Prion bills from Warham (1990)

D = Fulmar Prion

E = Fairy Prion
Cropped from above photo
Comparison with Fairy Prion (right) photographed off New Zealand, by Steve Wood. Note bill width.
Brooke (2004), quoting Harper (1980), records the following bill widths for the Fairy Prion; for example, of 69 males measured, bill width 11.5 5.81 using standard deviation, thus the bill widths sit between 10.1 to 12.9mm.
The measurements below are from HANZAB, the variation here from 9.0 to 12.5mm, some 40% difference on bill widths.
Bill measurements                 Males                         Unsexed                           Females
          P. turtur        20.4 - 24. 3    9.2 - 10.0    20.3 - 23.9    10.6 - 12.5    19.4 - 23.4    9.0 - 10.3
When seawatching, looking at prions from a ship's deck, the bill profile and width is important for the identification - the above just shows that these wide variations need to be taken into account also.....
Two more Fairy Prions for comparison - more usual colouration!
An invaluable link, a website by Edward Soldaat, showing seabird skulls, including prions: