1. Why the Bush Warbler Is Hard to See Even When You Hear It
Bush warblers rarely come out into open, sunny places; they live in thickets [1]. Even when their voices are clearly heard, their bodies are easily hidden by leaves. This habit matches the feeling of "hearing the voice but not seeing the shape." However, they are not birds that can never be seen. A guide from Niigata Prefecture states that they prefer to live in urban thickets, parks with many planted trees, and gardens [2]. This means they are near us, just hard to find. So, what kind of place is a "thicket" for a bush warbler?
2. Bush Warblers Prefer Bamboo Thickets Over Dark Forests
Dictionaries state that bush warblers prefer bamboo thickets, bamboo groves, and low shrub forests, and do not go deep into dark woods [3]. It seems that for bush warblers, what matters is not the depth of the forest, but having leaves and branches growing thickly at a low level. Their living area is wide, including plains, mountain forests, highlands with many bamboo and low shrubs, and urban thickets, parks, and gardens [2]. In Japan, they are found from Hokkaido to Kyushu and Okinawa; in the world, they are also found in northeastern China, the Russian Far East, and the Korean Peninsula [4]. According to an explanation by Shoji Hamao of the National Museum of Nature and Science, breeding grounds range from near coasts to alpine zones, but they especially like thickets at forest edges and open places in the mountain and sub-alpine zones [4]. An interesting recent change is noted: it seems the number breeding in hills and lowlands is increasing because thickets have grown in river improvement areas and abandoned farmland. However, this is a guess using the word "probably" [4]. Changes in human life may be changing where these thickets live. But do bush warblers stay in the same place all year? If you go to a park in winter, things are a little different. Because their distribution and environment are wide, it is safer to read about them by pairing location, season, and the specific area studied, rather than summarizing "how bush warblers live" in one phrase.
3. Winter Calls and the Move from Mountains to Plains
There is an image of the bush warbler as a spring mountain bird, but south of Honshu, it is known that some individuals that were in the mountains move to plains or warmer areas in winter [4]. From autumn to winter, they are also in low thickets in gardens and parks [4]. Surveys that attach tags (foot rings) to birds and release them have specific records. One bird released on Tobishima in Yamagata Prefecture was found on Ishigaki Island in autumn. A male released in a hill in Saitama Prefecture in May was caught again in Yokohama in November of the same year [4]. However, these are individual records and do not show the overall trend. In fact, not all bush warblers move. At Mount Tsukuba, the same individual was observed all year, and Niigata Prefecture materials list them as "resident birds" [4][2]. Depending on the place and the individual, some birds move and some do not. The voice heard in winter is a ground call that sounds like "jya, jya," called the "sasa-naki" (bamboo cry) [1]. Males also make the sasa-naki in winter, but they do not do so during the breeding season [4]. Even for the same bird, the sounds made in winter and spring are different. So, what is the spring "ho-ho-ke-kyo" for?
4. What Is the Male Bush Warbler's Spring Song "Ho-ho-ke-kyo" For?
"Ho-ho-ke-kyo" is the song of a male during the breeding season. It is said to be a voice to call females and mark territory [1]. It is a voice with a different role from the winter ground call, and it is often heard from early February to late August [5]. However, there are differences by region, so it is not a bird only for spring. Bush warblers are polygamous; males continue to sing throughout the breeding season to attract as many females as possible. There is a record of one male pairing with 6 to 7 females one after another in a single breeding season [4]. This is the highest recorded example, meaning one after another, not at the same time. The "tani-watari" (valley crossing), which sounds like "piruru... kekyo-kekyo," is also a male's voice. It was once thought to be an alarm call, but explanations state it is better not to decide it is definitely an alarm call [4]. If the male calls with his voice, what does the female do? Next, let us look at the nest secretly built in the thicket.
5. Low Pot-Shaped Nests and Female-Only Childcare in Bush Warblers
The nest is pot-shaped (like a rugby ball) made of dead bamboo leaves, with an entrance on the side or top [4]. The location is in low parts of the thicket. In a survey at Myoko Highlands, the height from the ground averaged 54 cm (range 5–140 cm) [4]. This is about knee-to-waist height for a young child. However, this is an example from one survey area; not all nests are at the same height. Nest building, incubating eggs, and raising young are all done by the female alone [4]. There are 4 to 6 eggs in one nest, and they are chocolate-colored. The incubation period is about 15 days, and it is about 13 days until fledging. Adding these, it takes about 28 days, or roughly 4 weeks, from the start of incubation to fledging. On the other hand, there is a low example of a 27% fledging rate, and the main cause is said to be predation by other animals. However, this is a value from a survey area, not a national figure [4]. Another relationship is with the lesser cuckoo (hototogisu). Nests used for breeding from late May to early July are often parasitized by cuckoos. Because bush warblers start breeding early, their first or second sets of young can fledge before the cuckoos arrive, according to explanations [3]. By the way, what does a bush warbler actually look like? Note that nest building, incubation, and chick rearing are done by the female only [4].
6. The Bush Warbler's Dull Greenish Brown Color vs. Bright "Uguisu Color"
The bush warbler's body is olive brown on top and white to beige on the bottom [4]. It is a dull greenish brown, different from the bright yellow-green so-called "uguisu color." The total length is about 14–15.5 cm, slightly shorter than a pencil. The weight is 14.8–22.3 g for males and 10.0–13.7 g for females [4]. Since a 1-yen coin weighs 1 g, females weigh as much as 10 to 14 coins, and males 15 to 22 coins. It may be that the bird is hard to see not only because it lives in thickets, but also because its body color blends easily with the background. However, this is a guess for this article and is not written in the materials.
7. How to Try Finding a Bush Warbler Near a Thicket
In nearby thickets, look at low trees in parks and gardens, or places with many bamboo, from a distance. If you hear a voice, stop and look at the direction of the sound or the movement in the thicket. Do not enter the thicket, and do not approach if you find a nest. To distinguish voices, if you hear sounds like "jya, jya" in addition to "ho-ho-ke-kyo," note the date and place. Arranging what voices were heard by month may help you see the difference between winter sasa-naki and spring songs in your own records. If you read materials, the explanation by Shoji Hamao of the National Museum of Nature and Science (Bird Research News) details movement, voices, nests, and childcare. It has an English summary and a Japanese main text. The next question is: How does cuckoo brood parasitism affect bush warbler childcare?