For Sri Lanka’s crows, following the trash is all part of the hustle

The International Crow and Raven Appreciation Day, on April 27, celebrates one of the least-liked but most intelligent groups of birds around.
In Sri Lanka, home to two crow species, the birds are widely seen as a nuisance — but one enabled by the continued practice of poor waste management, which draws crows in flocks of up to 500.
In “following the trash,” crows now occur even in wilderness havens such as Horton Plains National Park, where they feed on the trash left by human visitors, as well as on native wildlife species.
Scientists studying Sri Lanka’s crows say the best way to get their population under control is through better management of the country’s waste, rather than the more extreme options of killing them or destroying their eggs.

COLOMBO — Crows have long had a reputation as a nuisance and a pest, even though it’s widely recognized that they’re among the most intelligent of birds.

In Sri Lanka, that reputation goes back some 2,000 years, to the Jataka morality tales from Buddhist tradition. One of the tales tells of a “greedy crow” that spies a half-covered plate of fish in a nobleman’s kitchen and tries to steal it, only to be caught, plucked and thrown out. The moral being, of course, that greed is bad.

But if the modern-day crows of Sri Lanka are any indication, it would be safe to assume that the Jataka crow ended up just fine, ever hustling and thriving — the perfect avatar for the International Crow and Raven Appreciation Day that falls on April 27.

Sri Lanka is home to two crow species: the large-billed crow (Corvus macrorhynchos) and the house crow (Corvus splendens), also known as the Indian crow, a common bird found throughout much of South Asia. (The house crows found in Malaysia’s Penang state are thought to come from a population of 56 birds brought over from Sri Lanka in the 1890s for caterpillar control.)

While the large-billed crow abounds in rural settings in Sri Lanka, the house crow occurs mostly in cities. One thing they have in common, though, is that both thrive on trash, flocking to the many open dumpsites around the island. Another thing: both are among just seven bird species that aren’t protected under Sri Lankan law; the vast majority of the around 500 bird species found in the country are protected.

House crows are more abundant in cities than large-billed crows, and their population growth is strongly tied to the availability of garbage dumps and nesting sites. Image courtesy of Athula Edirisinghe.

Thriving on trash

Colombo, Sri Lanka’s commercial capital, appears to have the country’s biggest crow population. The Field Ornithology Group of Sri Lanka (FOGSL), the local affiliate of BirdLife International, initiated a study on Colombo’s crows in the late 1970s, and the effort continues today.

“Our study showed that the garbage is directly linked to the crow population fluctuations,” Nihal Dayawansa, the FOGSL president and a professor of zoology at the University of Colombo, tells Mongabay.

Crows have communal roosts, often a single tree, or several in the same area, depending on the size of the flock. This makes it easy to carry out a population assessment, which the FOGSL researchers do through roost counts at dusk. Over the decades, they’ve recorded the Colombo crow population growing from about 50,000 birds in 1980 to a peak of 124,300 in 2006, before dropping below the 100,000 mark, at 98,350, in 2012.

*Crows are notoriously smart birds and have adapted well to city life. Here, they’ve used clothes hangers to build a nest in the heart of Colombo. Image courtesy of Nirojan Donald.*

The turning point, Dayawansa said, was the Colombo Beautification project in 2010, which included shutting down several open dumpsites that were considered eyesores. But after 2014, the government largely abandoned the project, and new dumpsites began appearing.

The Colombo crows are predominantly house crows (only about 5% are large-billed crows), roosting in flocks of as many as 500 individuals — although figures like this aren’t common anymore, Dayawansa says.

*Crows have been known to attack raptors and are considered a threat to urban wildlife, especially smaller birds that nest in home gardens. Image courtesy of Thusitha Weerasighe.*

Impact on endemic wildlife

The inspiration to begin the crow study came from a public market near a school where the birds had become a nuisance, says Sarath Kotagama, an ornithologist and emeritus professor of ecology from the University of Sri Lanka.

“This market got shifted to another area and the crow population in the area too declined,” Kotagama tells Mongabay.

Over the years, the study has confirmed that the crows move with the garbage. And it’s not just open dumpsites that make for easy pickings. In Horton Plains National Park, one of the most ecologically sensitive areas of Sri Lanka and a UNESCO World Heritage Site, large-billed crows swoop in with every surge in human visitors, thanks to the large amounts of trash that the latter generate.

When visitor numbers to the national park drop significantly, as was the case throughout much of the COVID-19 pandemic, the crows lose their main source of food and began hunting native species. This includes endemic species like the Ceylon deaf agama, a lizard found only in Sri Lanka. That raises concerns about the impact on native species if the crow population increases unchecked, Kotagama says.

A native swallow attacks a large-billed crow trying to get its nest in Hortan Plains National Park. Crows numbers in the park spike with increased visitor numbers, as the latter generate large amounts of trash. Image courtesy of Shermi Nanayakkara.

Dayawansa says controlling crow populations can be done by managing the food resources that are available to the birds. This means an effective garbage management strategy, he says. Some countries, such as Singapore, have crow-culling competitions and other methods to eradicate the birds.

But if there’s an alternative moral to the Jataka tale from 2,000 years ago, one that applies to the present-day problem, it’s that the plate of fish in the nobleman’s kitchen should have been fully covered to begin with.

“Implementing artificial population control methods such as destroying eggs and culling of adults would not be necessary,” Dayawansa says, “if garbage sites are better managed.”

Citations:

Krzemińska, U., Wilson, R., Song, B. K., Seneviratne, S., Akhteruzzaman, S., Gruszczyńska, J., … Rahman, S. (2016). Genetic diversity of native and introduced populations of the invasive house crow (Corvus splendens) in Asia and Africa. Biological Invasions, 18(7), 1867-1881. doi:10.1007/s10530-016-1130-5

Karunarathna, D. M. S. S., & Amarasinghe, A. A. T. (2008). An observation of the jungle crow (Aves: Corvidae) feeding on Ceylon pygmy lizards, Cophotis ceylanica (Reptilia: Agamidae) at Horton Plains. Sauria, 30(4), 59-62. Retrieved from https://www.researchgate.net/publication/259500784_An_Observation_of_the_Jungle_Crow_Aves_Corvidae_feeding_on_Ceylon_Pygmy_Lizards_Cophotis_ceylanica_Reptilia_Agamidae_at_Horton_Plains_NP_in_Sri_Lanka

Banner image of two large-billed crows photographed inside the ecologically sensitive Horton Plains National Park in central Sri Lanka, courtesy of Uditha Wijesena.