Spotlight | Tides, twilight and the moon
by Deborah Buehler originally published in Wader Study 128(2)
Imagine that you’re a Hudsonian Whimbrel Numenius hudsonicus, a relatively large shorebird, about 45 cm from head to tail. The tide is up (which means your workday is over), the sun has set, and now you need a safe place to rest – you and nearly 20,000 others. Where do you go? Can 20,000 large birds hide?
In this issue of Wader Study, Sanders and colleagues describe a nocturnal roost site in South Carolina, which supports about 20,000 roosting Whimbrel.1 That’s nearly 50% of the eastern population and nearly 25% of the entire North American population.
A roost is a safe place where a group of birds can settle to rest. Unlike a nesting site, roosts are for respite, not reproduction. For shorebirds, roosting usually happens when the tide is high enough to flood areas where the birds feed. At that point, the birds will fly to exposed dry land to take a break. Because tide times vary, roost times vary, and shorebirds rest by day (diurnal roost) or by night (nocturnal roost). Nocturnal roosts are often farther from mainland beaches and forests than diurnal sites. This offers better protection from nocturnal predators, such as owls, but it also makes these sites harder for both birds and humans to find.
One wouldn’t think that 20,000 shorebirds would be easy to hide, but finding nocturnal roost sites is no easy task, even for large, well-studied species in a country relatively well-populated with ornithologists. Sanders and colleagues first discovered roosting flocks of Whimbrel on Deveaux Bank in 2014. Deveaux Bank is a sandbar at the mouth of the North Edisto River in Charleston County, South Carolina. It sits on the northeastern edge of the ACE (Ashepoo, Combahee, and Edisto Rivers) Basin, a 1,420 km2 multi-use conservation area. Though the island is about a kilometer squared at low tide, the ephemeral nature of the sandbanks means that only about a quarter of that remains dry at high tide for roosting.
After finding evidence of the roost, the researchers returned to the site in 2019 and 2020 to find out how many birds used the area and to refine their nocturnal bird counting methodology. The team consisted of experts experienced in conducting surveys and estimating the sizes of large flocks. One to two researchers were stationed at four counting locations around Deveaux Bank allowing them to scan in all directions for arriving Whimbrel (see Fig. 1c in the paper).
Because Whimbrel rest when their feeding areas are covered by the tide, the researchers chose survey dates when high tide was within 125 minutes of civil twilight (the period after sunset when enough natural light remains that artificial light is not needed), so that foraging areas would be underwater near sunset. Also, all but one of the survey dates fell within three days of a full moon, which proved helpful as it allowed counting by moonlight when the sky was clear. Sanders and colleagues began counting as soon as flocks flying towards the sandbar were visible over the water (about 0.5 km away). Once on the sandbar, if the birds were disturbed, for example by a predator, the team subtracted any birds that left from their totals. If they settled again elsewhere, they were counted at the new location. This ensured no birds were double counted.
Not surprisingly, the main challenge in counting birds using a nocturnal roost is darkness. Many nights the researchers could hear that birds were still arriving after civil twilight, but it was too dark to keep counting them. Their nightly counts ranged from 8,974–19,485, with the highest count on a night when the moon was nearly full, and the sky was clear. That night 34% of all Whimbrel counted arrived after civil twilight and were counted by moonlight. It was the only night when counting stopped because birds were no longer arriving, rather than due to darkness.
This study reports several important findings. First, Deveaux Bank – a previously unknown nocturnal roost site – supports at least 19,485 roosting Whimbrel during peak spring migration. Whimbrel numbers have fallen nearly 50% over a 15-year period since the 1990s. Deveaux Bank now presents an opportunity to conserve and study a large part of the Whimbrel population. However, it also presents a risk – a lot of birds relying on a single, ephemeral sandbar for their survival.
Second, the researchers found that Deveaux Bank is likely important over a large area. Birds flew into the roost from all directions over the course of the evenings, first arriving from nearby inland marshes then, hours later, over the water from the south. This suggests that some birds are flying into the roost from long distances, implying that ideal roost sites are limited. Together, these first two findings emphasize the need to discover or create a network of alternative roosting sites for the long-term conservation of the species.
Third, the study spotlights the fact that, though difficult to find, once known, nocturnal roosts provide a way to survey large numbers of birds that would otherwise be uncountable as they forage over huge swaths of saltmarsh. Optimizing methods to count birds at roosts could give researchers a way to get more accurate population estimates and to track how populations are changing over time.
Fourth, this study shows the importance of both tides and lunar phase when choosing dates to count birds at nocturnal roosts. The optimal nights are the ones when high tide happens just before nightfall. Finally, selecting dates close to the full moon can help to ensure that birds arriving after civil twilight can be counted by moonlight if the skies are clear. The researchers recommend survey dates near the spring tide (the highest tides in the month), within two days of the full moon, when civil twilight and high tide are 30–60 minutes apart.
Shorebird populations are in steep decline around the world. By publishing their work, Sanders and colleagues have given us a better understanding of the importance of nocturnal roost sites and how best to survey them once found. Whimbrel are well-studied birds and their roost was found in a conservation area frequented by ornithologists. For less well-studied species migrating through less well-studied areas, far less is known about where birds roost, or even the larger geographic areas that they visit on their journeys. In this issue of Wader Study there are several papers describing the quest to determine where birds go, and which areas are essential to them. For example, Summers and colleagues used geolocator trackers to determine areas important to Wood Sandpipers Tringa glareola on their migrations between Scotland and West Africa.2 Adha Putra and colleagues used ground surveys to discover that the eastern coast of Sumatra is a significant wintering area for Nordmann’s Greenshank Tringa guttifer – one of the most threatened shorebird species in the world.3
PDF version of this article is available for download here:
- Sanders, F.J., M.C. Handmaker, A.S. Johnson & N.R. Senner. 2021. Nocturnal roost on South Carolina coast supports nearly half of Atlantic coast population of Hudsonian Whimbrel Numenius hudsonicus during northward migration. Wader Study 128(2): 117–124.
- Summers, R.W., B. Etheridge, N. Christian, N. Elkins & I.R. Cleasby. 2021. Timing, staging, speed and destination of migrant Wood Sandpipers Tringa glareola. Wader Study 128(2): 145–152.
- Adha Putra, C., D. Hikmatullah, I. Febrianto, I. Taufiqurrahman & C. Zöckler. 2021. North Sumatra is an internationally significant region for non-breeding Nordmann’s Greenshanks Tringa guttifer. Wader Study 128(2): 157–164.
Featured image: Video frame of Whimbrel at Deveaux Bank, South Carolina. ©Andy Johnson/Cornell Lab of Ornithology.