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Avian species ‘sink’ into urban ‘traps’

TreeTake is a monthly bilingual colour magazine on environment that is fully committed to serving Mother Nature with well researched, interactive and engaging articles and lots of interesting info.

Avian species ‘sink’ into urban ‘traps’

Together with climate change, urbanization is considered one of the largest threats to wildlife, including the persistence of many bird species...

Avian species ‘sink’ into urban ‘traps’

Thinking Point

The urbanised world may even charm some species into coming over, but soon they get trapped and fall a prey to its various ills including high rate of nest predation, poor nutritional value of food sources, exposure to high pollution levels, and high incidence of collisions with windows and ACs...

AK Singh

Urban habitats and landscapes are markedly different from nonurban “natural” habitats. The major difference is the transformation of the land, from natural green areas to anthropogenic structures and impervious surfaces. To survive in the urban habitat, birds are forced to either accept or avoid the new conditions. In addition, the urban sprawl has led to a highly fragmented landscape, with islets of suitable bird habitat surrounded by highways and buildings that frequently act as barriers, even for mobile creatures such as birds. These altered conditions have changed the avifauna dramatically, with many species vanishing once an area is urbanized, thus resulting in a significant loss of local biodiversity. However, some species seem to thrive in the city, and these urban-dwelling species often show pronounced phenotypic differences (e.g., in behaviour, physiology, and morphology) to their rural conspecifics. These phenotypic changes have been linked to specific urban selective drivers such as air pollution, artificial light at night, noise, different kinds of food, different predation pressures, and human disturbances. However, these drivers are often confounded, and it is hard to separate one urban factor as the main driver for the differentiation.

Kaushalendra Singh, Wild-lifer and Ex-member, Wildlife Board, Uttarakhand, says: “If urbanization means chopping-off trees, filling-up natural water bodies, mindless construction and vehicular movement leading to pollution, then it definitely affects not just birds but the biodiversity of a place. As far as impact of urbanisation on birds is concerned, we need not go far. If we look at enormous construction activity going on at Shaheed Path in Lucknow, we will get all the answers:

1. Construction of Ekana stadium by filling up of a big wetland and razing of a natural acacia jungle is a big setback, it was home to micro-organisms, reptiles, birds and small animals, local eco system has been shattered beyond repair.

2. Little ahead of the stadium Awadh Shilp gram was constructed after filling up 2 natural bodies which had lot of aquatic life which supported many bird species.

3. Even now some construction is going on after filling up water bodies adjacent to river Gomti which will displace many bird species.

4. Large scale construction continues behind Ekana stadium is not only converting wetland into a concrete jungle but is already in the process of displacing lot of bird species.

5. I am witness to displacement of several pairs of sarus cranes from Shaheed Path area, while the construction advanced, they kept on moving and finally vanished, possibly wherever they went must have faced same concrete vastness.

Industries small, big and cottage also throw out toxic chemicals in the drains on which birds depend.”

Human exploitation of land dates back to our earliest settlements, with massive, yet local, destruction and deforestation. Already the Romans transformed the landscape to the treeless landscape of the Mediterranean that we are familiar with today, probably with devastating consequences for the wildlife at that time. Similar scenarios can be found throughout human history and across the world. However, it was not until the Anthropocene that the urban human societies started to grow significantly across the globe. Western industrialization started in the 1700s and with that urbanization and urban sprawl became a significant part of the landscape. Today, urbanization is a global phenomenon with implications for birds as well as for all other animals. Yet, developing countries are still in the early phases of industrial revolution; thus, the impact of global urbanization is expected to increase. Likewise, due to continued growth of the human population, existing urban areas in the western countries are also predicted to intensify and expand in the future. Together with climate change, urbanization is considered one of the largest threats to wildlife, including the persistence of many bird species. The foremost threat is probably habitat loss and fragmentation, which forces rapid decisions about emigrating (if possible) to more suitable habitats or stay and cope with the new conditions. The new urban conditions are not only through the process of urbanization per se but also the fact that the existing or remaining “green” areas are often changed, through plantation of non-native plant species, managed lawns, and removal of the mid-story canopy.

“Even common bird species that have benefited from association with humans in towns and cities in India are disappearing, e.g., the House Sparrow. The loss of open spaces in cities and towns, with the increase in human population and urbanization, advent of box-like architecture that does not provide nesting niches for them, concretization of pathways and compounds, the spread of supermarkets and their like where grain comes packed in plastic (thus cutting out food sources once available), and insecticide-laden pests thrown out into the open from homes. have all rung the death knell for sparrows. Along with the sparrow, our rapidly changing rural and urban landscapes are seeing declines in species of prinias, bulbuls, orioles, mynas, sunbirds, tailorbirds, babblers, and treepies. Only crows, koels, pigeons, and kites seem to be holding on in human habitation areas … but for how long?” wonders Asif Khan, Associate Officer, the Bombay Natural History Society. “Species (and habitat) extinctions are not new to planet earth, which has witnessed five mass extinction events since the first life forms appeared around 3.8 billion years. The causes for these extinctions were events such as climate change, glaciations, volcanic activity, and the impact of meteorites, and the process was spread over millions of years with new forms of plants and animals taking over. This is quite unlike the extinction process that is going on in this sixth species-extinction event. Never before in the earth’s history has the annihilation of so many species occurred in such a short span of time and as a result of one species, the Super Ape that tramples the earth,” Ranjit Manakadan, Deputy Director, Ornithology, Bombay Natural History Society, says. In temperate regions, birds can also benefit from the warmer climate caused by the so-called “urban heat island” effect, which is caused by the heat-absorbing properties of the impervious surfaces and buildings together with the scattering effects of air pollution, trapping heat irradiation within the atmosphere of the city. However, in warmer or tropical regions, the urban heating effect can be devastating for birds, leading to heat stress and dehydration, he adds.

Cities have a conservation role to play

The urban avoiders are the species that immediately vanish when an area is urbanized. These species are generally characterized by ecological features such as having low natal dispersal, migratory behaviour, fear toward humans (long flight-initiation distance), insectivory, and/or low yearly fecundity. However, cities that maintain native vegetation composition and structures will retain more native bird species than those that do not. Singapore is a good example of a megacity that has maintained a relatively high avian biodiversity. In a worldwide survey of bird species across 54 cities, 36 bird species were identified to be on the IUCN global Red List for threatened species, and 12 were recorded in Singapore. Thus, cities like Singapore have an important conservational role for maintaining these species in the future—to defeat the species homogenization effect and to maintain suitable habitats for them. Since the human population continues to increase, so will urbanization; hence, the urban threat for birds is likely to be even more alarming in the future, and urban city planners and conservationists have an important task to maintain existing biodiversity. The species that vanish immediately upon urbanization are relatively easy to identify.

City is either a trap or sink

However, many species show a slower response to urbanization, with population decline over time as a result. To identify these species can be more difficult and requires long-term studies of populations and their dynamics (e.g., dispersal, fecundity, and survival). For studies of population dynamics, it will be important to identify species for which the city act as an ecological trap, and to establish, whether the urban population is a sink or source population. Both scenarios can result in impoverishment of the urban bird populations through lower lifetime fitness compared to birds in the surrounding nonurban habitats. However, if the urban environment acts as an ecological trap, it can have implications for the persistence of the whole species. As the word trap indicates, an ecological trap acts by luring and attracting birds to a specific area. The city attracts many birds with its higher abundance of resources (i.e., food and nesting opportunities for cavity-nesting birds) and, in temperate regions, also with its milder winter climate compared to the surrounding nonurban habitats. These factors make birds to evaluate the city habitat as a “high-quality” habitat, thus a preferred habitat compared to more natural habitats. However, through the high rate of nest predation, predation at feeding tables by feral cats and dogs, poor nutritional value of the food sources, exposure to high pollution levels, and the high incidence of collisions with windows and cars reduce the overall fitness of the population in the urban habitat. Hence, if the ecological trap (city preference) is strong, the urban habitat will continue to attract rural birds to the city where they will suffer the negative consequences, ultimately reducing the species future chances of survival. The second scenario, the source-sink scenario, can result in both positive and negative effects on the population depending on whether the urban population is a source or a sink. However, if the urban population is a sink (without being a trap), birds do not prefer the urban habitat over nonurban habitats. Instead, surplus individuals can be forced into the city due to lack of free niches in the natural habitat. However, since the urban habitat is not preferred, there will not be as severe effects on the species level as with the ecological trap. That argument, however, relies on the existence of natural habitats; thus, the continuation of habitat destruction and deforestation could be devastating also for source-sink dynamics across the urban/rural landscape.

Urban environment as a barrier

A population needs to be reproductively isolated from other populations of the species to diverge genetically. For a long time, urban habitats were not considered a barrier for mobile species such as birds. However, habitat loss through intensification of urbanization or urban sprawl has been proven problematic for species with sedentary habits or limited vagility, i.e., the ability to move freely and migrate. As a result, populations get trapped. An Indian example that fits into the scheme is the House Crow. Why is it that the House Crow is such a successful species in human habitation areas in India? As Ranjit Manakadan : “Wastes from kitchens, restaurants, lunch boxes, weddings and parties all are thrown bewilderingly by the educated and the uneducated alike into the open. Then, there are the discards from the fish and meat markets and the filth of the open dump yards. Where else but in India do such bountiful conditions exist for the House Crow permitting them to proliferate? So, what is the harm if the crows increase in numbers? For one, the House Crow excludes many other birds, and other small animal, species from surviving in our rural and urban surroundings. Besides feeding on wastes, it is a predator of small birds, and the eggs and young of birds. Many of these species could survive with their own strategies if the crows were in normal densities that nature ordained them to be, but this is a case of dealing with a mob of intelligent predators. It has spread and colonised some parts of the Middle East, Africa and Southeast Asia, reaching these areas mostly by piggybacking onto ships. It is for this reason that governments of other countries pay a bounty for every immigrant ‘Indian Crow' (this is the term used, sometimes derogatorily, for the House Crow in colonised countries) shot — as it would otherwise disseminate the local fauna.” In Rajasthan, we have the example of threats faced by the already endangered Great Indian Bustard and the Lesser Florican. The Great Indian Bustard which was once seen very commonly at the dry plains of the Indian Subcontinent, has now been pushed to extinction. Reportedly, only 130 birds in India were there in 2019, as per the Wildlife Institute of India, which was down from 250 in 2011. As per the Wildlife Institute of India, the Great Indian Bustard had faced a whopping 75% reduction in population in the last 30 years. Besides being critically endangered by hunting and loss of habitat, that is, dry grasslands and scrub, one major challenge that the bustard faces are electrocution by high tension electrical wires and collision into wind mills. As reported by the Wildlife Institute of India, the bustard is dying at an annual rate of 15% due to collision with electrical wires. The bird has a weak frontal vision due to which it is unable to foresee electric wires. Since it is of heavy build, it means it cannot fly higher than usual.

“The males are especially large and majestic, and males holding territories make a grandiose display, giving out booming calls that reverberate across the landscape. Being an open grassland species, the GIB (and probably all grassland wildlife species like the Bengal Florican and the Lesser Florican) benefited from the early activities of humans, such as the clearing of forests and rise of subsistence agriculture, aided by low livestock grazing pressure in the cleared areas. Apart from the threat of predators, occasional hunting for the pot by local trappers was probably the only other danger it faced in early times (However, there is a reference to the savouries of its flesh in the memoirs of the Mughal emperor Babar, which suggests that it could have been a targeted species). Things changed with the arrival of the Europeans, guns, and later jeeps, especially for those who considered hunting a sport. One of the most despicable reports of hunting of GIB was in the defunct Oriental Sporting Magazine, where a writer signing himself as ‘Lover of all Sports’ claimed to have killed not less than 961 GIB from 1809 to 1929 in the neighbourhood of Ahmednagar in Maharashtra! Unforeseen issues like proliferation of power lines and windmills throughout the habitats of the species, ire of locals against the established sanctuaries due to crop depredation by Blackbuck that increased dramatically in numbers, loss of grazing lands, restrictions on right of way and developmental activities in the adjoining areas, encroachment of remote areas by ever expanding cities and towns, and intensification and modernization of agriculture are some of the reasons for the rapid decline of the GIB”, says Asif Khan.

Another dramatic decline among Indian birds is the case of three resident species of Gyps vultures. The loss of the Gyps species of vultures is attributed to the painkiller drug diclofenac used to treat sick cattle. Vultures die with time, on eating carcasses treated with diclofenac. Diclofenac was duly banned by the Indian Government for veterinary use and replaced by drugs safe for vultures in 2006 when advocated by birdwatchers. However, diclofenac continues to be used on the sly, there are other drugs that are toxic for vultures in the market, “New conservation issues may arise, and well-founded questions arise on the availability of safe areas with optimal food resources for vultures in the future in a developing/developed India with a billion plus human population,” says Dr Ranjit Manakadan.

Ray of hope

Hence, it can be said that urbanization has led to an immense change of the avifauna. Species have fled and vanished in response to urbanization but also flourished and changed. It is clear that urbanization is a huge threat to biodiversity and the existence of many bird species and urbanization is not expected to slow down in any close future, rather the. Thus, conservationists and city planners have an important task for the future. Their actions can in fact have great positive effects on the bird community, if the urban green space areas are managed well through plantation of native flora and enhanced complexity and if they enhance the urban green space or limit construction in key areas. Many political actions are also taken to reduce the impact of the different pollution sources, e.g., electric cars and LED lights that can be turned off or dimmed during sensitive periods, which will probably also show a positive effect on urban-dwelling species. However, much remains unknown about urban bird species resilience to urbanization and how plastic these species can be in their stress resistance responses to multiple stressors, before they reach a threshold with a population crash as a result. Future studies will entail, whether urbanization will be an opportunity for species radiation or if it will continue to be a habitat of species eradication and homogenization.


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