The riddle of the black-tailed godwits in Bangladesh

Every year, thousands of non-breeding black-tailed godwits can be found on the coast and in the freshwater wetlands of Bangladesh. Worldwide, four subspecies of black-tailed godwit have been identified depending on their different breeding grounds. But the question is, which of these four subspecies can be found in the wetlands of Bangladesh, and how do they migrate to and from Bangladesh? This is what Bangladeshi PhD student Delip Das is researching at Birdeyes in the University of Groningen.

_{By: Delip Das / Photos Henk Veenstra}

My name is Delip Das and I am a PhD researcher conducting ecological research on black-tailed godwits subspecies and their migratory patterns. The black-tailed godwit is a long-distance migratory bird species. Migratory birds lay eggs and raise their chicks in one place and then travel thousands of kilometers to rest and feed during their non-breeding period, only to return to their breeding place in the following season. From October to March, flocks of thousands of non-breeding black-tailed godwits can be seen at the coast and freshwater wetlands of Bangladesh. They breed separately in a vast area in the northern part of the world, ranging from Iceland in the west to the far east of Russia. Based on their separate breeding locations, four distinct black-tailed godwit subspecies have been identified, namely limosa, islandica, melanuroides, and bohaii. But the question is, which of these four subspecies can be found in Bangladesh’s wetlands? How do they migrate to and from Bangladesh? Do they migrate along the coast or over the Himalayan Mountains?

Challenging flyways

The fact that these questions remain unanswered is what makes them interesting for researchers like me, focused on migration research and conservation of the species. Bird migration fascinates us: how do birds know when and where to fly? Which route do they take? In general, these birds follow a consistent flight path from their breeding area to the non-breeding one and vice versa. So far, ornithologists (scientists who study birds) found nine major flight paths in the world and call them flyways. Bangladesh lies at the overlap between two of these flyways: the Central Asian Flyway and the East Asian-Australasian Flyway. This makes it an interesting location to study because migratory birds can come from both flyways, even though the Himalayan Mountains form a towering wall in the north of the country and are often considered a formidable challenge for migratory birds. However, the mountains also created the largest river delta of the world, which offers migratory birds vast wetlands with feeding and resting opportunities.

A new subspecies

The presence of godwits in Bangladeshi wetlands, under the overlapping flyways, piqued my curiosity about their origin and identity. It became even more interesting with the recent discovery of a new bohaii subspecies in Asia in 2021. I become curious to find out whether their presence in both freshwater and coastal wetlands across the country is related to their subspecies or migration pattern. In theory, populations of all three subspecies found in the Central Asian Flyway and East Asian-Australian Flyway could also be found in Bangladeshi wetlands, but previous research excluded the presence of melanuroides subspecies. Knowing the identity of the subspecies is essential for the proper management and protection of a species. This is particularly important for the black-tailed godwit because it is an endangered species.

Measuring and comparing

That is why I visited Bangladesh and, together with a team, captured black-tailed godwits from two wetlands. One wetland, Nijhum Dweep National Park, is located on the south-central coast of Bangladesh, where godwits forage on open mudflats. The other wetland, Tanguar Haor, is located in the north of the country, 300 km from Nijhum Dweep, where godwits forage on numerous shallow freshwater bodies. We gathered blood samples from each captured bird, measured their bill, wing, and tarsus (leg) length to verify their identity, and fitted them with satellite trackers to follow their migratory movement. We used the blood samples to sequence  mitochondrial DNA (mtDNA), which is passed on from mother to daughter, changes quickly, and is a reliable method to trace distinct evolutionary lineages such as subspecies in animal populations. We then aligned the sequence of each godwit from Bangladesh and compared it with sequences from four subspecies stored in GenBank, a database that stores global gene sequences from all organisms. This analysis confirmed that three subspecies are found in Bangladesh; the limosa, melanuroides, and bohaii black-tailed godwit. This finding is unique because, as far as we know, no previous record of three subspecies of black-tailed godwits co-occurring within the same region has ever been reported anywhere else in the world.

Breed from Kazakhstan to China

Our tracking data shows that black-tailed godwits in Bangladesh cover a large migration area—ranging from Kazakhstan in the west to China in the east. Their destination depends on the subspecies to which they belong. Birds belonging to the limosa subspecies migrated to the eastern Kazakhstan region near Mongolian border to breed, while the melanuroides subspecies migrated to eastern China, Mongolia, and areas close to the Baikal lake of Russia, and the bohaii subspecies migrated to the Sakha Republic of Russia. To our surprise, we found one bird belonging to the limosa subspecies that flew to the Sakha Republic — the breeding area of bohaii — and two birds belonging to the melanuroides subspecies that flew near the Mongolian border close to Kazakhstan — the breeding area of limosa. Did they make a mistake? Did they lose their way and end up in a wrong place? Did we witness a regular event that we are able to see now because we used advanced techniques, something that would not have been possible in the past? We are still unsure.

A pioneering study

Black-tailed godwits and migration patterns have been extensively studied in other parts of the world, particularly in Europe, but not in this interesting geographic location. In fact, this study is pioneering in Bangladesh, and its findings have significant implications not only for ecological and evolutionary research but also for the conservation of the species. The co-occurrence of three subspecies suggests that any disturbance or change in the wetlands of Bangladesh will have far-reaching consequences for multiple breeding populations across  Central and Northern Asia. Therefore, it is important to continue ecological research in this intersection from a global perspective to ensure the long-term viability of these migratory shorebirds.

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Delip Das