Slender-Billed Vulture

I've been focused on vultures for the past few days. So I'm going to leave Uganda and go back to Kaziranga NP in Assam, India, to look at some vultures I saw there. I'm having identification problems, so looking at them in detail may help. First, the slender-billed vulture. 

This is an illustration of the slender-billed vulture from Birds of the Worlds. 

The slender-billed vulture was first listed as Critically Endangered by the IUCN in 2002 and the IUCN has estimated a decline of 99% over three generations (36 years) as of 2020. As of 2021 the total population of mature birds was estimated at 730 to 870. A Wikipedia article on the Fauna of Kaziranga National Park notes, "Kaziranga was once home to seven species of vultures. About 99% of the stable vulture population was killed by kidney failure caused by consuming the veterinary drug diclofenac in domestic animal carcasses. Of these, the red-headed vulture, and Eurasian black vulture are still near threatened due to their large range, but the Indian vulture, slender-billed vulture and the Indian white-rumped vulture have suffered cataclysmic loss of numbers and are virtually extinct in the wild, including Kaziranga. The Indian populations of the other two vultures have similar losses — the griffon vulture and the Himalayan griffon, but they are still well represented outside India." This kind of information helps me to understand how privileged I've been to see these Critically Endangered birds in both India and Africa. I'm particularly blown away at the estimated living population of slender-billed vultures world-wide and the number of them that I was able to see in Kaziranga. 

The slender-billed vulture used to be classified as an Indian vulture with the subspecies name of long-billed vulture. However, they have non-overlapping ranges (except note that both were and maybe are both present in Kaziranga) and can be easily identified from long distance by trained observers which is why they split them into separate species. The Indian vulture is usually only found south of the Ganges river and nests on cliffs. The slender-billed vulture is found from the Gangetic plain of India north, west to Himachal Pradesh, south as far as northern Odisha and east through Assam (where Kaziranga NP is located). It is also found in parts of Nepal, Bangladesh, Myanmar and Cambodia. Unlike the Indian vulture, they only nest in trees, and are solitary at that, and will not nest in proximity to any other nests. 

This is the illustration of the Indian vulture from Birds of the World. While talking about similarities and differences, it should be helpful.

The slender-billed has a darker head than the Indian, and a thinner bill. It is mostly gray with a pale rump and gray under-tail coverts. Its thighs have whitish down, the neck is black, long, bare and skinnier than the Indian. The head is black, angular, narrow and dark, with a bill that narrows midway with a light spot on the culmen (the dorsal bridge on the upper mandible). The head and neck are also very wrinkled in contrast to the Indian. Juveniles are similar, except for white down on the back base of the neck which it loses as it matures. 

While driving through the Central Range of Kaziranga, our guide identified the following as a nesting slender-billed vulture. It has also been confirmed as a slender-billed by two other observers on iNaturalist in addition to me. 

The following have also been confirmed by two observers (in addition to me) as slender-billed vultures on iNaturalist. They were found in the Eastern Range of Kaziranga. There must have been carrion nearby. 

The following, also in the Eastern Range of Kaziranga, was identified by one observer (in addition to me) as a slender-billed on iNaturalist. 

The following vulture has not been identified on iNaturalist. I included it in a photo with another vulture that I identified as a Himalayan griffon and it has not had any response from other identifiers. With the review I've made today, I believe it is a slender-billed vulture and will send it to iNaturalist to see if I can get substantiation from others. 

It has really been fun to concentrate on vultures for a few days. I've learned a lot. And I feel very privileged to have seen four species of Critically Endangered vultures within the last year. It is conceivable that I could outlive these species and that is horrible to contemplate for our society and our planet.