The relationship between nest location selection of Barn swallows (Hirundo rustica) and human activity and residence

The breeding process of birds is dangerous, for both the offspring and the parents; birds exhibit sensitivity to conditions such as breeding season and nest site. In particular, there is a close relation between the location of the nest chosen by the parents and the success of breeding¹, and physical environmental factors², surrounding species, nest predation^3,4, and distance from food sources are important factors determining reproductive success^5–7. Consequently, birds have evolved for habitat selection where factors affecting reproductive success can be satisfied⁸ and for selection of various nesting locations such as cavity nest, shrub layer, water surface, and cliffs in order to suit the breeding ecology.

In general, while for many birds, selection of nesting locations is based on avoiding interference or influence of interspecific or conspecific species, barn swallows tend to build their nests within close proximity to human living spaces. Barn swallows, which inhabit locations worldwide, including Eurasia, America, and Africa, build their nests on the walls of doors, houses, barns, and warehouses, which are man-made structures, not natural elements such as trees or shrubs^9,10. Barn swallows are homing birds^11,12 approximately 40% of the population returns to their previous breeding grounds. They are friendly to human, so appearing frequently in historical events and traditional fairy tales in many countries. In medieval religions, swallows are sometimes regarded as being close to the gods, and in Europe, swallows are known as spring's herald¹³. They can also be helpful in farming by consuming pests and their species is so close to that of humans that a poem was written about swallows. In Korean legend, there is also a fairy tale about a swallow who gave a seed that will produce treasure to a good farmer who healed his injured leg.

Some bird species share a habitat with humans. By living close to humans, they gain a reproductive advantage. Humans have been farming for a long time; thus, sparrows, who eat mainly grains, are aided by the supply of food around humans. The great tit (Parus major) is a cavity nester, breeding in holes pre-made by other species and artificial nest boxes near locations of human habitation. The domestic pigeon, a representative species, has adapted to urban areas that are densely populated by humans. They can be observed breeding in various places such as crevices in buildings and under bridges. They do not share a habitat with humans, however, there are cases involving interaction with humans. In the case of the Greater honeyguide (Indicator indicator), honey in the hive is a staple food, however, they are not able to consume the honey from the hive themselves, therefore, they lure humans to dig out the hive, and then eat the remaining honey from the hive. The question of whether the barn swallow chooses to nest closest to humans because it can obtain any advantage associated with humans should be considered.

The barn swallow can build nests in various places, such as bridges and cliffs, as opposed to a species that only cavity nests, like the great tit. Nevertheless, considering the reasons why swallows choose to breed in close proximity to humans in human settlements and what the benefits might be, one possibility could be nest predation. Nest predation is the most common reason for reproductive failure in birds^14,15, and in order to avoid this, birds that are prey in the natural environment may select a breeding site located near a higher-level predator, a protective species, with the expectation that they will be protected from predators that pose a potential threat to their reproduction^16,17. In this way, it can be supposed that swallows utilize humans as a pattern of adaptation, recognizing humans as higher-level predators than their predators and reducing the rate of predation from predators that pose a threat to their reproduction.

In this study, a comparison of the characteristics of the nest location chosen by swallows as a breeding site and the reproductive ecology such as the success or failure of breeding was performed in order to examine the causes of barn swallow breeding around humans. In particular, evaluation of various factors, including direction, location, material, and distance from the nesting wall, and food source, was performed for analysis of the relationship between humans, such as human residence in the nesting place and human activity in the nesting place. In addition, an attempt was made to understand the reasons for inhabiting areas near humans through assessment of breeding results such as changes in breeding time and success rate in order to clarify the meaning of nest location selection.

In this study, barn swallow breeding was found to occur within close proximity to human dwellings, and the significant influence of humans on the reproduction of barn swallows has been verified. In comparison of human occupancy with the number of breeding nests, attempts at breeding were more likely to occur in buildings inhabited by humans during the breeding season. Even when a building was empty, human activity was observed in some cases, so that cases involving visible human activity and those not involving human activity were classified as an empty building; a significantly higher number of nests were located in buildings with human activity. Other factors that might affect selection of the nest location (building type, orientation, and wall material where the nest is located) were not regarded as having a significant effect on the selected location of the barn swallow nest. Swallow nests were located mainly in houses and senior centers, buildings with significant human activity. Most of the walls where the nest was located appeared to be facing south. Some birds choose a southern nest entrance orientation in order to gain the advantage of maintaining the temperature of the nest¹⁸, however, there are differences among species^19,20. In addition, according to the results of this study, approximately 96.6% of the barn swallow nests were located under the structure that serves as the eaves; due to its structure, sunlight does not reach either the inside or the outside of the nest, thus it is not possible to obtain this advantage. The wall material where the nest is located is not significantly different from wood, brick, and cement, therefore, it is presumed that it has no significant effect on selection of the nest location. This fact is supported by the findings of studies^21–23 that indicated no preference for directionality in nest selection of some birds. In this study, no significant relationship was observed between the number of barn swallow nests and the distance from potential resources available to barn swallows, whereas the result from comparison of the relationship with the number of nests according to the frequency of human activity showed a significant relationship. In light of these findings, it can be inferred that the presence or absence of humans exerts a more pronounced influence on the choice of a breeding site for barn swallows when compared to other factors. The following possibilities can be inferred from the current result showing that barn swallow nesting occurs in buildings where humans are active and in areas with significant human movement. It is hypothesized that building a nest near humans provides protection from predators and the rate of breeding success is high, with active selection of a location with significant human activity. Moller²⁴ reported that the nest predation rate of barn swallows nesting indoors was lower than the nest predation rate nesting outdoors. Therefore, the conclusions drawn from the correlation between human activity and nest count suggest a potential association with the presence of predators.

Predators have a profound effect on bird reproduction^14,25. Not only do they feed on eggs and chicks, but the presence of predators also limits the parental activity required for reproduction. In fact, many studies have reported on changes in prey species caused by predators^26–28. In addition, it is often observed in nature that various birds protect their nests by establishing species of predators as neighbors who may pose a threat to their potential predators²⁹. For example, there is variation in the number of nests of the azure-winged magpie (Cyanopica cyanus) depending on the rate at which Japanese lesser sparrowhawks (Accipiter gularis) expel their natural enemies³⁰. Another example is that of great snow geese (Chen caerulescens atlantica), who consider larid and snowy owls (Bubo scandiacus) as protector species that will protect their nests and therefore breed near them³¹. When choosing their breeding grounds, barn swallows build their nests in close proximity to humans because they will provide protection from predators such as snakes, rodents, owls, bats, and raptors^24,32. When breeding was attempted, there was no significant difference in the number of eggs in nests with and without human activity, however, a significant difference was observed between the number of offspring and the rate of reproductive success. This means that where there is a high level of human activity, there is better hatching of offspring as well as more successful fledging of offspring. Nesting within the range of human activity would limit the activity of potential predators of barn swallows, which would reduce the effect of predators on the brooder activity of barn swallows, thereby increasing the rate of reproductive success²⁴. In this case, it is possible that barn swallows were using humans as a means of avoiding predators.

Barn swallows not only build their nests near a door, through which humans often enter and exit, but also in locations that are within reach of humans. This probably occurs because barn swallows do not perceive humans as a threat to their nests. Barn swallows have traditionally been animals that have a close relation to humans. The barn swallow has a positive association in many European countries; it is a herald of spring and is familiar enough to make a frequent appearance in beliefs and superstitions of the past¹³. They stayed from March until September and believed that they would observe human behavior and report back to the gods. Therefore, according to a shamanic belief, swallows are objects of worship and if they overdo it, the family will be cursed with bad luck. These folk beliefs have faded, however, there is still a belief among local elders that barn swallows should be protected. In the case of our study sites, elderly people aged 65 and over account for 42% of the population, so that the sentiment toward protection of barn swallows is still high in the surveyed area. In addition, it is a representative animal associated with emotion that has been interacting with people for a long time due to its homing instinct^9,12 which memorizes the nest where it lived, so that it returns in spring.

The swallow (Hirundinidae) nest has various forms, including those of the studied species, Barn swallow³³. Tree swallow (Tachycineta bicolor) builds the nests using moss and grass, and the Bank swallow (Riparia riparia) makes holes in soil walls, building its nest with twigs and weeds. The cliff swallow (Petrochelidon pyrrhonota) builds its nest in the shape of a gourd with a narrow mouth in dry mud on the side of a cliff. Although they belong to the same family of swallows, nest material and shape vary according to species, thus they are capable of rapid adaptation to a specific breeding environment. Based on characteristics such as selectively building nests in areas with active human presence and exhibiting positive effects on reproduction in places influenced by humans, it is inferred that the barn swallow has also shown rapid adaptation.

J.K.L. and O.S.C. conceived the experiments. M.K. acquired the data. J.K.L. and M.K. analyzed and/or interpreted the results. All authors reviewed the manuscript.

This article was funded by Incheon National University.

The dataset is available from the corresponding authors upon request.

The authors declare no competing interests.