Spectacled Bulbul Pycnonotus erythropthalmos is a common resident throughout primary and secondary lowland dipterocarp and peatswamp forest, it occurs in lower montane forest as well.
Generally, most fledgings and young birds have yellow gape for ease of feeding by their parents, the yellow gape makes finding the target to feed easier, especially for cave nesters where it can be quite dark inside the nest. The gape will assume adult color in older birds.
However, the Spectacled Bulbul has a different scheme, its adult bird has bright yellow colored gape, contrasting to the younger birds which have duller pink gape. Bulbuls, building cup-shaped nest of dead-leaves, rootlets and vegetable fibres lined with fine dry grass, does not need to feed their chicks in the dark and thus do not depend too much on the assistance of the yellow gape in feeding. (Incidentally, chick of Yellow-vented Bulbul Pycnonotus goiavier shows cream instead of yellow gape.)
These three images are of adult birds, see the bright yellow eye-ring and gape.
These are immature birds, which have pinkish gape and eye-ring.
The fledgling has light pink gape and faint eye-ring. Lucky for me the adult bird came to feed so that its ID can be positively confirmed.