Agreeing on a taxonomy is not easy. Not only does bird taxonomy vary significantly from one authority to another, but it is in a state of constant change. There is no single authority to rely on; no one list can claim to be the list. However, the de facto standard for Wikipedia bird articles is the IOC World Bird List which is updated four times a year (currently version 6.3). This is preferred for all articles, although exceptions may be made in particular cases.
Major well recognised resources include:
For Africa, Roberts' Birds of Southern Africa has been recognised as the authoritative book on southern Africa's birds since its first publication in 1940. A new edition has been published. The list is available online here.
For Europe, the Middle East, and North Africa (i.e. the Western Palearctic), the nine-volume BWP or Birds of the Western Palearctic is considered the standard reference. It is also available as a two-volume concise edition. (See the publisher's site.) BirdGuides has combined the text of BWP, the text and plates of the concise edition, with text from the BWP Update journal, images, video and sound recordings into Birds of the Western Palearctic interactive DVD-ROM. The list does not seem to be available online, however.
There are also a number of family monographs (such as the Hayman "Shorebirds" and Harrison's "Seabirds" in the Helm Identification Guides series); some are available on line at Amazon using the Search Inside feature (which requires a free membership in Amazon). Although a mine of information, these books reflect the authors' idiosyncrasies and soon become dated.
A number of useful free to view online resources exist that are useful in writing bird-related articles:
Referencing (free scientific articles from ornithological journals etc.)
FreeFullText Alphabetical list of scientific journals with free full-text articles.
SORA Searchable online research archive, University of New Mexico. This site has decades worth of archives of the American ornithology journals, such as The Auk, The Condor, Journal of Field Ornithology, Ornitologia Neotropical, Studies in Avian Biology Pacific Coast Avifauna, and the Wilson Bulletin. Coverage ends around 2000, but still extremely useful. The ability to search almost all journals and browse issues exists on the front page.
Royal Society of New Zealand Free archive of several interesting journals until 2004. Link to Transactions and Proceedings of the Royal Society of New Zealand archive 1868-1961.
PALMM Textual Collections a State University of Florida project. Several monographs available via All Collections search, such as Pierce Brodkorb's "Catalogue of Fossil Birds".
Forktail Oriental Bird Club journal. Deals with South, East and Southeast Asia and surroundings. Several complete issues available.
Birds of North America, Cornell University's massive project collecting information on all bird species breeding in the ABA area. It isn't free, but available for 40 USD a year. Access is sometimes available via university libraries.
Malimbus, The Journal of the West African Ornithological Society; as well as the even older Bulletin of the Nigerian Ornithologists' Society. Mostly covers bird distribution in sites across the area, but also notes on behaviour and ecology of obscure birds from a little studied part of the world. The Journal is bilingual and several articles are in French.
Biodiversity Heritage Library is a large database that includes scans of much of the older ornithological literature. It is particularly useful for taxonomy and includes all 16 volumes of Peters' Check-list of birds of the world.
BirdLife International The Data Zone has species accounts for every species, although only threatened species have any detail beyond status and evaluation. But there is still a lot of information to be had there.
Zoonomen A great source of authors for the taxoboxes.