Let’s start with an easy one!
With the obvious bulk and long bill this is clearly a Christmas Island Frigatebird (CIF). Juveniles are not known to be sexable on plumage, but since females of all frigatebirds are larger and longer-billed than males, I would hazard a guess that this is a female. Full juvenile plumage would have a solid breast band (two ‘lobe shapes’ either side of the breast meeting in the centre). Moult into 2nd year plumage begins to replace the black breast band feathering with white, from the centre outwards, and this has begun on this bird. The belly patch on juv CIFs is hexagonal (triangular on LFs) and axillary spurs originate behind the line of the breast band, as here.
The angle of the photo makes id. trickier , as the bill proportions and belly patch shape are disguised by the slightly head-on perspective. Nevertheless, the belly patch is roughly hexagonal, the axillary spurs originate behind the breast band, and the ‘double-lobe’ shape of the breast band can all be seen, so I make this a juv CIF too.
This juvenile Lesser Frigatebird (LF) has a distinctly triangular white belly patch, the ‘base of the triangle is formed by the rear border of the breastband, which is more or less straight rather than formed by two obvious lobes. The axillary spurs originate from the front of the belly patch (extending outwards from the basal corners of the triangle), and are triangular rather than parallel-sided.
A slightly older juvenile LF. The tawny head feathers have become white as a result of abrasion, and the breast band has begun to whiten from the centre.
Apparently the same individual, but the axillary spur on its left wing is almost completely obscured.
A couple of second year LFs. This is in some ways the most confusing plumage, as at this stage, LFs share the white belly of adult female CIFs. In the 2nd year, the breast band disappears and the white belly (from juvenile plumage) starts to develop black, adult-like colouration.
The presence and shape of the axillary spurs is of critical importance for identification at this stage. Great Frigatebird (GF) lacks them; CIF has parallel-sided, forward-angled spurs, or lacks them; and LF has triangular, outward-angled spurs. 2nd year CIF has black ‘breast tabs’ which are absent in the other two species.
I think these are both 3rd year LFs. A pale throat and breast on an otherwise dark frigatebird is usually an id. feature of GF. However, both these birds have LF-shaped axillary spurs, and I think the pale throat can be explained by the fact that the birds are moulting from 2nd year (white head) to 3rd year (dark head) plumage. I think the upper bird is a female (with an adult female-like breast saddle), and a lower one a male (as the black on the belly comes too far up toward the breast for a female, starting to isolate the white axillary spurs).
Frigatebirds all sorted now? Try identifying, aging and sexing the birds in these four pics and see how you get on!
Left to right: