Urban areas are growing and their ecosystems are becoming increasingly important in ecological research and conservation management. These ecosystems are highly dynamic and in constant evolution. New species steadily invade cities and those that already occupy these ecosystems have to learn to live together with the new species. Therefore, the success of a species in an urban area is determined by mutualism, competition and predation, as well as preadaptations for living in cities. It creates an evolutionary laboratory all around us.
Species that are successful in urban habitats and whose abundance in towns exceeds numbers in natural biotopes are called “urban exploiters”. One of the most typical European urban exploiters among birds is the blackbird (Turdus merula). Its synurbanization started in Western Europe at the beginning of the 19th century and its density in urban areas has outgrown its numbers in the woods since the middle of the 20th century.
The blackbird is originally a forest bird. It started its conquest of cities in a comparable environment – parks and cemeteries. But as other species try to live in these spaces as well, the competition becomes quite high. Only a few species are able to adapt to more human-made conditions and the blackbird is one of them. It started to occupy residential areas with fewer and fewer trees and nowadays it is able to thrive in urban and residential developments with only a few shrubs. It has come a long way from the woods.
"Only a few species are able to adapt to more human-made conditions and the blackbird is one of them"
The urban population of the blackbird has differentiated noticeably from the forest norm. One distinction is migration behavior. In most parts of Europe, forest blackbirds migrate south during winter, but the urban ones stay in the city. This is because cities have a warmer climate than rural areas and blackbirds have changed their diet – they have become accustomed to human food and we can often see them scavenging through rubbish. A good rubbish bin is a valuable asset that is needed all the time. Therefore, urban blackbirds maintain their territories throughout the whole year, whereas forest ones only do so during the nesting season. As a result, we can see blackbirds fighting and singing (territorial behavior) in the city during autumn and winter too.
But the rubbish bin is not the only human creation used by blackbirds in the city. Clearly shining street lamps are another. In Central Europe, the best males occupy the brightest lamps from the beginning of the nesting season. The amount of light influences the breeding behavior of birds as males normally start to sing to attract females at dawn. At the whole bird community level, this results in all species singing early in the morning and ornithologists can go out and find which species are present. Each species starts singing at a different threshold of light, but within a species, the first singers are the most successful breeders. Blackbirds have found a way around the natural sunrise and compete for street lamps where there is enough light during the whole night. They can start to attract a partner at 2 or 3 AM; as a bonus, we can now admire their musical abilities during our trip home from a party. This is another difference between the forest and urban blackbirds. However, there could be a great variability among different parts of a city. For example, birds in lit housing estates could sing through the whole night, unlike the ones living in dark parks. Moreover, the street lights enable an extension of the nesting season to up to six months (February – July) compared to four in the forests (April – July) in South and Central Europe because the light conditions for the outset of the mate-attracting behaviors are better in the city.
Yet the city is not all paradise for the blackbird. Predators are among the new inhabitants of cities as well, which causes problems by changing the prey-predator equilibrium. There are two possible scenarios. Because of the lack of natural predators in urban biotopes, we can observe a lower predation rate in towns than in more natural habitats (safe zone hypothesis). Conversely, urban habitats may suffer from a higher predation rate. This could be due to higher abundances of some predators that lack natural enemies in these habitats (e. g. there are some martens in urban areas but no wolves regulating them, so called mesopredator release hypothesis); alternatively low species diversity has led to a situation in which predators are able to learn more effectively where to find prey because of less variability in possible locations where prey naturally occur. It is quite difficult to adapt generally to the predation pressure because urban areas represent heterogeneous units isolated from each other. Therefore, a considerable variability in predators’ community composition, and subsequently in predation pressure, is exhibited among cities and across a gradient of increasing urbanization.
This is exactly what has happened to blackbirds. Their abundance in cities has been falling since the 1980s. High nest predation, mainly by corvids (commonly known as the crow family), is the most frequently asserted reason. The increasing number of corvids, mainly magpies, is noticeable in towns throughout all of Europe and these nest predators can strongly influence passerine (perching birds) populations. Corvids are known as intelligent birds that use “searching imagination”, a mental tool that helps them find places where prey will most likely be present. This means, for example, that they look for blackbird nests in dense coniferous trees, rather than in deciduous shrubs where nests are less hidden but there are fewer of them.
The clash of these two species started an interesting evolutionary race coined the Red Queen’s Race. To maintain equilibrium in numbers, both species must rapidly change their behavior. Blackbirds have a tendency to nest in coniferous trees and shrubs. Magpies focused their search in these places. Blackbirds began to nest more often in deciduous shrubs where, although the nests are more visible, magpies hadn't yet foraged for prey. As a consequence, magpies gradually changed their searching imagination and the predation pressure on nests in deciduous species started to increase. Blackbirds tried to escape again, this time building nests on buildings, mainly balconies. Again, magpies followed. Then blackbirds started to nest inside of the buildings, in basements and pram rooms. This is their small victory nowadays. Nevertheless, magpies will surely find a way to overcome a fear of such close interaction with humans. This whole process took only 20 years in the Czech Republic.
Originally, the high predation pressure in parks caused by the presence of diverse predator communities pushed the blackbirds closer to human and built environments where nearly no predators were. However, dwelling in residential estates has been no more successful as avian predators of open landscapes also adapted and invaded these areas. Species suffering from such traps could defend themselves by utilizing different or rare nest sites as a means to preventing nest predators from being able to direct their search to a certain type of site. Blackbirds tried to escape on to buildings and, for the time being, in to buildings. It is really the safest place for them at the moment, but for how long?
Source: Tvergastein Journal
This article is culled from daily press coverage from around the world. It is posted on the Urban Gateway by way of keeping all users informed about matters of interest. The opinion expressed in this article is that of the author and in no way reflects the opinion of UN-Habitat.