- The black-headed ibis is found to roost in both rural as well as urban areas in southern Rajasthan, according to a new study.
- Roosting sites, where the birds congregate to rest, had three times higher numbers of black-headed ibises than nesting sites.
- Conservation measures must also take into account both rural and urban roosting areas, especially large native trees in urban sites, say researchers.
Waterbirds in southern Rajasthan, such as the black-headed ibis prefer not only to roost in rural and relatively undisturbed areas but also in highly disturbed urban sites that are close to human habitations and roads, finds a new study by a team of ornithologists based in Udaipur.
“Clearly, roost sites form a previously-ignored component of the birds’ life history and population, that need to be paid attention to if we are to conserve these species over the long run,” says K.S. Gopi Sundar of Mysuru’s Nature Conservation Foundation and senior author of the study.
Most studies on waterbirds have focused on feeding and breeding, but ornithologists hardly know much about another overlooked behaviour of birds: roosting. Like humans, birds also need to rest and they often wind down at night at a safe place such as perched upon a tree, sometimes congregating alongside many other bird species — known as communal roosting.
While many waterbirds roost in or near wetlands, where food is plentiful, the team had observed that the black-headed ibis (Threskiornis melanocephalus), residents of southern Rajasthan, tended to roost in trees and were spotted closer to cities in two incidences. The black-headed ibis, also referred to as the Oriental white ibis, is a medium-sized, near-threatened waterbird whose black neck, head and legs contrast its white plumage.
Curious, the team, which included assistant professor Vijay Kumar Koli and his Ph.D. student Sunil Chaudhary from Udaipur’s Mohanlal Sukhadia University, explored this further in a first-of-its-kind-study by surveying where black-headed ibis roost at night and comparing both rural and urban sites in Udaipur and five neighbouring districts; usually, studies focus only on rural or urban sites. Rural sites consisted mainly of cultivated lands — which are important foraging sites — with a few sparsely populated villages, whereas urban sites had clusters of buildings with relatively higher population densities. Trees dotted both rural and urban landscapes.
The team also located nesting sites and compared these with roosting sites. Going further, they assessed the difference in the composition of waterbird species in roost sites with and without black-headed ibis to understand its relationship with other co-occurring species.
Urban versus rural roosting sites
Roosting sites harboured three times more black-headed ibis than those found in nesting locations, and this can be attributed to the fact that roosts include non-nesting birds as well, said Sundar. Oddly enough, trees were larger at roost sites compared with nest sites and more so in urban areas. This may be because although there are fewer trees in urban areas, they are larger, boasting a higher canopy cover with ample space for large birds to roost at night, said Koli, lead author of the study.
Unexpectedly, the results showed that both rural and urban roosting sites had similar numbers of black-headed ibis. In fact, other waterbird species also found both types of sites attractive for roosting despite the fact that urban roosting sites experienced 2.3 times higher disturbance than rural ones.
“This, to us, underscores the high value of the human attitude in Rajasthan, and indeed many locations in India, in not deliberately disturbing birds and tolerating waterbird aggregations,” said Sundar, who is also the global Co-chair of the IUCN Stork, Ibis and Spoonbill Specialist Group. And “this toleration is not trivial,” he added, “since these aggregations are noisy, smelly and can wake you up very early in the morning!”
“Across India, many colonial waterbird species have traditionally been known to take advantage of the high levels of tolerance and local attitudes of people towards roosting and breeding habits of birds,” said Taej Mundkur, International Waterbird Census coordinator and senior technical officer at Wetlands International, who was not involved in the study. “This is despite the litter of vegetation, fish and other food and large quantities of guano from these birds that tend to drop on the ground around such roosts.”
Mundkur, who finds this study both fascinating and comprehensive, noted that “trees provide safe roost sites and nesting habitats for waterbirds in parks, gardens and along roadside trees in several urban and rural areas, often at a distance from their feeding habitats.” Consequently, as urban centres grow, he said, we need to understand “how colonial waterbirds are adjusting to the changes in the landscape, farming practices, infrastructural developments and other changes to better inform conservation planning for these species.”
Attention to conservation, stresses Koli, has to be paid equally in both rural and urban areas, noting that the “degradation of any site may affect another one also.” He adds that conserving urban sites is particularly important because “we are losing native and large trees in this area rapidly.”
This view is echoed by Mundkur who stated that “ensuring adequate protection and management of roosting trees within the proximity of foraging areas within urban and rural landscapes of these waterbird species is important.”
Surprisingly, both urban and rural roost sites were not always close to wetlands, as is often expected for waterbirds. This suggests that the black-headed ibis does not always rely on wetlands and rivers for foraging, but in a variety of different habitats, as was reported for these birds in Uttar Pradesh in an earlier study. They also use dumping sites, human food waste and nullahs for feeding reveals Koli. In two instances, he along with Chaudhary even spotted a few feeding on cow carcasses.
“The comparison of urban and rural roost site characteristics of waterbirds is interesting and perhaps not attempted earlier in the country,” said V. Santharam, the director of the Institute of Bird Studies & Natural History at Rishi Valley Education Centre, Chittoor, Andhra Pradesh.
Waterbirds are becoming more conspicuous in urban areas in other parts of the country such as in Chennai, he points out, where “the population of waterbirds is increasing and birds are using the wetlands in the outskirts of the city or in parts that are getting developed in recent times.” In Chennai, he reasons that the possible causes may be the increased availability of food from wetlands as they become more productive due to the discharge of sewage and greater water inundation due to unregulated urban infrastructure planning.
A close association with other species
Interestingly, the team found that the species of waterbirds in roosts with ibis were different from roosts without ibis. Although they counted 16 other species — some of which are also globally near-threatened — roosting alongside black-headed ibises, three species, in particular, were found to have a close association with the ibis: the lesser cormorant, great egret and cattle egret. This is the first time any study has reported that species roosting alongside the black-headed ibis roosts are not random.
According to Santharam, the finding of other globally threatened species co-roosting with the black-headed ibis is not surprising because some of these species are also spotted in large numbers in Chennai wetlands.
Santharam feels that co-roosting with other species might be influenced by the age of the roost site. “Initially, the roosts may start off with a single species and eventually may attract other waterbird species. This is also perhaps true for nesting sites (some of the nesting “sites may have started off as roosting areas).”
But what exactly goes on during bird roosts is largely a mystery and even more so for waterbirds, said Sundar. “Some scientists believe that roost sites are places where information is exchanged, and others believe that roost sites offer safety in numbers, among other benefits.”
According to Sundar, these findings raise questions that need to be explored further. “What interactions do black-headed ibis have at roosts with other species? Do they actively seek out some bird species because they get some specific advantages? Can these interactions help us understand why this species roosts in assemblages that are not formed of randomly occurring species and numbers?”
He hopes to address these questions in future studies, acknowledging that they will be tough to answer.
Koli, V.K., Chaudhary, S., Sundar, K.S.G. (2019). Roosting Ecology of Black-Headed Ibis (Threskiornis melanocephalus) in Urban and Rural Areas of Southern Rajasthan, India. Waterbirds, 42(1): 51-60.
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