Proposal to split Australian Tern from Gull-billed Tern - AOS

[Michael Szabo]

The American Ornithological Society is considering a proposal to split Australian Tern from Gull-billed Tern submitted by
Shawn M. Billerman, Cornell Lab of Ornithology, and Terry Chesser:

Treat Gelochelidon macrotarsa as a separate species from Gull-billed Tern G. nilotica

Background:
The Gull-billed Tern Gelochelidon nilotica has long been recognized as a cosmopolitan species
with breeding populations on every continent except Antarctica. Many taxonomic authorities had
recognized 6 subspecies, sometimes grouped into two distinct groups (Dickinson and Remsen
2013, Clements et al. 2022), with macrotarsa recognized by its larger size, overall different
structure, paler plumage, and darker and more extensive black around the eye in juvenile and
basic plumages, along with differences in molt, ecology, and behavior (Rogers et al. 2005,
Mlodinow 2023). These differences have led some global taxonomic authorities to recognize the
taxon macrotarsa as a distinct species (e.g., del Hoyo and Collar 2014, Gill and Donsker 2019).
In their assessment of the system using the Tobias et al. (2010) scoring criteria, del Hoyo and
Collar (2014) justified the split on the following basis:
Hitherto treated as conspecific with G. nilotica, but differs in its considerably larger
size, with effect size for bill 3.69 (published data (Rogers et al. 2005); score 2);
differently shaped bill, with culmen more decurved and negligible gonydeal angle (2);
nomadic, opportunistic and kleptoparasitic behaviour decoupled from tightly
scheduled migration patterns (1); paler grey upperparts (1); more extensive black
patch around the eye and ear-coverts in winter (1). Monotypic.
WGAC considered the split because of the differing treatments among global taxonomic
authorities, voting unanimously to split G. macrotarsa from G. nilotica. This has now been
adopted by the Clements checklist (Clements et al. 2023). Although G. macrotarsa has never
occurred within the NACC area, we are considering the issue to bring our concept of G. nilotica
into alignment with that of other global checklists.
New Information:
As far as I am aware, NACC has never considered this split. What follows is a brief summary of
the published information on differences between the macrotarsa group and the nilotica group,
mostly from Rogers et al. (2005), who studied the two groups in northwestern Australia, where
they overlap during the nonbreeding period.
Plumage Differences
Australian macrotarsa and the nilotica group are very similar, but do differ in some aspects of
plumage. Australian macrotarsa has much paler gray to almost white upperparts, including an
entirely white tail and rump, whereas nilotica has darker gray upperparts and a pale gray tail
with white outer tail feathers. In basic and juvenile plumages, macrotarsa has a much larger
blackish patch over the eye and auriculars. In addition, in basic plumage, macrotarsa usually
has some black speckling in the crown, whereas nilotica has a white crown.
148
Morphological Differences
Australian macrotarsa is larger in most measurements, with a significantly longer bill, deeper
bill, longer total head length, longer wings, longer tarsi, and greater mass (Rogers et al. 2005).
In addition to these size differences, bill shape also differs, with a decurved culmen and a
gonydeal angle close to the bill tip, giving the bill a decurved appearance, versus the relatively
straight appearance of the bill of the nilotica group, with a straighter culmen and gonydeal angle
closer to the base of the bill (Rogers et al. 2005).
Ecological Differences
Molt strategy differs significantly between macrotarsa and nilotica. Because they appear to
breed opportunistically when conditions are right (they breed at inland wetlands in Australia,
which are ephemeral, with specific breeding locations only occupied certain years), molt in
macrotarsa can occur at different times of year, and molt can be suspended in ways that is not
observed in nilotica, which has regular breeding and molting seasons. Although most breeding
occurs in macrotarsa during the austral spring and summer (September to April; Mlodinow
2023), breeding has also been recorded between May and August (Rogers et al. 2005). This
variability in breeding time can result in simultaneous waves and extensive overlap of pre-
alternate and pre-basic primary molt in macrotarsa, with it being able to suspend molt if the right
breeding conditions arise, whereas nilotica may only show slight overlap of pre-alternate and
pre-basic molts at specific times of the year.
In addition to molt and breeding timing differences, there are also strong behavioral and feeding
differences between macrotarsa and nilotica, with macrotarsa regularly engaging in
kleptoparasitim of the Whimbrel Numenius phaeopus; in one study, there was a significant
association of macrotarsa with Whimbrel, with macrotarsa waiting until a Whimbrel would catch
a large crab, at which point the tern would fly in and steal it from the Whimbrel. This behavior
was never observed among migrant nilotica in northwestern Australia.
Vocal Differences
Kimball Garrett and Kathy Molina, first author of the Birds of the World account for G. nilotica,
provided comments to RTC when WGAC considered separating G. macrotarsa last year (see
Appendix). Contact calls of adult macrotarsa were not available online, but they compared
sonograms of apparently analogous calls from macrotarsa (Higgins and Davies 1996) and
nilotica (Cramp 1985) and concluded that they were quite different. They also compared the
presumed begging call of a juvenile macrotarsa (https://macaulaylibrary.org/asset/352981901)
with those of the subspecies to be grouped under a split G. nilotica, noting that they had never
heard a remotely similar call from vanrossemi in California or Mexico. In contrast, they noted
that the begging call of a juvenile nilotica from India sounded very similar to those of juveniles of
vanrossemi. The sample sizes for the juvenile calls are small but nevertheless suggestive.
Genetic Differences
No genetic data are available for the two taxa, except for limited mtDNA barcode sequence data
that suggests the two represent reciprocally monophyletic groups, leading the authors to
designate them as “potential distinctive taxonomic entities” (Tavares and Baker 2008).
Recommendation:
Based on the pronounced morphological differences (different bill shape and significantly larger
measurements in macrotarsa), differences in molt timing (apparent ability of macrotarsa to
suspend molt and resume molt abruptly at different times of year in response to environmental
conditions that dictate breeding), differences in timing of breeding (ability to breed most months
of the year to take advantage of good conditions), and apparent vocal differences between both
adult and juvenile nilotica and macrotarsa, we recommend voting to split G. macrotarsa from G.
nilotica. Given the large difference in range size of the two species, continuing to use “Gull-billed
Tern” for G. nilotica is appropriate; G. macrotarsa has been given the English name Australian
Tern by other global checklist authorities, whereas Gull-billed Tern was retained for G. nilotica
(Gill et al. 2023, Clements et al. 2023). This decision would also bring the AOS Check-list in
alignment with WGAC and other global taxonomic authorities.
Effect on the AOS Checklist:
Because macrotarsa has never occurred in the AOS region, the only changes needed to the
Checklist would be (1) adjustment of the distributional statement for G. nilotica, (i.e., removal
from the statement of its Australian breeding range as well as its distribution throughout inland
Australia, as nilotica only occurs along the northern and eastern coast during the nonbreeding
season), and (2) mention of the split in the Notes.
References:
Clements, J. F., P. C. Rasmussen, T. S. Schulenberg, M. J. Iliff, T. A. Fredericks, J. A. Gerbracht,
D. Lepage, S. M. Billerman, B. L. Sullivan, and C. L. Wood (2023). The eBird/Clements
checklist of Birds of the World: v2023. Cornell Lab of Ornithology, Ithaca, NY, USA.
Cramp, S. (1985). The Birds of the Western Palearctic. Volume 4. Terns to Woodpeckers.
Oxford University Press, Oxford, UK
del Hoyo, J., and N. J. Collar (2014). HBW and BirdLife International Illustrated Checklist of the
Birds of the World. Volume 1. Non-passerines. Lynx Edicions, Barcelona, Spain.
Dickinson, E. C., and J. V. Remsen Jr. (2013). The Howard and Moore Complete Checklist of
the Birds of the World. Volume 1. Fourth edition. Aves Press, Eastbourne, United Kingdom.
Gill, F., and D. Donsker (2019). World Bird List, Version 9.2. International Ornithologists’ Union.
Higgins, P. J., and S. J. J. F. Davies, Editors (1996). Handbook of Australian, New Zealand and
Antarctic Birds. Volume 3. Pratincoles to Pigeons. Oxford University Press, Melbourne,
Australila
Mlodinow, S. G. (2023). Australian Tern (Gelochelidon macrotarsa), version 1.0. In Birds of the
World (N. D. Sly, editor). Cornell Lab of Ornithology, Ithaca, NY, USA.
https://doi.org/10.2173/bow.gubter3.01
150
Rogers, D. I., P. Collins, R. E. Jessop, C. D. T. Minton, and C. J. Hassell (2005). Gull-billed
Terns in north-western Australia: subspecies identification, moults and behavioral notes.
Emu 105(2):145-158.
Tavares, E. S., and A. J. Baker (2008). Single mitochondrial gene barcodes reliably identify
sister-species in diverse clades of birds. BMC Evolutionary Biology 8:81.

Submitted by: Shawn M. Billerman, Cornell Lab of Ornithology, and Terry Chesser

Date: 31 March 2024

Link to PDF: https://americanornithology.org/wp-cont ... 2024-C.pdf

Appendix:
External Comment on the proposed separation of Gelochelidon macrotarsa from G. nilotica

Regarding the Gull-billed Tern split, I would say that neither Kathy nor I previously held strong
feelings either way, but might now lean toward splitting for several reasons. Our thoughts are
outlined below, but I would hasten to add a couple of caveats. First, our experience is almost
exclusively with Gull-billed Terns in southern California and western Mexico (Gelochelidon
nilotica vanrossemi), and to a lesser extent with G. n. aranea in the se. United States and Cuba.
We saw Gull-billeds in Cairns, Queensland (Australia) in November 1990 but only at a distance
and we did not pay close attention to the subspecies issue (Kathy didn’t begin her field work on
these terns in California until 1991). The second caveat is that some of the “evidence” we point
to below comes from Macaulay Library/eBird and we can’t vouch for the subspecies
designations of the birds photographed or audio-recorded therein, though they make
geographical sense.
G. n. macrotarsa is really the only well-differentiated subspecies of GBTE -- all other named
subspecies vary only subtly from one another and individuals are often (usually?)
unidentifiable without knowledge of locality. In other words, all ssp. other than macrotarsa are
based on average differences that are real, but with much overlap in characters. We suspect
that essentially all individual macrotarsa would be diagnosable in the hand and probably in the
field.
Morphological differences in macrotarsa seem consistent and should distinguish virtually all of
them from virtually all individuals of all other ssp. Differences include overall size, bill size and
shape, foot size, dorsal coloration, and tail coloration, as well as basic and (see below) juvenal
plumage.
Here is an interesting comparison of bill size and shape of two birds in June at Cairns,
Queensland; the upper larger-billed bird is macrotarsa; the lower smaller-billed bird might be a
migrant (affinis/nilotica?) although the fully black crown would seem to be unusual for a northern
migrant in winter (June). So it might represent the small extreme of macrotarsa, which runs
counter to the “easily diagnosable” conclusion. [A fully black crown in winter might not be
unusual for macrotarsa, which can be nomadic and breed at all seasons.]
https://macaulaylibrary.org/asset/463685481
In any case, a perusal of Macaulay photos from Australia and elsewhere confirms the
morphological distinctness of (nearly all) macrotarsa.
151
We can't readily find any recordings of adult macrotarsa contact calls (none in XenoCanto, none
in Macaulay/eBird). HANZAB (v. 3, p. 581) presents a sonogram of the “Yelp” call (rendered as
‘kuh-wuk’) which we presume is equivalent to the common contact call of all GBTE. One can
compare this sonogram with that of Fig. IV in Birds of the Western Palearctic (v. 4, p. 14), the
analogous call of a nominate nilotica from Spain. They look pretty different in structure.
This recording is said to be of a presumed begging call of a juvenile macrotarsa following an
adult at Darwin, Northern Territory:
https://macaulaylibrary.org/asset/352981901
We have never heard any begging call remotely like this from vanrossemi in California or w.
Mexico. I don’t know how typical this recording might be. ...

... Also, the Birds of the World on-line species account has a good photo of a juvenile vanrossemi
with an adult on the “Subspecies” page.
Given the above and what is discussed in the 2005 Rogers et al. paper in Emu (including
differences in ecology, migration/nomadism, etc.), we would agree this is a reasonable species-
level split. However, it seems that more work should be done to document the vocal repertoire
of macrotarsa (field recording and/or diving more deeply into the literature and Australian
recording archives).

Kimball Garrett
Kathy Molina
Natural History Museum of Los Angeles Count

[Michael Szabo]

The proposed split of Australian Tern from Gull-billed Tern has now been accepted by the American Ornithological Society:
https://academic.oup.com/auk/article/14 ... 19/7716004

[Michael Szabo]

I see that eBird has now aligned with this split:

Australian Tern (Gelochelidon macrotarsa): https://ebird.org/species/gubter3/NZ

Gull-billed Tern (Gelochelidon nilotica): https://ebird.org/species/gubter2/NZ

Those who saw the putative Gull-billed Tern (Gelochelidon nilotica) at Big Sand Island in Jan/Feb 2023 can now list it as such:
viewtopic.php?f=9&t=12005&hilit=Gull+billed+Tern+Big+Sand+Island

[paradoxdinokipi]

Yup, eBird updates its taxonomy every October, it alligned with the Australian Tern split in 2023 so it's been in effect since late October last year, which leads to confusion from some birders erroneously putting down Gull-Billed instead of Australian..
eg.
https://ebird.org/checklist/S37782111
https://ebird.org/checklist/S160162330
https://ebird.org/atlasnz/checklist/S158747194