David M. Larson
Wood Thrush. Photograph by Sandy Selesky.
Have you ever thought about what birds do at night? I’m not talking about nocturnal owls and nightjars, but your average diurnal songbird. Well, they sleep. In fact, they sleep soundly. In the tropics on a night prowl with a headlamp, it is possible to walk right up and pluck a sleeping bird off a branch. And what is possible for tourists is possible for nocturnal predators. So, it seems that understanding where birds decide to sleep (roost) is actually an important part of understanding bird ecology, management, and conservation. We do know something about this topic, but mostly from studies on cavity-nesting species like woodpeckers. Turns out that most ornithologists are diurnal, too.
Jirinec and colleagues (2016) recently reported on an extensive study on the day and night activities of Wood Thrushes on their breeding grounds in coastal Virginia. Since Wood Thrushes are declining, the more we know about this species, the better we can craft conservation policies. The authors sought to determine if birds roosted in their daytime breeding territories, if birds selected roost sites of higher vegetation density (more cover), and if roosting locations varied with nesting status.
Using song recordings and mist nets, the authors captured territorial males and fitted them with VHF transmitters. Female mates of some of the males were captured and radio-tagged on territory as well. These transmitters allowed the researchers to follow the movements of the birds during the day, outlining the birds’ diurnal home range (DHR). At night, the transmitters helped reveal the nocturnal roosts of the same birds. Habitat characteristics were determined by light detection and ranging (LiDAR) measurements on breeding territories and roost sites. In order to address the nesting status, nests were checked every three days for evidence of eggs, nestlings, feeding, and other breeding evidence.
Interestingly, although female Wood Thrushes were essentially always found within the DHR (females do all of the incubation and so were on the nests), males often (31%) roosted outside of the DHR (average of 116 meters from the center). Most male roost locations were not consistent from night to night. Males roosted an average 6.8 meters off the ground in holly, beech, maple, and pawpaw trees. Consistently, males selected roost sites with higher vegetation density than randomly selected points in the areas, suggesting that either microclimate characteristics or predator-avoidance helped inform roost selection. Roost characteristics differed with bird age. Younger males roosted twice as far from diurnal activity centers than did the older males, presumably because the older males had better quality DHRs, perhaps including higher quality roost sites. Males might have chosen distant roost sites to avoid attracting predators to the nesting area. However, it is also possible, and perhaps likely, that use of distant roost sites allowed more opportunity for males to engage in extra-pair copulations (EPC) with neighboring females in the early morning or late afternoon hours when females were more receptive (Birkhead et al.,1996).
Roost locations of paired birds varied with nest status. If a pair had an active nest, the females roosted on the nest and males nested at a distance within or outside of the DHR. Pairs without active nests—after the young were independent or if the nest was predated—slept side by side, presumably so that the males could protect against EPC by the females during the time that they were most fertile. Although it is possible that roosting outside of the DHR allows for males to obtain more EPC, this hypothesis is confounded by the fact that most EPC are accorded to older males, but the younger males stray farther from home at night. Perhaps the older males do not need to stray as far due to greater experience.
This comprehensive report clearly demonstrates that male Wood Thrushes do not necessarily roost within their diurnal home range and that nesting status and age of the males are both variable. But it does not settle the question of why the males sleep away from home. More research is needed to resolve this issue.
- Birkhead, T. R., E. J. A. Cunningham, and K. M. Cheng. 1996. The insemination window provides a distorted view of sperm competition in birds. Proceedings of the Royal Society of London, Series B 263: 1187-92.
- Jirinec, V., C. P. Varian, C. J. Smith, and M. Leu. 2016. Mismatch between diurnal home ranges and roosting areas in the Wood Thrush (Hylocichla mustelina): Possible role of habitat and breeding stage. The Auk 133 (1): 1-12.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, Vice President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.