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gearrbhall m., gen. gearrbhuill, 
This is a reconstructed form, based on AFB˄’s own gearra-bhuill; the genitive is otherwise unattested.
While Lockwood early on sees gairfowl, garefowl as an anglicisaton of a Scottish Gaelic reflex of ON geirfugl, he later concludes that the second element of a Scottish Gaelic form gearrbhall would be unlikely to develop into Eng. fowl, instead proposing that ON geirfugl yields SG gearrbhall via an unattested Orkney ‘Norn’ intermediary, and that consequently ‘gairfowl, garefowl can be interpreted as essentially Orkney Norn, with gair-, gare- regularly corresponding to ON geir- (Marwick 1929, xlii) and -fowl as the expected anglicisation of -fugl’ (≈Lockwood 1978, 396).
ON geirfugl
From ON geir, the stem form of geirr m. ‘spear; point of an anvil’ + fugl m. ‘bird’.
Here with gearra, a compound form of SG geàrr, EG gerr (possibly a nominal form of the adjective geàrr (gerr) ‘short, ?mincing, ?quick’), + breac adj. ‘speckled’ and the genitive of gort ‘field’, respectively. EG gerr initially yields SG gearr, with its short vowel + long consonant developing in most dialects into gèarr, geàrr, with a long vowel + short consonant (cf. SGDS Item 470: geàrr ‘to cut’), e.g. geàrr-chlamhan ‘buzzard’ (cf. AFB˄), but the short vowel is retained in stressed epenthesis (svarabhakti) environments, e.g. gearrbhonn ‘the outer sole of a shoe’ (cf. Dieckhoff 1932), and when followed by a vowel, e.g. gearrachlamhan (cf. Wentworth 2003, s.v. kite). Note that there would be a tendency to shorten the long vowel of forms such as geàrr-chlamhan by virtue of its weakly stressed position.
Although *[ˈɡ̊ʲɑɍ͡ɑvɑɫ̪] would seem to be a likely outcome given the folk etymological connection with SG ball and the consequent spelling.
Alexander Carmichael (collector of CG) gives gearrabhul in Harvie-Brown and Buckley 1888, 158. Fergusson (1886, 82) has gearbhal; Forbes (1905, 30) gearrbhall, gearrbhuil, gearbhal; CG II, 284, gearr-bhall and gearra-bhall; Dwelly (1911) and AFB˄ gearra-bhall; Cunningham (1990, 135) garra-ball, in error; MacFhearghuis (1995, 59) and Garvie (1999, 63) gearra-bhall.
Pace Lockwood, it seems quite plausible that Martin’s gairfowl and Macaulay’s garefowl are partially folk etymologically driven scotticisations of SG gearrbhall, with its second element equated with Scots foul, fowl or Eng. fowl ‘bird’; indeed, Macaulay goes further in suggesting garefowl was a corruption of rarefowl, an explanation perhaps promoted by the bird’s relative scarcity: ‘The St Kildians do not receive an annual visit from this strange bird ... It keeps at a distance from them, they know not where, for a course of years’ (Macaulay ibid., 157).
Harris and Murray (1989, 27) state that the great auk bred in St Kilda ‘in the 17th century’. The last St Kilda bird is said to have been caught on Stac an Àrmainn ‘the stack of the hero’ (for the specific, see SG àrmann) NA151064 in 1840 (Harvie-Brown and Buckley 1888, 159), although MacKenzie (1911, 39) implies the bird was already extinct there by that year, and Wiglesworth (1903, 37–38) dates the killing on Stac an Àrmainn to between 1830–1835.
The folk etymological explanation that gearrbhall derives from SG geàrr ‘short’ and ball ‘spot’ is seen in Alexander Carmichael’s translation ‘the squat spotted one’ (CG II, 284; so also Forbes), although gearrbhallach, with nominal use of the adjectival suffix -ach, might be expected (cf. Black 2006˄). This may have influenced Carmichael's spellings gearr-bhall and gearra-bhall, and appears to be perseverated in AFB˄’s phonemic transcription /gʲaRəvaL/, which suggests a reduced form of (lenited) ball /bɑuʟ/.
In summary, SG gearrbhall (now generally gearrabhall) appears to be a loan-shift (with complete morphemic substitution) from ON geirfugl, while Scots gairfowl, garefowl appears to be a loan-blend (with part phonemic substitution and part morphemic importation) from *gèarful, an earlier Scottish Gaelic reflex of the Old Norse word.