Unhearthed

Human encroachment has created imbalance in the avian ecology. Better management practices are needed for the conservation of species diversity

 
By C S Malhi
Published: Saturday 15 December 2001

Unhearthed

-- (Credit: Photographs: Amit Shanker) Early morning twitter is music to ears. But soon this melody may turn into a requiem with the bird community slowly being driven out of its natural surroundings. Constant human encroachment on the forest cover is forcing these avian creatures to abandon their homes and explore alternate sites for shelter and sustenance. Large-scale deforestation and extensive agriculture have already altered their structure to a great extent since presence or absence of trees is directly linked to bird diversity. Birds exhibit a unique and intimate relation with their habitat, particularly the trees around them. But our knowledge on bird-tree association is meagre as only a few studies throw light on this aspect. Trees and their surrounding foliage play a significant role in determining the avian community structures that vary according to their specific requirements for foraging, roosting, courtship, and breeding. Birds select different sites for feeding purpose in an established community to avoid competition and in some species even male and female have a separate foraging niche.

Foliage structures, abundance of food, height, age, density of trees, competition for food and nesting are the main factors that determine establishment of a particular community. Some birds prefer dense foliage while others bare trees for roosting and nesting purpose. Exposed perches on bare trees are perfect sites for predatory birds to watch the moving prey. Conversely, these trees are used by other species for perching and roosting to keep a constant vigil on their predators. Birds like crows feed on the food gathered from the surrounding areas sitting on an isolated tree whereas the parrots select tall trees in the vicinity of the orchards to consume the food. Large bird species generally prefer tall trees for roosting whereas smaller ones favour trees with a lower canopy. Even dead and hollow trees are very important for this purpose. Almost all birds breed on trees, even the aquatic birds like egrets water herons and water hens use trees in the vicinity of the water body for breeding. Tall and old trees are preferred nesting sites for species of hawks and eagles and for cavity nesters like parrots and wood peckers. Old trees, particularly the banyan and peepal with rich foliage, accommodate majority of species including predatory birds like owls while thorny trees are home to communities like egrets and weaver birds.

Since a large number of birds are found in an ecosystem, the structure of the community is described as a unit. In general, three indices i.e., species richness, diversity and evenness or equitability are used to quantify the structure of bird communities. Although all trees attract some birds throughout the year, bird abundance and richness increase during the peak flowering or fruiting periods. Indian coral tree (Erythrina indica), when in its full bloom, attracts swarm of birds. Birds of all species, with the lone exception of ring dove (Sreptopelia decaocto) can be seen feeding on nectar from coral tree flowers, thus contributing towards cross pollination. House crow (Corvus splendens) is the most abundant species followed by common myna (Acridotheres tristis). Relative abundance of bird species and families in the morning and evening are similar and their species diversity also does not differ significantly at two different times of the day. However, average community density is higher in the morning and it displays a significant positive correlation to crown area of trees (thus indirectly with the number of flowers on trees).

Avian community on simbal (Bombax criba), banyan (Ficus bengalensis) and eucalyptus (Eucalyptus globulus) comprise 18 bird species. A major portion of the bird community consists of house crow, common myna and rose-ringed parakeet. Certain bird species show tree species specific preferences. Rufus-backed shrike, red-vented bulbul, little brown dove, hoopoe and green pigeon form a minor portion of bird community. Twelve species of birds constitute the community on woody and fruit trees. Among these species, overall, the house crow, common myna and common babbler constitute the bulk of the community structure whereas black drongo, cattle egret, red-vented bulbul and red-wattled lapwing constitute minor component of both fruit and woody trees. House crow is the most abundant species and appears to be present on all selected trees. The mean percentage of house crow is highest on eucalyptus, followed by simbal and mango trees. It appears that rose-ringed parakeet, the second most abundant species, mainly select wood trees that have natural grooves where they can establish their nests.

Species richness is nothing but the total number of species in a community. Species diversity, takes into account both relative abundance and the number of species in a community. Both the greater number of species and a more equitable distribution of individuals among the species, increase species diverstiy. Therefore, management practices are crucial to avian ecology. Logging, thinning, mechanical clearing and conversion of natural forests can change the structure of avian community. Deforestation can increase the population of granivorous bird species when the area under forests is subsequently used for agricultural practices. The increase and decrease in population of birds is directly related to the breeding success of birds which is connected to their nest site selection. The latter, in turn, mainly depends on the height and canopy size of the trees. In the absence of these suitable factors, some birds have been forced to establish their nests on electric poles and in and around human habitation structures like buildings. Watching its own interest the humankind has disturbed the ecosystem which has resulted in imbalance in several bird species. It is time to think collectively to initiate measures to rehabilitate these birds in the available forest cover. Monoculture in tree plantations should be avoided since diversification of tree species will help in encouraging and affording diversity in bird species.

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