Eastern Spot-billed Duck Ducklings, a Koi with Cranial Swelling, and a Newly Identified Brood in an Urban River
An unusual koi with a swollen forehead, Eastern Spot-billed Duck ducklings competing over a submerged leaf, the discovery of a previously unrecorded duck family, and confusion caused by several similar-looking broods made this urban river survey unexpectedly complex.
This field record documents the behavior, identification, movement, and development of several Eastern Spot-billed Duck families observed along the river. It also includes incidental observations of a Common Kingfisher, a possible Reeves’s turtle, an Asian House Martin nesting area, and other birds using the same river environment.
The observations presented here are not a formal scientific study. They are field notes and personal interpretations made by an independent wildlife observer without professional biological training. However, they are supported by continuous daily video recording, individual identification of mother ducks, comparisons with footage from previous and subsequent dates, and chronological evidence collected across multiple days.
Observation Video
The original field-recording video can be embedded here.
The first three minutes contain an edited highlight sequence. The remainder is presented with minimal editing and natural ambient sound so that the order, duration, and context of the observations can be reviewed.
Main Species Featured
カルガモ — Eastern Spot-billed Duck — Anas zonorhyncha
A common East Asian dabbling duck. Males and females have similar plumage, so identifying individual mothers often requires close comparison of facial markings, body shape, behavior, and the patterns of the tertial feathers.コイ — Common Carp / Koi — Cyprinus carpio
A large freshwater fish frequently seen in urban rivers. One red individual observed during this survey had a conspicuous swelling on its forehead, although the cause could not be determined from video alone.カワセミ — Common Kingfisher — Alcedo atthis
A small, vividly colored fish-eating bird. A male was observed near an area used for breeding and feeding.クサガメ — Reeves’s Turtle — Mauremys reevesii
A freshwater turtle native to East Asia. The individual filmed here was provisionally identified as this species, but its shell coloration appeared unusual and the identification remains uncertain.イワツバメ — Asian House Martin — Delichon dasypus
A colonial aerial insectivore that often nests beneath bridges and artificial structures. A known nesting site was checked, but no chicks were confirmed during this visit.スズメ — Eurasian Tree Sparrow — Passer montanus
A familiar bird of urban and agricultural environments. Sparrows were seen using the river, although bathing behavior was not successfully recorded on this occasion.
Searching the Usual Duck-Rearing Area
The survey began in the area where several Eastern Spot-billed Duck families had previously been raising their young. Despite checking the usual locations, including a small sluice gate, no duck families were initially found.
The family recorded as ケf2 had not been seen for several days. Because there was no clear indication of where the family had moved, I continued downstream rather than remaining in the usual observation zone.
At 03:01, a male Common Kingfisher appeared. Although the kingfisher was not the main subject of the survey, its presence confirmed that the area continued to provide feeding habitat for small fish-eating birds.
キf3-11 and the Temporary Uncertainty Over Duckling Number
At 03:49, I located the family キf3-11. Only three ducklings were visible, indicating that the brood had apparently decreased by one.
The family quickly entered dense vegetation. Because this mother frequently uses concealed areas and is unusually difficult to observe for long periods, it was initially impossible to determine whether the fourth duckling had been lost or was merely separated and hidden nearby.
This uncertainty is important. A missing duckling should not automatically be recorded as dead when visibility is limited. Dense vegetation, separated movement, and temporary concealment can all produce an incorrect count.
Later observations during the same survey provided stronger evidence that the reduction was real.
Ducklings Diving for a Leaf and Competing Over It
At 04:01, one of the ducklings dived and retrieved a fallen leaf from beneath the water. The other ducklings immediately became interested, and a brief competition developed over possession of the leaf.
The object did not appear to be an important food item. The behavior instead resembled exploratory manipulation or play-like competition.
Young waterbirds frequently investigate floating and submerged objects with their bills. Such behavior may help them develop feeding coordination, object recognition, and social responses, although it would be inappropriate to assign a precise function based on a single observation.
From a non-specialist observer’s perspective, the interaction appeared playful. At the same time, it may also have reflected the ducklings’ increasing physical ability, curiosity, and competition for objects that attract the attention of other members of the brood.
A Turtle with Unusual Shell Coloration
At 05:07, a freshwater turtle was recorded for an extended period.
Its general appearance suggested a Reeves’s turtle, but the coloration and overall appearance of the shell seemed unusual. Because lighting conditions, algae, mud, age, shell wear, and individual variation can all affect appearance, the video does not provide enough evidence for a confident diagnosis or even completely certain species identification.
For this reason, the turtle is recorded here as a possible Reeves’s turtle rather than a confirmed identification.
A Koi with a Swollen Forehead Approaches a Duck Family
At 07:06, a red koi with a conspicuous swelling on the forehead appeared near an Eastern Spot-billed Duck family.
The enlargement was clearly visible and gave the fish an unusual head profile. Possible explanations could include injury, infection, abnormal tissue growth, a cyst-like lesion, congenital deformity, or another medical condition. However, no reliable diagnosis can be made from external video footage alone.
The koi continued swimming and approached the resting duck family. There was no clear evidence that the fish intended to attack or disturb the ducks. Large carp often approach waterbirds because the birds’ movements can dislodge food or because both species are attracted to the same shallow feeding areas.
At the time of filming, I temporarily suspected that the duck family might be カf1. Because the unusual koi was visually distinctive and the ducks were partly obscured by shadow, I initially prioritized recording the fish while attempting to obtain enough footage of the mother duck for later identification.
A Mother Duck That Did Not Match the Known Families
At 08:08, the mother’s wing pattern became partially visible. The shadows made assessment difficult, but the bird did not appear to match カf.
At 09:15, the single duckling began to wake while the mother was temporarily away from its immediate side. This provided a better opportunity to observe the young bird’s size and feather development.
By 11:06, closer review indicated that the mother was not one of the familiar individuals I had expected to find in this location. Her facial appearance, wing pattern, and overall markings did not agree with カf, ウf, オf, or クf.
The family was therefore provisionally registered as:
サf1-25
This code represents mother サf with one duckling estimated to be approximately 25 days old.
The estimate was based partly on the appearance of dark brown juvenile feathers beginning to emerge around the duckling’s shoulders. Duckling growth can vary with season, nutrition, weather, and individual condition, so the age should be regarded as an informed field estimate rather than an exact measurement.
Summer ducklings can develop quickly, which initially made カf1-22 appear plausible. However, the mother’s markings did not match.
The possibility of later correction remains open. Individual identification in the field is provisional when lighting is poor, only one side of the bird is visible, or plumage patterns cannot be recorded clearly.
Why サf1 Was Treated as a New Family
Several previously recorded families had recently become difficult to locate, including オf, クf, コf, ウf, and Zf. This created a risk that the unfamiliar mother was actually a known individual observed under different lighting conditions or in an unexpected location.
I continued farther downstream than on most recent surveys and checked former resting sites used by duck families in previous years. None of the missing families was confirmed.
A known Asian House Martin nesting area was also inspected. No chicks were visible, although adult birds may simply have been resting away from the nests at that time.
The prolonged absence of several duck families is concerning. In some cases, families relocate beyond the regular survey range. In other cases, repeated failure to relocate a brood may indicate that the ducklings have been lost.
However, absence of evidence is not evidence of total brood loss. Unless the mother is later observed alone or other chronological evidence becomes available, complete disappearance cannot be established with certainty.
カf1-22 Was Found Separately
At 13:11, カf1-22 was located.
This observation was particularly important because it demonstrated that the earlier family and カf1 were not the same brood.
During filming, I had temporarily assumed that the first single-duckling family was カf1 and wondered whether it had moved rapidly upstream. Later review showed that I had encountered two different families:
サf1, seen during the outward journey
カf1, overlooked on the outward journey and found during the return journey
The shoulder feathers visible on the サf duckling were not yet obvious on the カf duckling. This supported the conclusion that the two young birds were different individuals and that the provisional age estimate of approximately 25 days for サf1 was reasonably plausible.
This is a useful example of why immediate field impressions should be separated from conclusions reached after reviewing the complete footage.
When many similar duck families occupy the same river system, location alone is not sufficient for identification. A family may move several hundred meters or more, and two broods of similar age may overlap in the same section of river.
Survey Limitations Caused by Extreme Heat
The downstream search produced relatively little footage compared with the distance walked.
The heat became severe, and my concentration appeared to decline. I repeatedly located potential subjects, including kingfishers and bathing sparrows, but failed to record them effectively.
This is not only a personal difficulty but also a limitation affecting observation quality. Heat stress can reduce patience, visual attention, decision-making, and the ability to maintain accurate counts.
By the later stages of the survey, continuing would have increased both health risk and the likelihood of recording errors. Ending the survey was therefore the correct decision, even though several duck families remained unconfirmed.
Of2-51 and the Development of Older Juveniles
At 14:10, Of2-51 was observed.
The two juveniles had developed into large, well-proportioned young ducks. At approximately 51 days of age, they were approaching the stage when distinguishing them from unrelated juveniles and adults becomes increasingly difficult.
Their appearance illustrates one of the practical limitations of long-term duck-family identification. As juvenile plumage develops and family cohesion weakens, reliable visual tracking becomes harder.
Confusion During an Encounter Involving イf4-50 and Of2-51
At 14:47, イf4-50 was seen on the opposite bank. A sudden disturbance then created a confusing sequence involving the mother, her four juveniles, an unrelated duck, and the approaching Of2 family.
The event was difficult to interpret during filming.
Reviewing the footage later suggested the following sequence:
An unrelated duck approached イf4.
The イf mother entered an aggressive or emergency response.
In the confusion, she appeared to rush toward one of her own young.
The unrelated duck fled rapidly downstream.
Members of Of2 approached separately.
The four イf juveniles reunited on the opposite bank.
The unrelated duck was driven away by Of2.
During the observation, I mistakenly thought the fleeing unrelated duck might be one of the イf juveniles. When all four juveniles later appeared together, the situation seemed inexplicable.
Frame-by-frame review showed that the initial confusion probably began because the mother’s aggressive movement was directed through a mixed group of birds, making family membership difficult to judge in real time.
This episode demonstrates the value of video evidence. Complex interactions involving several visually similar ducks can be misinterpreted in the field but partially reconstructed afterward.
Even so, the description above remains an observer’s interpretation rather than a confirmed account of the birds’ intentions.
Tf8-46: Changing Resting Behavior in Older Ducklings
At 17:33, Tf8-46 came ashore to rest.
The young ducks no longer remained tightly grouped beneath or immediately beside the mother. Instead, they spread out and rested with greater independence.
This change is consistent with their increasing size and more advanced feather development. Very young ducklings depend strongly on the mother for warmth and protection, while older juveniles can regulate body temperature more effectively and often rest at greater distances from one another.
At 20:41, one juvenile was recorded preening and loosening the sheath around a developing pin feather. The feather opened from the tip in a brush-like form, making the origin of the term “pin feather” visually apparent.
This is a valuable developmental detail because it captures the transition from down-covered duckling to feathered juvenile.
キf3 Reappears Upstream
At 22:29, キf3 unexpectedly appeared in the main duck-rearing area.
Earlier, the family had been observed farther downstream, so its reappearance indicated that it had moved a considerable distance upstream during the survey.
The brood still contained only three ducklings.
Because the family had traveled between river sections and the missing fourth duckling did not reappear, the reduction from four to three became much more likely to be genuine rather than a temporary counting error.
It is still not possible to determine what happened to the missing duckling. No loss event was witnessed, and no physical evidence was found.
The correct record is therefore that the brood number was confirmed as three on this date, not that the cause of the reduction was confirmed.
Summary of the Day’s Findings
The most significant result of this survey was the recognition of a new single-duckling family, provisionally designated サf1-25.
The separate discovery of カf1-22 later in the same survey confirmed that two similar single-duckling families were present in different parts of the river.
Other notable observations included:
a red koi with a conspicuous cranial swelling
ducklings diving for and competing over a fallen leaf
a possible Reeves’s turtle with unusual shell coloration
the confirmed reduction of キf from four ducklings to three
developing shoulder feathers in the サf duckling
pin-feather sheath removal in a Tf juvenile
increasingly independent resting behavior among older ducklings
a complex multi-family interaction involving イf4 and Of2
The day also demonstrated how incomplete visibility, similar plumage, rapid movement, environmental heat, and observer fatigue can affect field identification.
For this reason, the records are deliberately written with uncertainty where uncertainty exists. Provisional identifications may be corrected when later footage provides stronger evidence.
About This Long-Term Observation Record
This article forms part of a continuous, day-by-day wildlife observation project.
The videos are preserved with minimal editing, and the recording dates are linked to the publication records. Observations from previous and subsequent days are compared to track brood size, movement, feather development, family identity, disappearance, and reappearance.
This approach does not replace professional ecological research. It is an independent field archive created by a non-specialist observer who records the river almost every day.
The value of the archive lies in its chronological continuity. A single scene may be ambiguous, but observations across multiple dates can provide supporting evidence about whether a family moved, whether a duckling count changed, or whether two similar-looking families were actually different.
Observation Codes
The filming-date code for this record is:
0722F
In this system, 0722 represents July 22, and F represents the year 2025.
Duck-family codes combine:
an identifying symbol assigned to the mother
a lowercase letter representing the observation year
the number of ducklings
the estimated age of the ducklings in days
For example:
サf1-25
This means:
mother duck サ
year code f
one duckling
approximately 25 days old
The katakana identifier サ must not be interpreted as the Latin letter combination “Sa.” Katakana mother-duck symbols and Latin alphabet symbols are maintained as separate identifiers.
A detailed explanation of the filming-date codes, duck-family identification system, and the purpose of this long-term observation project is available here:
https://okasinaikimono.blogspot.com/p/welcome-to-wild-duck-diaries.html
The identification chart used to compare the mother ducks recorded during this period is available here:
https://okasinaikimono.blogspot.com/2025/07/0722f.html
The next day’s video record is available here:
05:07 Japanese Pond Turtle...? I think so, but the shell color looked unusual, so I filmed it for a while.
07:06 A deformed...? Red carp with a swollen forehead... Wait, Spot-billed Duck... a family!!
* Maybe Kaf1? I wasn't sure what to film first, so I focused on the carp for a while.
08:08 The mother's wing pattern... maybe it's not Kaf after all? Hard to tell because of the shadows.
09:15 The ducklings look like they're about to wake up! Watching them wake up... but the mother doesn't return.
11:06 An unfamiliar mother duck. I'll designate this new family as Saf1-25.
* At first I thought it was Kaf, but after checking carefully, I realized it was an unfamiliar mother. Based on the location, duckling size, and brood size, it could have been Kaf1-22, but both the wing pattern and face were different. Uf1-28 was another possibility, but didn't match either. It wasn't Of or Kuf, so this is a new family. (Only Kuf had somewhat similar wings, but the ducklings were far too small for a 12-day-old family.)
* Since dark brown juvenile feathers had begun appearing on the ducklings' shoulders, I estimate they were about 25 days old. (Ducklings grow faster during summer, so that also makes Kaf1-22 somewhat suspicious... If Kaf still turns out to fit later, I'll reconsider then.)
* After this, I searched as far downstream as I've ever gone this year, hoping to find the missing Of, Kuf, Kof, Uf, or Zf families, but found nothing. I even checked every place that had been used as duck sleeping sites in previous years. I can't confirm it, but when I fail to find this many families, it often means they unfortunately didn't survive.
* I also checked the Barn Swallow nests but didn't see any chicks. (The parents may simply have been away.)
13:11 Found Kaf1-22!!
* While filming, I mistakenly believed the earlier Saf1 family was actually Kaf1, so I thought, "Wow, they moved all the way upstream already!? What a huge range!" In reality, I couldn't find Saf1 on the return trip, and instead encountered Kaf1, which I had overlooked on the way downstream.
* The adult feathers on Kaf's ducklings' shoulders still hadn't appeared. When I wrote the earlier memo, I never imagined I had actually seen both Saf and Kaf on the same day. My estimate of around 25 days old for Saf may have been fairly accurate after all!
* On the way back, I carefully searched for Kif3 again but couldn't find them. I also spotted kingfishers and sparrows bathing, but failed to film them every time. There were very few ducks worth filming, so I spent most of the time just walking around. The heat became overwhelming and I started feeling light-headed, so I gave up. (At that point I'd only recorded about 10 minutes... It might have been my worst filming efficiency ever considering how far I walked—and that's even including the turtle and carp footage.)
14:10 Of2-51 – They've really grown into impressive birds.
14:47 If4-50 was on the opposite bank... A fight? Then Of2 arrived, making everything confusing.
* At the time, I couldn't understand what was happening. Looking back now, it seems another duck approached If4, causing confusion. The If mother mistakenly charged at her own ducklings, triggering an emergency response. The other duck fled downstream, and Of2 then approached from different directions. If4 regrouped on the opposite bank, while the other duck was chased away by Of2. While filming, I mistakenly thought the fleeing duck was one of If's ducklings, so I was worried. Then I suddenly saw all four If ducklings together, which made no sense at the time. (Considering the mother had accidentally charged her own ducklings first, it's no wonder I couldn't figure it out. )
17:33 Tf8-46 – Time for a nap on land! Even the ducklings are now sleeping separately.
20:41 A Tf duckling is preening its pin feathers. The feathers are just beginning to unfurl from the sheath at the base, looking exactly like tiny paintbrushes.
22:29 Huh? Kif3 is back in the breeding area... They came upstream! Still only three ducklings...
* I had seen them farther downstream earlier, so this surprised me. Since the missing duckling still hadn't returned even after such a long move, it now seems almost certain that one duckling has been lost.
* By this point I was becoming concerned about heatstroke, so I ended the observation. I can't believe I filmed so little of Kif3 after finally finding them again... (Looking back now, I want to ask my past self why I recorded so little.) I must have been much more exhausted than I realized, so heading home was probably the right decision.
▲▲▲ End of timestamps.
So... while writing these notes, I was surprised to realize that the "new family" was indeed Saf1. I enjoyed walking around the downstream area for the first time in a while, noticed some unusual carp and turtles, and encountered several unfamiliar sights—but there were hardly any duck families or birds at all. The heat was really getting to me as well. Maybe the birds were hiding from the heat too...
The family I'm most concerned about is Lf4. I haven't seen them recently... I wonder where they've gone.
* There are simply too many duck families to identify by memory alone, so I keep track of them using each mother duck's tertial feather pattern and identification code.
Reference list:
https://okasinaikimono.blogspot.com/2025/07/0722f.html
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