The McCaulay Library Cornell has an amazing collection of bird sounds. Greg Budney show’s you inside the library (YouTube 2011)
xeno-canto Bird songs from the Americas and around the world. My recording here.
North American Bird Sounds An wide selection of bird sound with a few sentences about each.
Western Palearctic Region – European Birds Songs, Calls and Sonagrams.
Link to Bird Sounds – an impressive array of links to bird sounds sites.
Bird Life International’s Bird Links to the World – Bird Sound page. Many links to world sounds.
Nathan Pieplow blog “earbirding” Learn to study sonograms and lots more!
Also read about a great app from Pocket Chirp for making spectrograms in the field on your iPhone.
Listening to Nature – A Sound Walk Across California From the Oakland Museum