Request for Info: Bird Feather Colour

Hello again,

Does anyone know, offhand, what class of chemical is responsible for 
the colours found in bird plumage?  Beta-carotenes, perhaps.  Or do the 
colours we see arise from  optical effects such as interference, liquid 
crystals (as in beetles, I believe).   Alternatively is there a good 
reference which may be of assistance.

Thanks and cheers,

Llew Rintoul