Like a scene out of Alfred Hitchcock's horror film The Birds, a flock of more than 20 crows soared above Sungei Buloh Wetland Reserve with their wings outstretched, before coming to rest on a naked tree, squawking all the way.
That was what greeted The Straits Times on the morning of May 8, during a visit to the reserve in Kranji.
Fortunately, unlike the movie, the crows there left humans well alone.
But their growing numbers over the past one to two years may pose a problem for other inhabitants of the reserve, including birds such as little herons.
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"House crows are not native to Singapore. Their mobbing habits can have a detrimental effect on other birds if their population gets too large," said Mr Wong Tuan Wah, group director for conservation at the National Parks Board (NParks), which manages the reserve.
"For example, they may harass other bird species at a feeding site, or raid their nests," he added.
Nature enthusiasts told The Straits Times that they have observed crows disturbing other birds at the reserve.
"I've seen crows harassing shorebirds at Sungei Buloh, although it is not clear why they did so. They could have been checking to see if any of them was weak and could be targeted as prey," said nature guide Ivan Kwan, who visits the reserve at least once a month.
NParks to clear crow nests within reserve
Mr Kwan, 34, added that other people have seen crows raiding little heron nests in Sungei Buloh.
"If this continues, it could ultimately lead to some bird species choosing not to nest in the area because of the high risk of nest failure."
Birdwatcher and market manager Lim Kim Chuah, 54, who has observed flocks of up to 50 crows in Sungei Buloh, said their numbers seemed to have gone up over the past one to two years.
It is not immediately clear why the crows are congregating there.
Wildlife consultant Subaraj Rajathurai, 53, said one possible reason is that the birds could have been driven from their roosts and habitats in Johor Baru located across from Sungei Buloh.
"There may also be a food source nearby. With their presence, nesting birds may be in danger as these crows also raid nests to feed on eggs and chicks," he said.
To control the crow population, NParks will seek out and remove any house crow roosts and nests within the reserve, said NParks' Mr Wong.
Mr David Tan, a bird scientist at the National University of Singapore, said NParks' move to reduce available nest sites for crows in the reserve is a good way to reduce their numbers.
"But more scientific studies, such as long-term monitoring of the crow populations there, should be done to gauge the effectiveness of such strategies, and to inform future control measures."