1. Swallows eat insects that are flying
From around 6 a.m., parent swallows leave the nest to find food. According to Honda Woods, swallows catch flying insects such as mosquitoes, crane flies, and dragonflies while they are in the air, a method called "flying catch" [2]. The parents take turns bringing food back. When they approach the nest and call, the chicks open their mouths. A weather news article lists flies, horseflies, dragonflies, and flying ants as the main diet of swallows [3]. The basic answer to the question is that swallows catch flying insects while flying.
However, it is difficult to see exactly which insects are being brought and in what quantities. Small insects disappear into the nest in an instant. The next section explains how students in Nara overcame this challenge.
2. Video was not enough, so they analyzed DNA from droppings
Students at the Junior High School attached to Nara Women’s University observed swallow breeding in a restaurant district in Ikoma City, Nara Prefecture, from 2018 to 2024 [1]. In 2022, they tried to identify the food using videos of feeding and filters for droppings, but the contents were not clear. Therefore, for the 2023 and 2024 academic years, they switched to analyzing parts of the insect DNA found in the chicks’ droppings. They asked a specialized company to analyze the DNA sequences and compared them with known data to identify insects down to the species or genus level. The insects found were mostly those that move by flying [1].
This method has weaknesses. When chicks are small, parents carry droppings out of the nest, so droppings could only be collected on 14 days in 2023 and 8 days in 2024 [1]. Also, this was observation at just one location. It cannot be said for certain that this represents the diet of all swallows in Japan. Still, even from a few collections, changes in insect types became visible.
3. Detection rates of termite relatives and changes in dragonflies and butterflies
When droppings were compared by month, the types of insects were clearly different [1]. Insects from the orders Odonata (dragonflies) and Lepidoptera (butterflies and moths) were rarely found in May but were found often in June and July. Other insects were also detected more frequently as the season progressed. Reading the graphs in the report, relatives of termites (labeled as order Blattodea in the report, with a note that these are all termite relatives) were detected at about 45% in May and about 65% in June. Flies (order Diptera) were high in May and July, reaching about 90% in July. These numbers are approximations read from bar graphs. The report does not give a precise definition of these percentages, so they should be understood as "insects frequently found in that month’s droppings."
The flying termites were likely winged termites. As the observation period got later, the number of times food was brought to the nest decreased, and the days until fledging became shorter. The researchers suggested in their discussion that the nutritional value of the insects may have increased. However, this is just a suggestion. This study cannot determine if swallows changed their choices or if the insects in the sky changed.
4. A study in a Canadian airport examined swallow stomachs
The contents of the stomachs of 31 barn swallows examined at Vancouver International Airport in Canada in 2013 were reported in the academic journal *Canadian Field-Naturalist* [4]. The average percentage of insects by number was: order Hymenoptera (including ants) 40%, order Diptera (flies) 31%, order Hemiptera (true bugs) 15%, and order Coleoptera (beetles) 12%. Ants alone made up 29% of the food. Ants are included in Hymenoptera because they are classified in the same order as bees and wasps. In the Nara students’ study, termite relatives were common, but these ants are a different group from termites.
It would be an overgeneralization to say "40% of a swallow’s diet is bees and ants." The study only looked at 31 birds around an airport. If the location or method changes, the breakdown would likely change too. The next question is: did the proportions of insects in the stomachs match the proportions of insects flying in the air?
5. 80% of trapped insects are tiny, but swallows chose 4-8 mm ones
In this study, insects in the air were collected with traps and compared with the contents of swallow stomachs. "Choosing" here means comparing the proportion of insects in the air with the proportion in the stomach [4]. The result showed a strong preference for insects 4 to 8 mm long. About 80% of the insects caught in the traps were smaller than this. Swallows avoided insects that were too small or too large. By type, they strongly preferred Hymenoptera (mostly ants) and tended not to choose Diptera, Coleoptera, or Lepidoptera.
In the stomachs, Diptera (flies) made up 31%, which is noticeable, but this does not count as "choosing" because flies are also very abundant in the air. If swallows just ate what was most common, the stomach ratios would match the air ratios. Only Hymenoptera were eaten more often than their abundance in the air. This answers the question from the previous section. However, since this is a result from a Canadian airport, we cannot say for certain that Japanese swallows also "love 4-8 mm insects."
Safety Note: Hymenoptera (mostly ants) were eaten at a rate higher than their availability in the air.
6. Compare 4-8 mm on a ruler with insects outside your window
Look at 4 mm and 8 mm on a ruler. Each mark is 1 mm. This is the size range of insects chosen by the Canadian swallows [4]. If you see a small insect outside your window or in a park, try to guess if it is larger or smaller than 4 mm without touching it. You can see how the number "most trapped insects are smaller than 4 mm" matches what you see.
If you see a swallow nest, count how many times the parents return to the nest in 10 minutes from a distance. Do not approach the nest, and do not touch the droppings underneath. The Nara study reported that male parents fed the chicks more times per hour than females [1]. You can count the visits, but you cannot see what insects are being carried. The original reports are available in Japanese. The report from Nara Women’s University includes graphs on feeding behavior by sex that were not mentioned here [1].