Most birdwatchers visiting Azores are aware of the occurrence of many bird subspecies on the islands.
   There are as in many other isolated areas local endemic subspecies, more or less distinguishable from nominate forms. Most well known is the Azores Bullfinch Pyrrhula pyrrhula murina, which now by most people are considered being a genuine species Pyrrhula murina. Other very obvious subspecies are the local Yellow-legged Gull Larus michahellis atlantis, and of course the three different subspecies of Goldcrests.
   There are many more local subspecies, but there are also probably more or less unknown vagrant subspecies occurring in the islands - some of them probably missed since birdwatchers do not pay attention to them, or maybe not are aware of the possibilities.
   For this reason the following "list" might be of help to put focus on the situation. Hopefully this will lead to a more careful documentation of some observations.
   And even if separation is impossible today, a good photo or description might well turn out to become a proof later on when there might be more knowledge accumulated.
   Note that the list is not a complete list of possible subspecies - just a list of most of those that might be possible to detect in the field. The list says nothing about how to separate different subspecies. Some guidance is however given here about where to find more of this kind of data in recent ornithological literature. A lot of information can also be found in many general handbooks and in special books covering groups of species.
   Note also that in Birding Azores we follow the list Holarctic Bird Species, which is just one of many lists where the nomenclature and separation into species or subspecies may differ from each other. Some subspecies mentioned on this page might be considered genuine species by others.

Species groups and subspecies which are mostly already recognized

In this category are a number of very similar species, where some of them not long ago were considered subspecies, and some already mostly well known subspecies.

These include: Most of the Anser and Branta Geese, Common/Green-winged Teal Anas crecca/Anas carolinensis,
Common/Black Scoter Melanitta nigra/Melanitta americana,
Fea´s Peterel/Zino´s Petrel Pterodroma feae/ Pterodroma madeira,
Cory's/Cape Verde Shearwater Calonectris diomedea borealis/d. edwardsii,
Manx/Balearic Shearwater Puffinus puffinus/Puffinus mauretanicus,
Little Shearwater Puffinus assimilis baroli/a. boydi,
Great Egret/American Great Egret Egretta alba alba/a. egretta,
American Golden Plover/Pacific Golden plover Pluvialis dominica/Pluvialis fulva,
Common/Wilson´s Snipe Gallinago gallinago/Gallinago delicata,
Black-tailed Godwit Limosa limosa limosa/l. islandica,
Whimbrel/Hudsonian Whimbrel, Numenius phaeopus phaeopus/p. hudsonicus
most larger gulls (Larus; fuscus graellsii, f. intermedius, smithsonianus, argentatus argentatus, a. argenteus, michahellis michahellis, m. atlantis, glaucoides glaucoides, g. kumlieni, g. thayeri),
Barn Swallow/American Barn Swallow, Hirundo rustica rustica/r. erythrogaster,
Yellow Wagtail Motacilla flava flava/f. flavissima, f. iberiae, f. thunbergi,
White Wagtail Motacilla alba alba/a.yarrelli, and
Northern/Greenland Wheatear, Oenanthe oenanthe oenanthe/o. leucorhoa.

Species where local subspecies are breeding in the Azores

There is a possibility that other subspecies than those breeding might turn up as vagrants in the Azores. This is mainly nominate forms from the European mainland, but also Cory´s Shearwaters from the Mediterranean islands, and "Laughing" Moorhens from North America.
   These subspecies could be more or less difficult to separate from the breeding birds, but in some species the time of the year and location could give some clues. For example an autumn observation of a European Robin in Flores or Corvo (where the local "form" is missing) would most probably be a vagrant from Northern Europe.
   The following subspecies would have possibilities to reach the Azores:

Common Quail Coturnix coturnix coturnix,
Cory's Shearwater Calonectris diomedea diomedea,
Common Buzzard Buteo buteo buteo,
Common Moorhen Gallinula chloropus cachinnans,
Common Wood Pigeon Columba palumbus palumbus,
Grey Wagtail Motacilla cinerea cinerea,
European Robin Erithacus rubecula rubecula,
Common Blackbird Turdus merula merula,
Blackcap Sylvia atricapilla atricapilla,
Goldcrest Regulus regulus regulus,
Common Starling Sturnus vulgaris vulgaris,
Common Chaffinch Fringilla coelebs coelebs.

Vagrants where more than one subspecies may occur

Greater Scaup Aythya marila marila/m. nearctica,
Common Eider Somateria mollissima mollissima/m. borealis/m. dresseri,
Goosander Mergus merganser merganser/m. americanus,
Great Cormorant Phalacrocorax carbo carbo/c. maroccanus/c. sinensis,
Hen Harrier/Marsh Hawk Circus cyaneus cyaneus/c. hudsonius,
Rough-legged Buzzard/Rough-legged Hawk Buteo lagopus lagopus/ l. sanctijohannis,
Merlin Falco columbarius columbarius/c. aesalon/subaesalon,
Peregrine Falcon Falco peregrinus peregrinus/p. calidus/p. tundrius/p. brookei,
Red Knot Calidris canutus canutus/c. rufa, c. islandica,
Dunlin Calidris alpine alpina/a. schinzii,a.arctica, a. hudsonia,
Common redshank Tringa totanus totanus/t. robusta,
Ruddy Turnstone Arenaria interpres interpres/i. morinella,
Common/Mew Gull Laruscanus canus canus/Larus c. brachyrhynchus,
Little/Least Tern Sternula albifrons/Sternula antillarum,
Black Tern Chlidonias niger niger/ n. surinamensis,
Sandwich Tern Sterna sandvicensis sandvicensis/s. acuflavidus,
Buff-bellied Pipit Anthus rubescens rubescens/r. japonicus,
Common Stonechat Saxicola torquatus rubicola/t. hibernans/t. maura,
Chiffchaff complex including Iberian and Canary Islands Chiffchaff Phylloscopus collybita collybita/c. abietinus/ c. tristis/Collybita ibericus/Collybita canariensis,
Common Redpoll Carduelis flammea flammea/f. cabaret/f. rostrata/f. islandica,
Arctic Redpoll Carduelis hornemanni hornemanni/h. exilipes,
Lapland Longspur Calcarius lapponicus lapponicus/l. subcalcaratus,
Snow Bunting Plectrophenax nivalis nivalis/n. insulae.

Other Azorean specialities

Although a little beside the focus of this list, it is difficult not to mention the two forms of Madeiran Storm-petrel Oceanodroma castro breeding in the Azores. By some already given full species status with the names proposed to be Grant´s Storm-petrel for the cool-season and Monteiro´s for the hot-season breeder. So far locality, time of the year and calls might be used for "field" identification.
   Furthermore the melanistic form of Blackcap Sylvia atricapilla gularis is so special that it deserve being focused on, and sightings being reported.

References:

The frontline when it comes to field identification is constantly moved forwards, and many facts and details are often found in new books concerning field identification in general, or for special species groups like ducks and geese, waders/shorebirds, pipits etc.
   Furthermore many articles are published in European or American birdwatching magazines. Some sources in English language (by no means complete) are listed (in scientific order) below:

Garner, M. 2008. Female-type Green-winged and Baikal Teals, pp: 138-145. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Scott, M. 1999. Identification of female Green-winged Teal. Birding World 12:81.
Millington, R. 1998. The Green-winged Teal. Birding World 11: 430-434.
Garner, M. 2008. Northern, American and Pacific Eiders, pp: 168-173. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Garner, M. 2008. Common and Black Scoter, pp: 154-160. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Waring, D. 1993. Identification Forum: female Black Scoter. Birding World 6: 78-79.
Alderfer, J. 1992. Immature Black Scoters. Birding World 5: 193-194.
Garner, M. 1999. Identification of Common Merganser. Birding World 12: 31 - 33.
Shirihai, H. 2008. An April expedition to Madeira and the challenge of Zino´s Petrel at sea. Birding World 21: 282-288.
Zino, F., Brown, R. & Biscoito, M. 2008. The separation of Pterodroma madeira (Zino´s Petrel) from Pterodroma feae (Fea´s Petrel). Ibis 150: 326-333.
Howell, S. and Patteson, B. 2007. Moult and Fea´s Petrel identification. Birding World 20: 201-202.
Steele, J. 2006. From the Rarities Committee's files: Do we know what British 'soft-plumaged petrels' are? British Birds 99: 404-419.
Harrop, A.H.J. 2004. The 'soft-plumaged petrel' complex: a review of the literature on taxonomy, identification, and distribution. British Birds 97: 6-15.
Tove, M. 2001. Verification of suspected field identification differences in Fea´s and Zino´s Petrels. Birding World 14: 283-289.
Gantlett, S. 1995. Field separation of Fea's, Zino's and Soft-plumaged Petrels. Birding World 8: 256-260.
Fisher, A. & Flood, B. 2004. A Scopoli's Shearwater off the Isles of Scilly. Birding World 17: 334 - 336.
Madge, S. 2003. Identification matters: Three of a kind. Birdwatch 12, no. 7: 22 - 25 (Cory´s Shearwater).
Porter, R., Newell, D., Marr, T. & Jolliffe, R. 1997. Identification of Cape Verde Shearwater. Birding World 10: 222 - 228.
Gutierrez, R. 2004. Identification of Yelkouhan, Balearic, and Manx Shearwaters. Birding World 17: 111-122.
Crochet, P.-A. 2006. Little Shearwater: underwing pattern. Birding World 19:19-20.
Garner, M. 2008. Cormorant races, pp: 99-103. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Millington, R. 2005. Identification of North Atlantic and Continental Cormorants. Birding World 18: 112-123.
Newson, S.E., Ekins, G.R., Russell, I.C. & Sellers, R.M. 2005. Separation of North Atlantic and Continental Cormorants. Birding World 18: 107-111.
Martin, J.P. 2008. From the Rarities Committee's files - 'Northern Harrier' on Scilly: new to Britain. British Birds 101: 394 -407.
Martin, J. 2008. Ringtail harriers (inc. Marsh Hawk), pp: 179-183. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Wallace, D.I.M. 1998. Identification Forum: Marsh Hawk - the end of a 41-year hunt? Birding World 11: 454-457.
Jensen, J.-K. 2006. Are dark morph Rough-legged Hawks overlooked in Europe? Birding World 19: 208-209.
Mullarney, K. & Murphy; J. 2005. The Rough-legged Hawk in Ireland. Birding World 18: 503 - 504.
Garner, M. 2008. Taiga Merlin, pp: 184-191. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Crochet, P.-A. 2008. A Taiga Merlin on the Azores: an overlooked vagrant to Europe? Birding World 21: 114-116.
Garner, M. 2002. Identification and Vagrancy of American Merlins in Europe. Birding World 15: 468-480.
Bourne, W.R.P. & Curtis, W.F. 2005. Letter: 'North American' Peregrine Falcons in Britain. British Birds, vol. 98: 260 - 261.
Robinson, P. 2004. Letter: 'North American' Peregrine Falcons in Britain. British Birds 97: 478 - 479.
Harrop, A.H.J. 2004. The 'North American' Peregrine Falcon in Britain - A review on behalf of the British Ornithologists' Union Records Committee. British Birds 97: 130 - 133.
Garner, M. 2008. American (Laughing) Moorhen, pp: 104-107. In: Garner, M. 2008. Frontiers in Birding. BirdGuides Ltd, Sheffield.
Vinicombe, K. 2007. ID in a depth - American Golden Plover and Pacific Golden Plover. Birdwatch 16, no. 9: 32 - 35.
Johnson, O.W. & Johnson, P.M. 2004. Biometrics and field identification of Pacific and American Golden Plovers. British Birds 97: 434 - 443.
Stoddart, A. 2007. An apparent Hudsonian Dunlin on the Isles of Scilly. Birding World 20: 464-466.
Reid, M. 2008. Identification of Wilson's and Common Snipe. British Birds 101: 189-199.
Lidster, J. 2007. The Wilson´s Snipe on the Isles of Scilly. Birding World 20: 432-435.
Leader, P. 1998. Identification forum: Common Snipe and Wilson´s Snipe. Birding World 12: 371-374.
Roselaar C.S., Gerritsen, G.J. 1991. Recognition of Icelandic Black-tailed Godwit and its occurrence in the Netherlands. Dutch Birding 13: 128-135.
Vinicombe, K. 2007. Hudsonian Whimbrel. Birdwatch 179: 28-31.
Alfrey, P. & Ahmad, M. 2007. Short-billed Gull on Terceira, Azores, in February-March 2003 and identification of the 'Mew Gull complex'. Dutch Birding 29: 201 - 212.
Muusse, T., Muusse, M. Luijendijk, B.-J. & Altenburg, R. 2005. Identification update: moult variability in 3rd calendar-year Lesser Black-backed Gulls. Birding World 18: 338 - 348.
Eskelin, T. & Pursiainen. 1998. The status of Lesser Black-backed Gulls of heuglini, graellsii and intermedius type in Finland. Alula 4: 42 - 54.
Baxter, P. and Gibbins, C. 2007. Identification of Kumlien´s and American Herring Gull and other large gulls at St John´s, Newfoundland. Birding World 20: 162-175.
Adriaens, P. & Mactavish, B. 2005. Identification of adult American Herring Gull. Dutch Birding 26: 151-179.
Lonergan, P. & Mullarney, K. 2004. Identification on American Herring Gull in a western European context. Dutch Birding 26: 1-35.
Howell, S.N.G. & Mactavish, B. 2003. Identification and variation of winter adult Kumlien´s Gull. Alula 9: 2-15.
Howell, S.N.G & Elliott, M.T. 2001. Identification and variation of winter adult Thayer´s Gull, with comments on taxonomy. Alula 7: 130-144.
Chandler, R.J. & Wilds, C. 1994. Little, Least and Saunders´s Terns. British Birds 87: 60-67.
Massey, B.W. & Atwood, J.K. 1978. Plumages of the Least Tern. Bird-Banding 49: 360-371.
Massey, B.W. 1976. Vocal differences between American Least Tern and the European Little Tern. Auk 93: 760-773.
Mullarney, K. 2006. The American Black Tern in Co. Wexford. Birding World 19: 279-282.
Andrews R.M., Higgins, R.J. and Martin, P. 2006. American Black Tern at Weston-super-Mare: new to Britain. British Birds 99: 450-459.
Hallam N. and Lewington, I. 2009. Identification of American Black Tern . Birding World 22: 383-388.
Garner, M., Lewington, I. & Crook, J. 2007. Identification of American Sandwich Tern (Sterna sandvicensis acuflavida). Dutch Birding 29: 273 - 287.
Jiguet, F. & Zucca, M. 2005. The American Barn Swallows on the Azores - a new Western Palearctic bird. Birding World 18: 475-478.
French, P. 2006. Identification of American and Siberian Buff-bellied Pipits in the Western Palearctic. Birding World 19: 499-515.
Lee, C. & Birch, A. 2002. Notes on the distribution, vagrancy and field identification of American Pipit and 'Siberian Pipit'. North American Birds 56: 389-398.
Alström, P. & Mild, K. 1996. The identification of Rock, Water and Buff-bellied Pipits. Alula 4: 161-175.
Dubois, P. 2007. Yellow, Blue-headed, 'Channel' and extra-limital Wagtails: from myth to reality. Birding World 20: 104-112.
Siddle, J.P. 2006. Letter: Which subspecies of Common Stonechat breeds in coastal Portugal? British Birds 99: 372 - 374.
Hering, J. 2005. Erstnachweis einer “Schleiergrasmücke” Sylvia atricapilla auf Flores, Azoren. Limicola 19: 217-224.
Collinson, M. and Melling, T. 2008. Identification of vagrant Iberian Chiffchaffs - pointers, pitfalls and problem birds]. British Birds 101: 174-188.
Millington, R. 2006. Identification of a Siberian Chiffchaff in Norfolk. Birding World 19: 14-16.
Dean, A.R. & Svensson, L. 2005. 'Siberian Chiffchaff' revisited. British Birds 98: 396-410.
Reid, J. & Riddington, R. 1999. Identification of Greenland and Iceland Redpolls. Dutch Birding 20: 261-271.
Pennington, M. & Maher, M. 2005. Greenland, Iceland & Hornemann's Redpolls in Britain. Birding World 18: 66 - 78.
Votier, S.C., Steele, J., Shaw, K.D. & Stoddart, A.M. 2000. Arctic Redpoll (Carduelis hornemanni exilipes): an identification review based on the 1995/96 influx. British Birds 93: 68 - 84.
Millington, R. 1996. Identification forum: Arctic Redpolls revisited. Birding World 9: 65 - 69.
Mjøs, A.T. 2007. Lapland Bunting origins. Birding World 20: 348.
Garner, M. 2007. Where do our Lapland Buntings come from? Birding World 20: 203-207.