The rufous hornbill (Buceros hydrocorax), also known as the Philippine hornbill and locally as kalaw (pronounced KAH-lau), is a large species of hornbill endemic to the Philippines (the largest hornbill in the country). It is called the "clock-of-the-mountains" by locals, because of its loud booming call which typically occurs every hour. Rufous hornbills inhabit moist tropical lowland forest areas, and are now regarded as a threatened species. Reasons for decline include habitat destruction, hunting and poaching for the illegal pet trade.

It is illegal to hunt, capture or possess rufous hornbills under Philippine Law RA 9147.

The bird is endemic to the Philippines, where it occurs in primary, mature secondary, and disturbed forests, and is found across majority of the country except the regions of Mimaropa, Sulu and Western Visayas.

Its described as a large bird with a dark body, rufous legs, chest and neck, an off-white tail and a black fack faced and a yellow thorat. The bill of the nominate subspecies is entirely red, while the bill of the subspecies semigaleatus and mindanensis are pale yellow on the distal half. Birds are sexually dimorphic in which females have striking electric blue eyes, a smaller casque and no eye ring while males have yellow eyes and a larger casque.

Its striking red bill is caused by the oxidation of oils produced by the uropygial gland.

Three subspecies are recognized:

The HBW and BirdLife International Illustrated Checklist of the Birds of the World has split this into two species with the Northern Rufous Hornbill consisting of the nominate and the Southern Rufous Hornbill consisting of the semigaelatus and mindanensis subspecies.

It is sometimes called "the clock of the mountains" because of its periodic noontime call which can be heard up to 1.5 kilometers away. Due to its large size, its wings make a distinct whirring sound in flight. It has been recorded to have been preyed upon by the Philippine eagle.