add nests into the mix and things become even more interesting. Nesting females spent the night in the nest cup--not surprising, since female Wood Thrushes are exclusively responsible for incubating eggs and nestlings. While nests were active, males didn't stick around at night, continuing their usual routine of moving to safer roosting spots that were often outside their daytime territories. However, once a nesting attempt ended (either because the nestlings reached independence or because they were killed by a predator) males roosted side-by-side with their mates, inside their daytime range. Rather than being motivated by romance or loyalty, males were probably guarding their mates to prevent other males from surreptitiously mating with them in the dark.
Tracking birds at night led to some adventures. "Thankfully many of our study birds were in large forest tracks or chose not to roost near human dwellings, but there were still a considerable number of individuals who did just that," says Jirinec. "Although I had the necessary permits, explaining that to homeowners at 2 a.m. after they spotted a guy creeping around their property, antenna at hand, was not what I was about to try. Therefore, in those instances, I followed the radio signal entirely in the dark, feeling my way around the forest very slowly and turning on my headlamp only at the last instance to confirm bird position, then quickly absconding with my precious roosting data as newspaper headlines like 'Graduate student shot in the buttocks by Colonial Williamsburg resident' swirled in my mind."
"This is an important study that uses robust data collection from extensive radio-tracking to answer a simple question that has important conservation implications: how often do Neotropical migrants use different habitats for roosting and day-time activity on their breeding grounds?" adds Kara Belinsky of the State University of New York, New Paltz, an expert on the behavior and ecology of thrushes. "The use of different roost habitat should inform conservation plans for this declining species--we will need to protect high-quality roosting habitat as well as high-quality nesting and daytime habitat if we want to slow the decline of this species, and the same is also likely to be true for many of the other Neotropical migrants we care about."
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"Mismatch between diurnal home ranges and roosting areas in the Wood Thrush (Hylocichla mustelina): Possible role of habitat and breeding stage" will be available October 21, 2015 at http://www.aoucospubs.org/toc/tauk/133/1.
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.