1. Counting Birds Flying Past Cape Tappi at Night Under Streetlights
At Cape Tappi on the tip of the Tsugaru Peninsula in Aomori Prefecture, a survey supported by Bird Research Japan took place for about two months each in the autumns of 2021 and 2022. The method involved visually counting birds passing through the night sky illuminated by streetlights, identifying them by species [2]. In 2022, preliminary results recorded 84 species and 16,074 birds, including those identified only by their calls [2]. Because only birds within the range of the streetlight’s glow are counted, this does not represent all birds flying through the night sky.
Even birds typically seen during the day have a "night face." The survey poster lists hawks, cranes, geese, and swans as birds that migrate during the day, while thrushes and ducks are listed as birds that migrate at night. It describes night migration as a "mystery full of things we cannot see directly" [2]. Despite this, the counts included 40 greater white-fronted geese, 176 tundra swans, 67 mallards, and 103 Eurasian wigeons [2]. It appears that geese and swans, often categorized as daytime migrants, also pass through the night sky. What kind of journey are these night-flying geese undertaking?
2. Bar-headed Geese Crossing the Himalayas Mostly at Night and Early Morning
Research teams from the UK and other countries attached GPS transmitters to 91 bar-headed geese crossing the Himalayas to track them. They found a bias in the times when positions were received during migration: the geese crossed mainly at night and in the early morning [1]. Note that while 91 birds were tagged, detailed analysis was performed on 38 of them [1].
The idea that they fly at 8,000 m (about 26,247 ft) routinely was not supported by this tracking. The highest altitude recorded was 7,290 m (about 23,917 ft) when heading south and 6,540 m (about 21,457 ft) when heading north. 95% of the received positions were below 5,489 m (about 18,008 ft), and many were close to the terrain in valleys rather than above mountain peaks [1]. The highest altitude is the maximum recorded value, not the average. They choose lower altitudes and fly at night. Why Might Birds Fly at Night?
3. Why Cold, Dense Night Air May Make Flying Easier for Birds
Researchers argue that because night air is cold and dense, the altitude calculated using the International Standard Atmosphere is several hundred meters lower at night than during the day, even at the same actual elevation [1]. The actual height does not drop; rather, the altitude calculated against the standard atmosphere becomes lower due to air density. Since air is denser at lower altitudes, flying at the same height at night is similar to flying at a lower altitude during the day. The research team argues that this reduces the burden of flying at high altitudes [1].
Denser air provides more resistance when wings push against it, similar to how it is easier to move through water than air. However, the relationship between air density and flight power is shown through more complex calculations. This discussion applies to flights over high mountains and cannot be said to be the sole reason for all nocturnal waterfowl flight. General explanations for the benefits of night migration include cooler temperatures, fewer daytime predators, less air turbulence, and the ability to use daytime for feeding and rest [3]. These are general explanations and may not apply directly to all waterfowl. How do ducks, which do not cross high mountains, fly at night?
4. Ducks in the Netherlands Flying Between Resting and Feeding Areas at Night
In the IJsselmeer area of the Netherlands, researchers used ship radar to study the night sky. In February and March 1995, common pochards and tufted ducks flew mainly in the dark. Most of their flights were concentrated in the approximately 1.5 hours after sunset and the approximately 1.5 hours before sunrise [4]. The flight altitudes were a maximum of about 75 m (about 246 ft) above dikes and about 50 m (about 164 ft) over open water. Altitudes dropped further when headwinds were strong [4].
These flights are trips between roosting and feeding areas, distinct from long-distance migration. It looks like a waterfowl "commute," going out in the evening and returning in the morning. However, this was studied in only two species around a lake and may not apply directly to ducks in Japan. How do waterfowl that migrate thousands of kilometers prepare their energy for the journey?
5. How Geese Eat Enough to Migrate Thousands of Kilometers
Greater white-fronted geese are said to sometimes migrate thousands of kilometers without resting. Satellite tracking has reported cases where East Asian populations traveled from northern Miyagi Prefecture, via lakes in Akita and Hokkaido, directly to the Kamchatka Peninsula (Takekawa et al.) [5]. "Thousands of kilometers" is noted as "sometimes," so they do not always fly without rest.
Plant-eating migratory birds like geese have small, simple digestive systems. To sustain long migrations, greater white-fronted geese select high-quality food and eat large amounts. Energy stored in stopover sites is also used for breeding [5]. Even for waterfowl that fly at night, the power to fly is thought to come from feeding time. How do waterfowl know which direction to go in the dark night sky?
6. How Migrating Birds Find Their Direction in the Dark
The indigo bunting, a small bird that migrates at night, uses the center of the rotating starry sky (near the North Star) as a directional clue. In Emlen’s 1967 experiment, when the "north" of the starry sky shown in a planetarium was changed, the birds’ orientation changed accordingly [6].
In the brains of juvenile sooty shearwaters, "head-direction cells" were found that activate when the head points in specific directions. These cells are distributed with a bias toward the north, suggesting they act like a compass (Doshisha University and Nagoya University, 2022) [7].
The migration route might be a "culture" learned from parents. Juvenile light-bellied brent geese tend to return to places they were led by their parents in their first year, suggesting migration routes are transmitted as culture from parents [8]. However, these are examples from small birds, seabirds, and light-bellied brent geese. These three examples alone do not determine what geese and ducks use as clues at night.
7. How to Watch Geese and Ducks Take Off at the Water's Edge in the Evening
Go with an adult to a pond, river, or beach where geese and ducks are present. Look up at the sky for 15 minutes after sunset and observe whether birds take flight. Do not enter the water, and stay with your adult and leave while you can still see your way safely. There may be days when geese and ducks are not present; not seeing them is normal. If you see them, write down the number of birds that flew, the size of the flock, and their calls during the 15 minutes after sunset, so you can compare them later. In the Netherlands, flight was concentrated about 90 minutes after sunset, but this may not be the same at Japanese watersides [4].
If you want to read more, start with the Bird Research Japan poster on the night count in Japanese. The paper by Hawkes et al. is in English, but adults and children can enjoy looking at the figures together [1].