Ageing and sexing resources

Identification Atlas of Birds of Aragon - The best online species guide for Europe
Species, Age and Sex Identification of Ducks Using Wing Plumage
Tabular Pyle - A user friendly presentation of the first Pyle guide. Some Non-passerines are also covered.
Ageing North American Landbirds by Molt Limits and Plumage Criteria

Photograph collections

Piranga - Photographs from North America
McGill Bird Observatory - Photographs with ageing and sexing tips from Montreal, Canada
University of Puget Sound - Slater Bird Wing Collection - Photographs of museum specimens
LaBORINg - Photographs from the Western Palearctic
Bluebonnet Bird Monitoring - From Louisiana, USA

Journals regularly publishing ageing and sexing information

Ringing and Migration - Published by the British Trust for Ornithology, content over 3 years old is free
Bird Study - Published by the British Trust for Ornithology, content over 3 years old is free. Some relevant information in older issues
Journal of Field Ornithology - Published by the Association of Field Ornithologists, mostly studies from North America
SORA - Free achives of old editions of many North Amerian bird journals

Ageing and sexing tips

Sorby Breck Ringing Group - Large active group of bird ringers from South Yorkshire, United Kingdom
Merseyside Ringing Group - Tips from bird ringers near Liverpool, United Kingdom