1. What did researchers find in the stomachs of Black-tailed Gulls at a river mouth?
Encyclopedias explain that gulls are omnivores. They eat animal carcasses washed up on the coast, sea fish, and even small mammals and insects inland. They are also said to steal prey caught by other birds [1]. So, what is actually inside their stomachs?
On May 4, 1973, the stomachs of 6 Black-tailed Gulls caught about 1 km (about 0.6 miles) upstream from the mouth of the Sakogawa River in Kanagawa Prefecture were examined. The contents included fish such as horse mackerel, cutlassfish, and silversides (small silvery fish), a type of shrimp, sea slaters, and soldier fly larvae [2]. All of these are creatures found near water.
The weight of the stomach contents ranged from 26.4 to 100.2 grams, with an average of 47.9 grams. Since one egg weighs about 60 grams, the average amount is lighter than a single egg. This survey was conducted because a local fishing cooperative was worried that gulls might be eating sweetfish (ayu). No sweetfish were found in the stomachs [2].
However, these results come from only 6 birds on a single day. We cannot say this represents the diet of all Black-tailed Gulls. If a bird that eats insects as well as fish is found in one place, what does it eat in other places? Next, we look at rice paddies in Hokkaido.
2. Why were Great Black-backed Gulls eating tadpoles in Hokkaido rice paddies?
It is reported that Great Black-backed Gulls began coming to rice paddies in Shimamaki Village, Hokkaido, in 1994, and their numbers increased year by year [3]. Sea birds started visiting rice fields.
In June 1999, the stomach contents of 25 gulls were examined. For males, by weight, fish made up 79.7% and tadpoles made up 20.2%. For females, fish eggs made up 62.0%, tadpoles made up 29.0%, and fish made up 8.4% [3]. Of the 25 gulls, 9 had eaten tadpoles [3].
These percentages are based on the weight of the items in the stomach, not the number of items eaten. The fact that fish eggs made up more than 60% for females is also a striking number.
The researchers who conducted the study speculate that the behavior of eating tadpoles is likely passed from gulls that have eaten them to those that have not [3]. This is strictly a guess. Still, if the way of eating is passed to others, it suggests that gulls are birds that can learn by watching.
On the coast, shellfish are also an important food. How do they break the hard shells?
3. How do gulls in Japan drop shelled prey to break it open?
"Food dropping behavior," where prey with shells is dropped from the air to break it, has been frequently reported in crows and gulls. In Japan, shell dropping has been observed in 17 prefectures, from Hokkaido to Okinawa. Among gulls, Black-tailed Gulls, Herring Gulls, and Great Black-backed Gulls have been confirmed [4].
The height from which they drop the food is often 3 to 10 meters, though there are examples of 2 meters or less and 15 meters. If one floor is about 3 meters, this is roughly the height from a first-floor roof to a three-story building. They dropped the food onto hard surfaces like paved roads and seawalls at 40 locations, and onto soft surfaces like sandy beaches and grassy areas at 9 locations [4]. In Australia, gulls have been reported dropping food from about 25 meters, but in Japan, no examples exceeding 20 meters have been observed [4].
The shellfish dropped include blue mussels, scallops, clams, pond snails, and whelks. In 22 locations, gulls used shellfish discarded from fishing operations or those being unloaded [4].
This paper considers that mastering shell dropping likely requires advanced learning, such as identifying edible items and choosing the dropping location and height [4]. However, out of 46 locations surveyed through interviews, gulls were confirmed at only 7 locations, while crows were confirmed at 45. Shell dropping is not a skill unique to gulls.
If they learn by watching, who is a familiar "model" for gulls? Next is an experiment about gulls watching human food.
4. Do Herring Gulls in Brighton choose food by the same color people are eating?
Finally, we introduce an experiment with gulls living near people. On a beach in Brighton, UK, Herring Gulls were shown two types of food that differed only in color [5]. When nearby people ate food of one of those colors, it is reported that 95% of the gulls' pecking was directed toward the food that matched the color of the human food [5].
This 95% is the ratio of pecking times, not the ratio of the number of gulls. The study was published in the scientific journal Biology Letters in 2023 (a study by Graham et al.). Here, we are checking the publicly available abstract [5].
Watching people eat and choosing the same color. If gulls use human food as a clue that it is safe and edible, this is also a form of "learning by watching." Tadpoles in rice paddies, shell dropping on the beach, and human food. Behind the wide variety of gull diets, there seem to be examples involving observation and learning.
We cannot say from this experiment alone that gulls understand all human food. Still, from the examples so far, we can see that the phrase "bird that eats fish" is not enough.
5. How can you observe gulls at ports and beaches?
If you see gulls at a port or beach, try observing the following from a distance.
What to look for: Is what they are holding in their mouth a fish, a shellfish, or something else? If a gull is rising into the air and dropping something, look at whether the dropping spot is hard or soft, and try to estimate the height. Whether other gulls or crows are doing the same thing in the same place is also a clue.
For safety: Do not give food to gulls or get too close to them. At the seaside, be careful of waves and footing, and go with an adult.
Read the original sources: Among the materials for this article, those readable in Japanese are the survey of Black-tailed Gull stomachs at the Sakogawa River mouth [2], the survey of rice paddies in Hokkaido [3], and the distribution survey of shell dropping [4]. Some are short reports, so looking for the source of the numbers helps you understand the strength of the words in the article.