Analysis of Dark-billed, Dark-legged Common Tern (Sterna hirundo) Records in Massachusetts
New England has proven to be quite the hotspot for vagrants originating from Oriental Asia in recent years. Whether this is due to a per capita increase in birdwatchers actively looking for such species, or rather just its physical location on a map, the coastline between western Connecticut and southern Maine has shown itself to be well-suited in collecting these few, far-dispersing migrants. From the second and third records of Gray-tailed Tattler (Tringa brevipes) in the contiguous United States (Trimble, et al 2012)(Johnson 2017), to the almost annual occurrence now of Kamchatka Mew Gull (Larus canus kamtschatschensis) in Massachusetts (Trimble, et al 2014) (Kardell, 2021), it is clear that even medium-distance Eastern Palearctic migrants (as is the case with the latter species) are capable of reaching these coastal areas. Reports of dark-billed, dark-legged Common Terns (Sterna hirundo), presumably of the Siberian form (S. h. longipennis) have also been described in the Northeastern United States as far back as 2003 (pers comm S. Mitra), with records concentrated primarily around Suffolk County, New York and Nantucket County, Massachusetts (pers comm J. Trimble). The identification of these individuals has been clouded by a lack of understanding in the separation of S. h. longipennis from the nominate subspecies, S. h. hirundo, with several arguments having been made to dismiss these birds as either hybrids with Arctic Tern (S. paradisaea) or (more commonly) examples of young nominate birds in a stage of advanced body molt that have retained juvenile bare parts coloration. (pers comm S. Mitra)
It is the aim of this paper to provide the ground work for state-level avian record committees to assess these reports critically, and to outline the potential status of S. h. longipennis within the geographical scope of the commonwealth of Massachusetts.
S. hirundo is a widely-distributed seabird that breeds throughout a variety of habitats across temperate regions in the Northern Hemisphere. (Arnold, Oswald, et al 2020) (Brazil, 2009) Three field identifiable subspecies are recognized by the majority of contemporary sources, with the nominate S. h. hirundo breeding in much of the Nearctic and Western Palearctic realms. S. h. tibetana occupies a wide area across most of the Tibetan Plateau and east towards Mongolia, whereas S. h. longipennis occupies mainly east-central Siberia and northeastern China as a breeder, with accepted records in western mainland Alaska and the Pribilof Islands. (Arnold, Oswald, et al 2020) (eBird) Populations across Eurasia exist on a cline latitudinally, with darker, shorter-billed, and longer-winged birds increasing in relation to how far East a given nesting site is. S. hirundo is both a coastal and inland nesting species, although conservation concerns have been raised over the decline of breeding success at inland sites reflected in asymmetric gene flow towards coastal populations. (Arnold, Oswald, et al 2020) In Massachusetts, the species is a strictly coastal breeder, also listed as a “Species of Special Concern” by the state. (Kamm, Walsh, et al 2013) Major nesting areas within the commonwealth are primarily low-lying, offshore islands devoid of large mammalian ground predators such as Monomoy, Penikese, and Muskeget Islands. (Kamm, Walsh, et al 2013) An increasing number of both S. hirundo and Roseate Tern (S. dougallii), apparently young individuals, have been observed over-summering at traditional staging areas during the early breeding season, where they are joined by post-breeding aggregates in mid-July from other colonies in Massachusetts. (Veit, Perkins, 2014) This trend has allowed for closer observations of S. hirundo over a longer period of time by both the casual birdwatcher and field ornithologist alike during the months of June and July. On Tuckernuck Island (an island adjacent to Muskeget Island and an included part of Nantucket County), the mixed staging aggregations at sites like Whale Point have grown steadily in number over the last few years, with flock composition consisting overwhelmingly of S. dougallii.
There are a minimum of two disputed accounts of S. h. longipennis from Nantucket County, and a third account that may represent a separate individual. These three accounts make up the entirety of the unaccepted records in Massachusetts, for which there are currently no records that are confirmed. Two of these accounts involve photographic documentation, and were observed within larger mixed Sterna flocks. For the breadth of this work, only these three accounts will be considered. Reports of similar individuals described by other observers either lack adequate documentation or were dismissed initially as birds belonging to the nominate subspecies, S. h. hirundo. (pers comm S. Williams) The following three accounts will be the focus of this body, listed chronologically in the order of [# of individuals; age class; dates encountered (first seen — last seen); location (site, county, state); names of observers]:
- 1; adult; 2–8 July, 2011; Muskeget Island, Nantucket County, MA; Richard R. Veit, et al.
- 1; adult; 2 August, 2011; Muskeget Island, Nantucket County, MA; Richard R. Veit, et al.
- 1; adult; 20–21 July, 2020; Tuckernuck Island, Nantucket County, MA; Skyler K. Kardell.
The first individual from Muskeget was observed at least three times by several trained observers over the course of 2–8 July, and was photographed using a telephoto lens. The bird was also banded on its right leg. According to the primary observer of the second account in August of 2011, this bird was wearing a band on its left leg. Because this lack in continuity can not be discredited through a perceived lapse in memory, this report stands as a possible third account of S. h. longipennis. A more recent account comes from 20–21 July, 2020 in which a single bird was observed loafing with an estimated 200 S. h. hirundo and 300 S. dougallii on Whale Point, an extremity of Tuckernuck Island. This bird was also banded on its right leg.
An obvious issue that must be addressed is the fact that all three accounts involve banded birds with metal field readable bands. Banding operations of S. hirundo are presumably carried out in very small numbers in eastern Russia (pers comm I. Nisbet), although the lack of data thereof suggests that actual values could be much lower. One explanation that may justify this statistical disparity of having three different banded birds show up within the same county and in subsequent years would be that both the July 2011 and 2020 accounts represent the same returning adult individual. In this scenario, a bird from Siberia could hypothetically arrive in the western Atlantic and acclimate to seasonal movements of nominate S. h. hirundo. If this is the case, a vagrant S. h. longipennis would not need to be banded in Russia as a pullus (chick) to show up in New England as a banded adult. S. hirundo exhibits high site fidelity toward over-wintering locations. (Arnold, Oswald, et al 2020) Those breeding in Massachusetts are believed to occupy an 8000-km stretch of coastline in South America during the non-breeding season which spans from northwest Venezuela to northeast Argentina. (Nisbet, Mostello 2015) Numerous banding operations of S. hirundo exist in South America, with 3,590 individuals banded at Punta Rasa, Buenos Aires Province, Argentina between 1996–2002. (Hays, Lima et al 1999) Given that S. h. hirundo and S. h. longipennis appear extremely similar in adult basic plumage, it is unlikely that a vagrant bird from Russia would be detected given the lack of understanding in this identification. (Brazil 2009)
The individual described in the 2020 account of S. h. longipennis was photographed in flat light on 20 July. Photographs of this bird were taken using a 400mm lens at a shutter speed of 1/2500 frames per second. Bare parts coloration is consistently cited as a primary means of separating the two subspecies in alternate plumage. This bird shows a uniformly dark bill with dull red undertones. The legs and feet are a similar shade of dark ochre that may have appeared more black in the field due to body shadow. The issues posed by overlapping features in bare parts coloration exhibited by potential S. h. longipennis candidates from eastern North America and adult nominate birds with asynchronous molt timing is the source of much disagreement in assessing these birds. (pers comm S. Mitra) Structurally, this bird is similar to that of surrounding S. h. hirundo and S. dougallii, however the head appears slightly smaller in relation to the body and the chest more barrel-shaped. These features are noted by several authors in separating S. h. longipennis from S. h. hirundo including Higgins and Davies 1996 and Brazil 2009. The binomial namesake for S. h. longipennis is drawn from the Latin words longus (long) and penna (feather/wing). This trait should be evident in both the field and in photographs with individuals that are in direct comparison with S. h. hirundo. In this case, we see in pictures of the 2020 bird at rest that the tail streamers appear to project just past the length of the folded primaries. It is difficult to judge the true extent of the primary projection in relation to the body from these pictures, however the flight shots depict the length of the outermost primary (P10) as marginally longer than the primary (P9) proximal to it.
An additional argument that can be made regarding this particular individual is that the extent of gray on the breast and upper chest is too extensive for nominate S. hirundo at this age and date, and that this feature is more suggestive of S. h. longipennis. Several sources also suggests that the mantle and wings of S. h. longipennis are markedly darker than nominate birds (Arnold, Oswald 2020) (Brazil 2009). It is difficult to assess these characteristics in the photographs due to the haziness of the setting in which they were taken. Overall, we conclude that the 2020 account of S. h. longipennis stands as a weaker candidate given the inferiority of lighting and atmospheric conditions in which it was documented, as well as several features that seemingly pose at odds with this identification. The bill is not fully black, but instead a dark shade of red for much of the base contrasting weakly with a pure black tip. The legs are also not entirely black, although this trait is variable (Brazil 2009). Since we are unable to completely eliminate the possibility of either a second/third-year nominate bird with retained juvenile bare parts or rather an adult with retarded basic-plumage bare parts from matching the bare parts described in alternate-plumage S. h. longipennis, consideration of these features must not be heavily weighted. However, aforementioned individuals that exhibit these traits are either extremely rare or not well documented.
A much more convincing candidate of S. h. longipennis is shown in the July 2011 account from Muskeget Island. This bird was photographed on 8 July by Peter Trimble using a 400mm lens at a shutter speed of 1/800 frames per second. The bare parts appear more uniformly black, and the head is more obviously domed than the 2020 account. In the flat light provided by the atmospheric conditions, it appears the mantle and wings are also marginally darker than surrounding S. h. hirundo. Ventral coloration appears similarly dark in comparison to the nominate subspecies, and this wash continues up into the upper chest and throat. Bill shape is also noticeably different from both S. h. hirundo and the 2020 account. The Muskeget bird has a relatively shallow gonydeal expansion as well as a more rounded culmen, recalling the bill of S. paradisaea. This feature is particularly noticeable when the bird is in profile and against a uniform background, as it is with the flight shot showing the underwing of the bird in flight. Additionally, the legs and feet appear wholly black, unlike the 2020 individual, which shows contrastingly orange parts at certain angles.
By analyzing these two individuals (we can not take into account the August 2011 bird due to lack of photographic documentation), we conclude that the assessment of these dark-billed, dark-legged S. hirundo specimens in eastern North America should be approached only with the utmost care and skepticism. A positive identification should not be concluded based on bare parts coloration alone, but also bill length (perhaps more importantly bill shape), wing length, and dorsal/ventral pigmentation. In order to assess these accounts critically, excellent photographic documentation must be required, as in-the-field study may not be sufficient in placing these birds into the category of subspecies. A priority for future research should be in documenting S. h. hirundo with retarded bare parts coloration that are known banded individuals.
Works Cited
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Nisbet, I. C. T. and C. S. Mostello. (2015). Winter quarters and migration routes of Common and Roseate terns revealed by tracking with geolocators. Bird Observer 43:222–230.
Veit, R.R. and S. A. Perkins. 2014. Aerial Surveys for Roseate and Common Terns South of Tuckernuck and Muskeget Islands July-September 2013. US Dept. of the Interior, Bureau of Ocean Energy Management, Office of Renewable Energy Programs, Herndon, VA. OCS Study BOEM 2014–665. 13 pp.
Walsh, Joan M., et al. The Massachusetts Breeding Bird Atlas 2. Mass Audubon, 2013.
