The Asian Openbill Anastomus oscitans is a relatively new addition to the Singapore Bird Checklist. It was added to the list in 2013, when a small group of birds stayed briefly in January. It was previously considered a vagrant in Singapore, as its sightings were seasonal yet sporadic, with arrivals taking place during the winter months. 

Records of Asian Openbill in Singapore

 

Review of status

Any status definitions are only an attempt to simplify a whole collection of complex factors relating to bird movements into singular words. Movements can be driven by a multitude of possible explanations, from climatic patterns to habitat change and food availability. Birds also don’t always obey our terminology perfectly, so assigning them into statuses is not always straightforward. This can be especially tricky in the cases of visitors, migrants, and vagrants. Our role as a Records Committee is to provide the best possible categorization of each species to reflect, as far as possible, the background behind its presence in Singapore. We use the following definitions to guide our classifications:

Visitor: Species with generally resident populations within the Greater Sundaic region, but not regularly recorded in Singapore
Migrant: Species that are known to undertake annual (or near-annual) migration to Singapore
Vagrant: Species that are not known to regularly migrate to Singapore or the immediate region

The movements of Asian Openbills in the region are essentially dispersive in nature. Seasonal weather patterns in Indochina contribute to varying food availability in the region, driving birds out of their core range in search of food. Breeding patterns and associated post-breeding dispersal also contribute to these movements. Based on these dispersive patterns, some might suggest that they are non-breeding visitors (“Visitors”, in our checklist) to Singapore. As highlighted in the definitions above, though, we prefer to restrict Visitors to those species which undertake short-distance movements within the Greater Sundaic region. We consider longer-distance seasonal movements to be more in the realm of migration. Many migratory bird species are only partially migratory; some individuals may remain in their breeding areas throughout the year, quite like the openbills. An example of this is the Horsfield’s Bronze Cuckoo Chalcites basalis, a species where many individuals remain in Australia all year round, while others move northward during the driest months when suitable habitat is most scarce; we consider the species a migrant because the movements are not short-distance in nature. Likewise, since Asian Openbills do not breed in the Malay Peninsula, migration or vagrancy would explain the movements better, and the critical factor differentiating these two is the regularity of appearances – not just in Singapore, but in the immediate region. These are what push species like the Grey-tailed Tattler Tringa brevipes to the Migrant category, even when their sightings are so infrequent.

Records in Malaysia on eBird back up the theory that a small proportion of openbills undertake seasonal movements down the Malay Peninsula during the northern winter and spring. The recent increase in openbill records in Singapore and the immediate region could be attributed to a growing population in the core range in Indochina as discussed by this 2015 paper in the journal Forktail. The invasive golden apple snail has become a major food item of the Asian Openbill since its introduction in the region too, and the successes of these two species are also likely to be heavily linked. 

Six full years passed between the first and second appearances of openbills in Singapore, which led to the previous treatment of the species as a Vagrant, rather than a Migrant. Given the increasing number of records of openbills in Singapore (indeed at least one new arrival every year since 2022), nearly all clustered within the winter and spring months, we now believe there’s enough evidence to “upgrade” openbills to the status of Migrant. Perhaps it could even go one step further into the Resident box in the future…