Bravery among birds

Behaviour is the first thing a bird changes with an environmental change. Escape is one of the most important defence mechanisms used by animals in the lower parts of the food chain. It is highly effective in an encounter with a predator, but it is very energetically demanding and time consuming as well. If an animal has to run away too often, it might affect population density of some species. Escape is also the usual reaction to an encounter with humans. Fearfulness of individual species is consistent with their tolerance towards humans and is reflected in their escape distance, which is yet unexplored in the tropics.

The European stonechat, author: Peter Mikula

The data in this study was collected by researchers in nature. When they noticed a bird, they measured how far away it was and then started directly approaching it. The distance at which the bird flew away was measured and compared to other species. It turned out that the strongest predictor of avian tolerance was the starting distance of the approaching human. If an intruder was approaching from a longer starting distance, the bird escaped early after spotting him. That is because monitoring potential danger costs some energy and the individual in danger cannot fully engage in foraging, for example. Fleeing saves the bird from stress caused by a potential predator in sight. Knowing the maximal escape distance is important in defining buffer zones, where species could live without disturbances from human activities.

It was shown that the type of habitat, where tropical birds lived, was another important factor reflecting on their tolerance. The escape distance was longer in rural areas and areas with a lower human footprint index, which means that the tolerance was lower. This shows that the presence of humans changes how birds assess risk. This pattern has already been proven outside the tropics. While brave birds settle down in urban areas, shy individuals prefer areas with lower human activity. Interestingly, fearfulness is to some extent heritable, which means that it is difficult for some individuals and species to get accustomed to humans.

The history of species spreading is another factor explaining different tolerances across bird species – some populations lived with people for a longer time period. The presence of people shortens the escape distance of birds and there are lower activities of predators in urban areas – both of these factors increase bird tolerance. When birds and humans coexisted for a longer period of time, it might have led to local adaptation within the population and therefore higher tolerance towards humans. That would explain why birds differ in their escape distances within species.

Body mass and wing shape were also crucial in explaining bird tolerance towards humans. Bigger birds escape earlier than smaller birds. Larger animals also live longer, have slower reproduction, and prioritise survival over current reproduction (they exhibit a so-called slow pace-of-life). By the way, this is one of the reasons why bigger birds have elevated extinction risk, as a consequence of negative human activities. Individuals with longer wings escaped earlier, probably because flight isn’t as energetically demanding for them, therefore they choose it sooner after spotting a predator. Faster escape was also detected for long-distance migratory birds who were less familiar with local environments.

Larger species of savannah birds, such as the Hagedash ibis (Bostrychia hagedash), usually escape predators earlier than smaller species. Author: Peter MIkula

Furthermore, bird tolerance differed during the year. Birds were less tolerant towards humans in the wet season when compared to the dry season. The breeding season of tropical birds is not yet well described, but a possible explanation for the varying bird tolerance during the year is, that the tropical birds studied mainly bred during the rainy season, and therefore had longer escape distances during that time. Birds escape earlier in the breeding season in order to eliminate predation and to lower the of revealing the nest’s position.

It was due time to study the effect of human exploitation of ecosystems on tropical birds. This study, the first of its kind, has shown that bird tolerance is most importantly affected by the starting distance of the approaching human, the extent of human presence in the area, biometric traits, and the annual cycle. The results of this study may be used in nature conservation, especially in defining buffer zones for birds, where they wouldn’t be disturbed by humans.

Eliška Leštinová

Mikula, P., Tomášek, O., Romportl, D., Aikins, T. K., Avendaño, J. E., Braimoh-Azaki, B. D. A., Chaskda, A., Cresswell, W., Cunningham, S. J., Dale, S., Favoretto, G. R., Floyd, K. S., Glover, H., Grim, T., Henry, D. A. W., Holmern, T., Hromada, M., Iwajomo, S. B., Lilleyman, A., Magige, F. J., … Albrecht, T. (2023). Bird tolerance to humans in open tropical ecosystems. Nature communications, 14(1), 2146. https://doi.org/10.1038/s41467-023-37936-5