[Frigatebirds] Great Frigatebird 黑腹軍艦鳥

Great Frigatebird

Congratulations to all photographers for a great set of pics, and to Paul for finding this bird at long range in the mist. Identifying juvenile frigatebirds is not easy, but these photos clearly confirm the field identification as Great Frigatebird.

By its much larger size and wingspan compared with the two nearby Black Kites, it could be seen from far off that this was one of the larger species [ie Christmas Island or Great], and not the smaller Lesser Frigatebird which is nearer to Black Kite in size. Seen closer the bill was also too long for Lesser. Christmas Island has the longest bill on average followed by Great. However there is a large overlap which is further complicated by females of each species being bigger than their respective males. Thus a female Great is similar to a male Christmas Island.

The main feature seperating juvenile Great from Christmas Island is the shape and extent of the white underparts. From directly underneath, the shape appeared classic for typical Great with a curved [convex] boundary with the dark breast band, tapering sides which narrowed towards the rear, and a rounded or square-cut [not pointed] cut-off across the lower belly in front of the feet and ventral area. The clincher is the absence of prominent solid white axillary spurs extending out from the sides of the middle of the white underparts onto the axillaries and leading forward onto the underwing area, which is a diagnostic feature of Christmas Island.

However, when viewed from the side, the shape of the white underparts can be seen to be more complex and variable. On the right side there is a clear short white spur extending out across the upper flanks, but not across the axillaries onto the underwing. Curiously, this is assymetric and not matched by a similar strong feature on the left. The spur is accentuated by the dark rear flanks cutting in behind. Individual Great Frigatebirds can show short spurs or even extended scalloping across the underwing, and the pattern on the HK bird is within the range observed for Great and not Christmas Island.

A final point is that the alar bars [the pale diagonal areas across the inner upperwings] appear buffish and not whitish. All juveniles have prominent alar bars, but Christmas Island has the most marked which often appear whitish after bleaching and wear. The less prominent alar bars on the HK bird further reinforce its identity as Great.

It is interesting to note that the date fell within the same period [14 April to 20 May] as the three previous accepted records.

Mike Chalmers