Elevate Fregata ariel trinitatis to species rank

Proposal (768) to South American Classification Committee

Elevate Fregata ariel trinitatis to species rank

Effect on South American CL: This proposal would add an Atlantic Lesser Frigatebird, Fregata trinitatis to our main list.

In the South Atlantic, Lesser Frigatebirds once bred on both Saint Helena and Trindade (this second, over 1,100 km east of mainland Brazil). The Saint Helena population disappeared in ancient times and is only known from subfossil remains, estimated to be a few hundred years old. More recently, it disappeared as a breeding bird from the main island of Trindade, effectively restricting its known breeding range to a small rocky islet off the main island. It is unclear whether it breeds on the Martin Vaz Islands, another part of the Trindade archipelago (Olson 2017). The Atlantic populations are starkly isolated from their nearest relatives in the Indian and Paciﬁc oceans.

Miranda-Ribeiro (1919) described Fregata ariel trinitatis (as subspecies, valid name) only on the assumption that it would have smaller proportions. This Brazilian author provided only bill and wing measurements (81 mm and 510 mm, respectively) vs. a small table of measurements of Australian F. ariel (83–92 mm and 523–533 mm) from Mathews (1915).

However, the birds from the South Atlantic cannot be diagnosed from other Lesser Frigatebirds based on length measurements from skins, or length measurements of rostrum and wing bones (Olson 2017), although means of length measurements of culmen in males are signiﬁcantly smaller. In the Atlantic birds, two measurements of bill width indicate a considerably stouter bill than in F. ariel, and bill lengths of males are statistically signiﬁcantly smaller than in F. ariel.

Examining osteological data, Olson (2017) found that length measurements of humerus, ulna, and rostrum do not differ between the two taxa, but the width measurements conﬁrm that the wing elements and rostrum of F. trinitatis are appreciably more robust than in F. ariel.

Certain plumages differ consistently between Fregata ariel and F. trinitatis, although I have detected none between adult males. With the larger series available to Olson (2017), he found that the subadult plumage of the South Atlantic population is distinctive: the top of the head and hindneck are dark brownish and the throat a smoky gray.

The Atlantic birds evidently have a juvenile plumage with a rufous head as in Indo-Paciﬁc F. ariel, but this appears to be very evanescent and must be quickly lost as there is no hint of rufous in any of the subsequent plumages, whereas in F. ariel there is some rufous in the plumage of all but adult males (Olson 2017).

These differences should be sufﬁcient an argument for raising the Atlantic Lesser Frigatebird, Fregata trinitatis, to full speciﬁes rank (given the minor differences among other taxa ranked as species in Fregata), separate from Indo-Paciﬁc Lesser Frigatebird, F. ariel. Unfortunately, Fregata trinitatis now remains on Trindade in extremely small numbers and is in imminent danger of extinction.

Recent combined studies suggest that the remaining South Atlantic population is tiny, possibly numbering less than 20 breeding pairs (Olson 2017). If recognized as a separate species, as proposed in 2017, this South Atlantic endemic qualifies as Critically Endangered, as already dealt with by Brazilian official list of threatened animals.

Recommendation: I recommend a "YES" vote on accepting Fregata trinitatis as species-level taxa to our list.

Literature Cited:

Miranda Ribeiro, A. 1919. A fauna vertebrada da Ilha da Trindade. Archivos do Museu Nacional do Rio de Janeiro 22:169–194.

Olson, S.L. (2017) Species rank for the critically endangered Atlantic Lesser Frigatebird (Fregata trinitatis) Wilson Journal of Ornithology 129(4):661-675.

José Fernando Pacheco, January. 2018

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Comments from Stiles: "YES. In view of its wide geographical isolation as well as plumage and morphological differences, species status for trinitatis seems justified."

Comments from Steve Howell: " This may well be a valid split, but information on plumage sequences was potentially misrepresented and should be clarified.

"While I am not qualified to comment on the taxonomic or biological significance of small statistical differences in robustness of some skeletal elements, I draw attention to some aspects of plumage differences reported. The sample of trinitatis is inevitably small, and the plumage sequences of neither it nor Indo-Pacific Lesser are fully understood. However, evidence from careful studies of Magnificent, Great, Lesser (Indo-Pacific), and Christmas Frigatebirds (Howell 1994, James 2004) indicate that all of these species have a juv/1st-year plumage (sexes similar), a 2nd-cycle plumage (sexes similar), and then 3rd- and 4th-cycle “immature" and “subadult" plumages in which sexes are distinguishable. These 2nd (especially) through 4th cycle plumages are seen (and likewise photographed and collected) infrequently at best, even on the common and easily observed ‘mainland' Magnificent Frigatebird, and I don’t know that they are fully known for Lesser.

"Regardless, it is erroneous to state that in Indo-Pacific Lesser “the head feathers molt directly from the rufous juvenile plumage into the glossy black adult plumage” (Olson 2017: 673-674), as this skips several years of molts and plumages (a little like saying a juvenile Herring Gull molts directly into adult plumage), unless perhaps the terms "juvenile" and “adult" are being used loosely, which would be unfortunate in a scientific paper. The term “subadult" is also imprecise, although a suggestion that the brown-headed phase of trinitatis occurs in both sexes might indicate it to be a 2nd-cycle plumage.

"Much of the proposal’s weight seems to rest on a distinctive immature plumage, but, as noted above, until plumage sequences are carefully documented and like compared to like for the 6+ predefinitive plumages (1st, 2nd, 3rd male and female, 4th male and female cycles), the unique “subadult” plumage of trinitatis remains to my mind unproven. Is it real, or is it like comparing a 2nd-year plumage to a 3rd-year plumage of a single large gull species? For example, the frontispiece image for Olson (2017) may well be comparing a 2nd-cycle trinitatis with a 3rd or 4th cycle ariel; hence not a direct, valid comparison. Moreover, image B in figure 3 (Olson 2017) showing a “female changing from juvenile to adult” is likely a 4th cycle bird, and thus much older than a juvenile, as well as being likely of a different age to the proposed unique “subadult” plumage of trinitatis.

"If the age/plumage cycle of brown-headed trinitatis can be ascertained and compared directly to same-age ariel that would strengthen the case.

"The plumage differences reported in adult females are average at best (clean white in trinitatis, tinged rusty in some ariel but clean white in others; might a larger sample find some trinitatis with rusty staining?), and no differences are known for adult males, so the existence of a unique immature plumage requires careful documentation. Might it even be simply a morph? Plumage morphs are not certainly documented in immature frigatebirds, but apparently occur in adult Ascension Frigatebirds, perhaps also in juveniles, but data are scant.)

"In terms of its biogeographic isolation, trinitatis seems likely a good candidate for species status (although perhaps no more so than the Atlantic Great Frigatebird, or tens of other pelagic birds = think brown boobies, tropicbirds). However, given the small samples of trinitatis, are the few “statistically significant” differences found in measurements also biologically significant differences?

"Were I in a voting position, I would vote NO pending a critical evaluation of plumage sequences.

"References:

Howell, S N G. 1994. Magnificent and Great Frigatebirds in the eastern Pacific—a new look at an old problem. Birding 26:400-415.

James, D. 2004. Identification of Christmas Island, Great, and Lesser Frigatebirds. Birding Asia 1:22-38."

Comments from Areta: "NO. The comments by Steve have helped me strengthen my initial doubts on the validity of the proposed split. Given the complexities and little knowledge on plumage sequences, potential misassignation of birds to different ages and the minor morphological differences of trinitatis vs. nominate ariel I am not convinced that the Atlantic birds should be recognized as a different species from the Indo-Pacific ones. This is a border-line case and one that puts some extra pressure on our shoulders, given the critical conservation situation of the Atlantic population. It is clear that this population deserves protection regardless of its taxonomic status (whether a subspecies or a species), and that further taxonomic work is warranted.

"As a side note, Storrs's paper also suggests that our current Fregata magnificens might deserve to be split in two: Fregata magnificens in the Galapagos and Fregata rothschildi in the remainder of its range."

Comments from Zimmer: “NO. Given the complexities of the extended plumages in frigatebirds in general, and the under-sampling of trinitatis in particular, I’m hesitant to place too much stock in the described differences in “subadult” plumage. As Steve Howell states in his comments, “subadult” is an imprecise term, particularly when dealing with a large seabird whose molt sequence is essentially continuous. Apparent discontinuities between populations in “subadult” plumage characters could merely reflect the inadequacy of the samples of like-aged birds. As Howell notes, the geographic isolation of trinitatis makes it a likely candidate for species status, something that genetic analysis would likely reflect, but the putative plumage and morphometric differences are unconvincing to me, given the weaknesses of sample size.”

Comments from Stotz: “NO. Olson et al. may very well be right about trinitatis deserving species status, but even if they have not confused different aged subadults, I am not completely convinced that this leads inexorably to considering them as distinct species. Given that there are potential problems regarding the plumage sequences in these birds, I definitely think we should not split these taxa at this time.”

Comments from Jaramillo: “YES, with hesitation. The plumage sequences are yet unknown for trinitatis or the nominate. However, this distinctive plumage (whatever age it actually is) seems to have been detectable in a small sample of trinitatis, while it is unknown (trusting the paper here, I don’t know) in a much larger set of specimens from the rest of the distribution. Numerically this does not make sense unless the specimens happen to “hit the jackpot” and detect the rare plumage, that the larger set of specimens of ariel does not. So, statistically speaking, it seems much more likely that it indeed is a unique or much more common plumage in trinitatis than ariel. Again, irrespective of what age state it is from. This in addition to the highly isolated distribution of trinitatis, and the fact that it is smaller but is more robust in bill thickness, and some bone elements just tips me over the fence on the YES. As Nacho comments, there are others in Fregata that need to be thought about.”

Comments from Pacheco: “NO. In the face of Steve's comments, doubts about the validity of the proposal become strong. I think we should not assume the division of these taxa at this time.”

Comments from Remsen: “YES. Alvaro’s reasoning has convinced me. I think burden of proof is on the assertion that this plumage occurs in ariel. Furthermore, the bar is set very low for species limits in Fregata.”

Comments from Robbins: “NO. Trinitatis may merit recognition as a species, but given Steve Howell’s comments and the rather minor morphological differences between trinitatis and nominate ariel, I vote NO for now.”