Oriental Cuckoo
Cuculus optatus (Myer :
C. horsfieldi) and Himalayan Cuckoo
Cuculus saturatus are both locally common non-breeding winter visitor to lowland and hills of Borneo, accordingly to Mann's Checklist, they are here in equal number during migration.
Life was easy before their split in 2005 as all birds would be ticked as Oriental Cuckoo, resulting in a list of sightings pre-2005 which were not identified to race.
Oriental Bird Club does not follow the split by Payne (2005) and King (2005), and continues to list both as Oriental Cuckoo in its image database, so referring to OBI for identification tips is of not much help.
Different authors use different scientific name for Oriental Cuckoo as seen in the first paragraph, Mann and Phillipps follow Payne while Myer follows King.
Oriental and Himalayan Cuckoo are virtually identical and best distinguished by call and size, however, migrating birds are usually not vocal in their wintering ground, again, size is another unreliable method in the field if only a single bird is present, thus the identification of this species here is more often a calculated guess.
I think this bird is Oriental Cuckoo and not Himalayan, based on the markings on its vent. Himalayan sometimes has vague dark splodges on its vent while Oriental sometimes has spots on undertail-coverts (Brazil, 2009), the undertail-coverts of this bird sure looks more spotty then splodgy to me.
