Valley-floor censuses of the critically endangered Yellow-crested Cockatoo Cacatua sulphurea occidentalis on Komodo Island, East Nusa Tenggara province, Indonesia, point to a steep population decline over a six-year period

The population of the Critically Endangered Yellow-crested Cockatoo Cacatua sulphurea occidentalis in Komodo National Park, Komodo Island, East Nusa Tenggara province, Indonesia, is thought to be second largest, but has been little studied. In September-October 2005, we surveyed cockatoos from vantage points overlooking five coastal valleys, each one on three consecutive days, and in September 2006 counted cockatoos in Loh Sebita valley on five consecutive days. Our method reduced the possibility of double-counting birds because on each day only the single largest count of cockatoos was used. We compared our 2005 and 2006 data with population census counts from the same valleys using the same method in September-October 2000. We also collated opportunistic counts of the species on Komodo made between 1996 and 2015 and checked whether temporal trends were apparent between two 10-year periods. Nest and breeding data were also collected. A total of 137 cockatoos was recorded in 2005 compared with 340 in 2000, with census counts declining by an average of 60%. In Loh Sebita valley the population declined by 41% between 2000 and 2006. A total of 19 active nests was located, with 25 nestlings/ juveniles recorded, mostly in tall and smooth-trunked Sterculia foetida, S. oblongata and Corypha utan trees, which are apparently selected to reduce nest predation. Vantage point census counts are a suitable method on Komodo because inland topography renders cockatoo flocks easily detectable from ridges, although greater survey effort is needed to reduce margins of error.The cause(s) of the sharp population decline remain unclear but trade is the most likely driver, with other factors such as breeding failure possibly involved. Annual population and habitat monitoring is needed on Komodo to confirm the causes of decline and specific patrolling is needed to monitor nests.