Ageing:
Young birds are only possible to age before their first moult. Skull neumatisation can not be used because of the black skin on the head, making it impossible to see the process of calcification of the skull. I checked some non-feathered parts like the bill (color of the palate, tongue, etc), iris color and none had a correlation with ageing (or at least to my eyes).
Rwbulbul (with the data we collected) do a Postbreeding complete moult and a Postjuvenile complete moult. What does it mean? Easy, Juveniles do a moult changing all the feathers and adults as well after their breeding time or fledging of the nest. So after this moult all signs of juvenile or adult are gone away (like many other birds in Mauritius). So that makes you able to age them for a short period of time and always carefully. If you follow the next instructions you won't age them wrong. If you have doubts make pictures of wing an head of the birds and I will help you, otherwise I strongly recommend you to put 1k+:
Adult (2k+): Worn primaries, secondaries and tertiaries. Presence of red whiskers.
Juvenile (1k): Fresh Primaries, Secondaries and Tertiaries. No presence of red whiskers.
Indeterminate (1k+): Fresh Primaries, Secondaries and Tertiaries. Presence of red whiskers. These birds are Juveniles or Adults that moulted completely and there is no way to find out. At the end of the breeding season you will get many of these ones. But remember, birds in Mauritius have a long breeding period so birds that breed in October will be moulted by now, specially in the low areas and some birds will be quite worn so you could age some moulted Juveniles as unmoulted adults.
Moulting birds: Take special care when ageing these birds. Remained old, brown feathers at the whiskers are from Juveniles. Anyhow, Juveniles could moult the whiskers and still be moulting the remiges. Then you have to compare (After a while it’s easy) the new Remiges (moulted or moulting) and the old ones (From the nest, so relatively fresh). You won't see a big difference betwen old and new feathers. Adults birds you should be able to see a big difference between new feathers and old ones (these old feathers would be from the last moult a year ago so very worn, specially.

Click here To SEE photos from telling the sex from Red-wiskered Bulbul / VER fotos de los sexos del Bulbul de mejillas rojas / Veure fotos del sexe del Bulbul de galtes vermelles

If you want to see more photos from birds, we invite you to visit our gallery from several birds from diferent parts of Spain and the Wolrd: the Ebro Delta, Ports Mountains, Extremadura, Andalucia, Pyrenees, Morocco, Turkey. With hundreds of images from birds, other wildlife, landscapes and traditions