Insect decline may affect migratory birds, including those that stop in Singapore

SINGAPORE – The season for bird migration has begun, with cooler weather descending upon the Northern Hemisphere, sending birds on their long journeys south to places like Singapore to escape the winter chill.

Between August and March, these feathered voyagers draw plenty of attention from birdwatchers and groups campaigning for the conservation of wetland habitats where the birds stop and refuel.

But a new United Nations report has highlighted a critical aspect of migratory bird survival – the decline of insects. The report has said that dwindling insect abundance, biomass and diversity are contributing to population losses among migratory birds, especially those that rely on insects as their main source of food during their migration.

According to the report, which cited other studies, 10 per cent of insects are threatened with extinction globally.

It also noted that about 11 per cent of all migratory land birds and waterbirds are listed as threatened or near threatened on the International Union for Conservation of Nature Red List of Threatened Species, or IUCN Red List. In addition, bird species dependent on insects for food have declined drastically over the last 50 years.

Insect decline worldwide is mainly driven by land-use change, climate change and pollution, the report prepared by the Convention on the Conservation of Migratory Species of Wild Animals – a biodiversity treaty of the UN – said.

Though Singapore is better known for hosting migratory shorebirds – which are less impacted by insect decline due to their diet of worms and crustaceans in wetlands – insect decline may affect the migratory land birds, as well as have an effect on ecosystems here.

Migratory land birds can be found in places such as the Singapore Botanic Gardens and Pasir Ris Park.

Common migratory land birds here are bee-eaters, shrikes, flycatchers and warblers, said Mr Alan OwYong, committee member of the Nature Society Singapore Bird Group.

Some stop to refuel for a day or two, while others visit during winter and stay for a long period.

“They will compete for insects with resident species during the migratory season, so any decline in insect life will impact their ability to continue with their migration,” said Mr OwYong.

Birds like the brown shrike and Oriental reed warbler have shrunk in numbers here based on the Nature Society Singapore’s long-term data and are listed as vulnerable in the Red Data Book. But it is not known if the reduction in food sources directly contributes to this as the decline is also a result of habitat loss and hunting along the flyway. 

Migratory birds are particularly vulnerable to contractions in food supply as they expend a lot of energy during their journeys, said Dr Tan Yen Yi, assistant senior curator of birds at Lee Kong Chian Natural History Museum at NUS.

“These migratory birds need to eat enough calories to sustain and survive their arduous migration flights, so it is likely that the documented declines in insect populations worldwide will have negative repercussions on migratory birds,” she said.

In comparison, shorebirds such as the common redshank and common sandpiper, which usually refuel in coastal wetlands such as the Sungei Buloh Wetland Reserve, feeding on marine invertebrates like worms and molluscs, may be less impacted by insect decline, said Dr Yong Ding Li, regional head of species conservation and flyways for non-governmental organisation Birdlife International.

Insect decline, however, affects more than just the birds that feed on them. If birds begin dying out as a result, the animals that feed on these birds would also be impacted.

And with the interconnectedness of ecosystems, the effects will be felt across entire ecosystems and not just affect singular groups of animals in isolation.

“Changes to any point within the food web will affect its prey or predator, or both, and trigger a chain reaction of events which may affect those not directly connected,” said Dr Tan.

According to the UN report, while there is evidence of an overall global decline in insects, the largest declines in terrestrial insect biodiversity were found in North America and Europe. A loss of 9 per cent per decade in the abundance and biomass of terrestrial insects has also been reported, although these patterns are also mainly based on trends in North America and Europe.

While it is hard to determine whether insect populations have declined in Singapore, experts said there are factors that could lead to it.

Dr Hwang Wei Song, senior curator of entomology at Lee Kong Chian Natural History Museum, said insect populations will decline where their natural habitats are developed, and they are unable to adapt to a more urban environment. However, the overall status of local insect populations has yet to be studied, he noted.

Aside from habitat loss, the increasing use of pesticides and other chemicals as a result of agricultural intensification is believed to be a major cause of insect declines, said researchers.

A highly urbanised city like Singapore can also create an inhospitable environment for insects.

While there is no hard data on overall insect populations declining, there have been reports of butterflies going extinct in Singapore in the past few decades.

Almost half of Singapore’s butterfly species have disappeared from the island over the past 160 years, with the loss of specific plants and deforestation being key drivers of the local extinctions, according to a 2020 study published in the scientific journal Biological Conservation.

Aside from the decline in insects, other reasons leading to the decline of migratory birds are unsustainable and illegal hunting across Asia and the loss of their coastal wetland habitats, Dr Yong said.

He said migratory shorebirds have declined steeply in the region over the past 20 years, with the highest declines in the marsh sandpiper, curlew sandpiper and Terek sandpiper. Terns, such as the white-winged tern, and black-capped kingfishers have also decreased in numbers.

To address losses and protect migratory species, including birds, the UN report recommends insect conservation measures and calls for more monitoring efforts, engagement between stakeholders, as well as guidelines for the most urgent actions to address insect decline.

Dr Tan said: “There is no doubt that global declines in insects will affect not just migratory birds, but also the entire ecosystem.

“However, we have to first understand our insect communities, which scientists are working hard to resolve – to that end, biodiversity discoveries and inventories are a prerequisite.”

Where to spot migratory birds

Check out some avian hot spots here during the bird migration season, which happens between August and March every year. Hailing all the way from places like Russia and China, feathered travellers such as the Tibetan plover and Blyth’s paradise flycatcher can be spotted at some of these places highlighted by Chin Hui Shan and Lim Yong.

Sungei Buloh Wetland Reserve

Singapore Botanic Gardens

Pasir Ris Park

Marina East Drive