Waterbirds persisted while terrestrial birds collapsed: contrasting habitat-guild responses to reservoir desilting in a wintering bird assemblage, western India

In the seasonal tropics, shallow lakes and reservoirs act as vital sponges, soaking up rain and storing water for agriculture and drinking. Over time, these basins naturally fill with mud and silt, reducing their capacity to hold water. To fix this, engineers often drain the water completely and scrape out the sediment, a process known as desilting. For decades, conservationists have watched these projects closely, but they have usually focused their attention on a specific group of animals: the birds that swim, wade, or dive in the open water. These waterbirds are easy to spot and count, and they serve as a standard measure of a wetland's health. This focus has created a quiet assumption: that the ecological value of a reservoir lies entirely in its water, and that draining it will primarily hurt the birds that need the water to survive.

However, a reservoir is not just a pool of water; it is also a complex edge where water meets land. Around the shoreline, tall grasses, reeds, scrub, and scattered trees form a fringe that supports a different community of birds. These are terrestrial species that nest in branches, hunt from perches, or forage in the grass, relying on the structure of the vegetation rather than the water itself. When heavy machinery moves in to dredge a lake, it often clears this entire fringe to make way for trucks and to dump the excavated mud. The question remains whether removing the water is the only thing that matters, or if the destruction of the vegetation around the edge might be even more devastating for the bird population.

In western India, a team of researchers set out to test this idea at the Pariyej reservoir, a large artificial lake in the state of Gujarat. This site is recognized as a critical habitat for birds and is part of a massive state program to remove silt from over a thousand water bodies. The scientists monitored the bird life at Pariyej across four consecutive winter seasons, from late 2022 through early 2026. This timeline was perfect for observation because the reservoir was drained and dredged during the middle of this period, specifically starting in February 2024, before being refilled after the monsoon rains. The team visited the site ten times each winter, using powerful binoculars and spotting scopes to count every bird they could identify without disturbing them. They did not just count the total number of birds; they sorted each species into groups based on where they lived and fed, separating those that depended on open water from those that relied on the scrub, trees, and grassy edges.

The results of this long-term watch revealed a story that ran completely opposite to what conventional wisdom would predict. When the reservoir was drained and the surrounding vegetation was cleared for the dredging work, the total number of bird species dropped dramatically. In the seasons before the work began, observers recorded an average of more than 117 species per visit. During the height of the construction, that number plummeted to fewer than 48 species. Yet, when the water returned and the site was surveyed again a year later, the bird community bounced back, returning to nearly 116 species per visit. On the surface, it looked like a temporary dip followed by a full recovery, a pattern that might have led an observer to conclude that the project was harmless to the birds in the long run.

But a closer look at the different groups of birds told a very different story. The researchers found that the birds did not all react the same way. The species that depended on the open water, the mudflats, and the shallow edges actually thrived after the project. By the end of the study, these waterbirds were not just back to their original numbers; they were more abundant and diverse than they had been before the dredging started. The removal of silt had deepened the basin, creating more open water habitat, which clearly benefited the ducks, geese, and wading birds that rely on it.

In stark contrast, the birds that lived in the scrub and trees suffered a catastrophic collapse. These terrestrial species, which needed the woody vegetation for perching and nesting, lost three-quarters of their variety and nearly nine-tenths of their total numbers during the construction. When the vegetation was cleared to make way for the machinery, these birds had nowhere to go. While the waterbirds flourished in the new, deeper lake, the forest and scrub dwellers were left with nothing. The researchers noted that the site had actually held more terrestrial bird species than waterbird species overall, meaning that the part of the bird community that was most damaged was the one that most people were not looking at.

The data showed that the change was not a matter of different birds replacing one another, but rather a simple loss of the terrestrial community. The birds that disappeared were those that needed specific structures, such as starlings and parakeets that nest in tree cavities or raptors that hunt from high perches. These species vanished entirely during the dredging phase. Meanwhile, the waterbirds, including species like the northern pintail and the ruddy shelduck, saw their numbers surge. By the time the study concluded, the wetland was dominated almost entirely by water-dependent birds, whereas before the work, the mix had been much more balanced. The recovery of the entire bird community was only possible because the water returned and the vegetation eventually began to grow back, but the scrub and tree birds had not fully recovered to their original levels even a year after the water returned, suggesting that their habitat takes longer to regenerate than the water itself.

This finding challenges the standard way of assessing the impact of such engineering projects. If conservationists had only counted the waterbirds, as is common practice, they would have concluded that the desilting project was a success, causing only a brief, temporary disturbance before the birds returned in greater numbers. They would have missed the fact that the project effectively wiped out the terrestrial bird community for a season. The study highlights that a reservoir is a dual habitat, supporting both aquatic and land-based life, and that clearing the vegetation fringe is just as destructive as draining the water. The researchers suggest that future projects should be planned with this in mind, perhaps by leaving strips of vegetation untouched to serve as refuges, ensuring that the birds that live in the trees and scrub are not sacrificed for the sake of engineering efficiency. The recovery of the Pariyej reservoir shows that nature can be resilient, but it also reveals that the cost of that resilience is often paid by the species that are hardest to see and easiest to overlook.