Popular Science: Cemetery or urban park? Where do birds have more fear?
You can surely imagine that living conditions in nature and urban areas are very different and birds living close to humans thus need to modify their behavioral patterns. This includes their response to potential predators, specifically, how close they allow an intruder to approach them before they fly away. This level of risk-taking is very often measured as flight initiation distance (FID) and it quantifies the compromise between the cost of too early (and maybe not necessary) escape and benefits from staying (but including the possibility of being threatened). As the abundance of predators is usually low in towns and cities, the key factor enabling successful colonization and exploitation of urban areas, while reducing the energy and other costs of unnecessary escapes, is adaptation to human disturbance.
Despite the known negative impacts of urbanization on biodiversity, many bird species can actually benefit from living in human proximity, as they may find more food and encounter fewer predators there (which is reflected in the shorter FID in urban than in rural areas). Above all, parks and cemeteries, with respect to their infrastructure (buildings, monuments, lamps) and usually very diverse vegetation, offer space for new breeding and food opportunities.
The team sampled 2139 FID estimates of 44 common bird species in 79 parks and 90 cemeteries in the Czech Republic (Prague), France, Italy and Poland. Both these types of urban environments are frequently visited by people, but in parks they are usually more active, louder and potentially accompanied by other bird stressors such as screaming children or pets. The microhabitat structure of these two places can also differ, for example, with respect to their vegetation structure and the number of structures created by people. Even though all this can have an influence on the anti-predatory behavior of birds, the differences in escape behavior of birds between cemeteries and parks have so far not been so deeply studied.
The actual experiments consisted in a researcher approaching a bird individual (usually sitting or foraging on the ground) from a distance of more than 30 meters. The FID was then the distance between the observer and the point, where the bird began to flee.
In all countries, birds allowed people to approach closer in cemeteries than in parks. This can also be associated, however, with the fact that natural predators, such as raptors, are also less present in cemeteries, which may result in a shorter FID there. It was also shown that the larger the area of the park or cemetery, the closer one could get to the birds; on the other hand the higher the tombstone and human density, the earlier they flew away. Regarding the tombstone coverage, the higher vigilance and earlier escape are probably caused by the fact that a tombstone itself is not a convenient place, where one could eventually hide (to squeeze oneself for example directly into a cavity in a tombstone is usually not possible) and at the same time the higher tombstone density is connected with a lower probability of the existence of natural hiding places in the proximity (tree cavities, dense shrubs).
Even though the FID is usually shorter at places with a higher human presence and activity, because the birds adapt to them and can profit from them, the results of this study show that the situation can be a little more complex. The longer FID found in parks might have been influenced by other unmeasured factors, such as the louder behavior of people and more frequent occurrence of pets (dogs and cats) in parks than in cemeteries.
The results were similar for the most common bird species, countries and level of surrounding urbanization. The study thus shows that birds are able to adapt their behavior to different types of urban areas, based on local environmental conditions, including the character of interactions with humans.
Morelli F, Mikula P, Benedetti Y, Bussière R, Jerzak L, Tryjanowski P (2018): Escape behaviour of birds in urban parks and cemeteries across Europe: Evidence of behavioural adaptation to human activity. Sci Tot Env, 631-632: 803-810.