Now down to the nitty-gritty of frigatebird identification! For this I relied pretty much exclusively on the seminal identification paper by David James, Identification of Christmas Island, Great and Lesser Frigatebirds, which came out in the very first BirdingASIA back in 2004.
The first thing which surprised me was how different Christmas Island Frigatebirds (CIFs) and Lesser Frigatebirds (LFs) were structurally. CIFs (left two birds) were visibly bigger, stockier and broader-winged even with the naked eye, and the bill was markedly longer.
Now to plumages. The above image shows two adult males. Male CIFs have a distinctive squarish white lower belly and male LFs just have thin ‘spurs’ that angle back from the axillaries to the body. Adult male Great Frigatebirds (GF) are all black, but I didn’t see any of those! This looks like one, from Jason’s blog.
An adult male CIF (top 2 pics). I think the lower image may show a 4th year bird, as the lower edge of the breastband, the belly-patch and axillary spurs all show a shape reminiscent of younger birds, and the gular pouch appears pink rather than red. But it may just be an adult variant.
Adult female LFs have a black hood, white (usually) breast with a concave lower edge, black belly and white triangular spurs. The tawny wash on the breast is said by James to be present on some birds, but not typical. It seemed a common feature of Mantanani birds however.
Juveniles and subadults get a lot trickier!