A COMPARISON BETWEEN COMMON AND SOUTH GEORGIA DIVING-PETRELS        
The photographs below show Common Diving-Petrel (left) and South Georgia Diving-Petrel (right)

These birds were unfortunately found dead on the decks of Explorer whilst we were on a cruise in the southern Atlantic Ocean (November 2002). They had come aboard attracted by the ship's lights during a severe storm.

The study skins were prepared on board for the purpose of these photographs. The identification of these birds was based on 'Seabirds - an identification guide' (Harrison 1983) and 'Seabirds of the World - a photographic guide' (Harrison 1987), with reference to Murphy and Harper (1921) and Payne and Prince (1978) - see Harrison for bibliography.

The South Georgia Diving-Petrel was identified by the posterior black line down the tarsi,
shape of bill from underneath, and measurements in the hand    
Photo 1

(1) This South Georgia Diving-Petrel (above right) does not show distinct pale tips to the scapulars, which can often be a good pointer to specific identification in the field. Compare the South Georgia Diving-Petrel below.      
(2) White tips to the secondaries are often quoted as being distinctive on South Georgia Diving-Petrel - you can see on Photo 1 there were some white tips to the secondaries on both birds (note right closed wing on specimen, and the bird above), but the upper photo shows how difficult these are to see (open left wing)

(3) Neither of the birds in Photo 1, when flying and seen from above i.e. a ship's deck, could have been specifically identified at sea
Photo 2

(1) This photograph shows the undersides of the bills. The drawings in 'Seabirds of the World' are better than those in 'Seabirds - an identification guide'.

(a) The South Georgia Diving-Petrel (right) shows the so-called 'Gothic arch' shape, a broad-based bill converging gradually. The Common Diving-Petrel (left) shows the more parallel sides to the bill converging more nearer to the tip
(b) In the hand Common Diving-Petrel has a longer bill (South Georgia 5.0 - 5.7mm, Common 6.0 - 7.3mm)

(2) The mottling to the throat and sides of neck is variable in both species        
Photo 3

The underwing coverts are said to be white on South Georgia Diving-Petrel and grey on Common Diving-Petrel ('Photographic Handbook to the Seabirds of the World', 'Seabirds' etc)  - this is not borne out with these two birds, both having very similar underwing colouration. The small size of these birds, flying low with rapid whirring wingbeats make seeing the underwing in the field virtually impossible anyway.        

Comment

These photos show how difficult these two species can be to tell apart. In the hand there are measurements, the shape of the bill, and South Georgia Diving-Petrel has a black line running completely down the back of the tarsus. In some regions of the southern oceans only the Common Diving-Petrel is found, and there is no confusion species. Where both Common and South Georgia Diving-Petrels are sympatric then field identification can be very tricky indeed. There may be a combination of features that allude to one species or the other, but on current knowledge the specific identification of most, if not all, at sea would seem impossible.