A long tern view: distribution of small terns (Sternula) in Western Australia and implications for their conservation

Keywords: breeding range expansion, conservation, Fairy Tern, Little Tern, management units, meta-population, Sternula, superspecies complex, range extension, tropicalisation.

Tropical terns (Roseate Tern Sterna dougallii, Sooty Tern Onychoprion fuscata, Bridled Tern Onychoprion anaethetus) and noddies (Common Noddy Anous stolidus and Lesser Noddy Anous tenuirostris) were well established at the Houtman Abrolhos Islands during the colonial period (most ornithological records in the region commenced from the 1880s), having occupied or re-occupied the islands earlier in the Holocene (Fig. 1; Dunlop et al. 2015; Surman 2019). The sub-tropical latitude of these colonies for tropical tern species has been attributed to the influence of the warm, southward flowing Leeuwin Current (Dunlop and Wooller 1990; Surman 2019). Since the beginning of the historical record in Western Australia, there has been a southward shift in the distribution of several tern species – populations, particularly south of the Houtman Abrolhos Islands (Serventy et al. 1971; Dunlop and Wooller 1986, 1990). The species include the Bridled Tern (Dunlop 2009), Roseate Tern, (Dunlop and Wooller 1990), Crested Tern Thalasseus bergii (Dunlop 1985), Sooty Tern (Dunlop and Mitchell 2001), and Common Noddy (Dunlop 2009). The Lesser Crested Tern is also, purportedly, expanding southwards from northern Australia and is now known to breed as far south as Shark Bay (Burbidge and Fuller 2000; Johnstone et al. 2013). The range extensions south of the Houtman Abrolhos Islands since 1900 are likely to have been driven by the marine climate changes of the Anthropocene (Dunlop 2009).

Fig. 1.  Map showing localities of breeding and nuptial Little Tern Sternula albifrons sinensis. Inset with symbol (▪) shows the area between Roebourne and North West Cape, where annual breeding records were made between December 2016 and December 2021.

The Little Tern Sternula albifrons is a coastal species found on the Australian mainland and surrounding continental islands and two subspecies have been identified in Australia (BirdLife Australia 2019). The Indo-Pacific Little Tern Sternula albifrons sinensis consists of an Australian breeding population that nests in northern Australia (Garnett et al. 2021) and Asian migrants that visit Australia in their non-breeding period during spring and summer (Johnstone and Storr 1998). These non-breeding Little Terns are found around Australia, except between Perth, Western Australia, and Adelaide, South Australia, and are considered vagrants south of Onslow in Western Australia (Higgins and Davies 1996; Collins and Jessop 1997; Johnstone and Storr 1998; Garnett et al. 2021). A second subspecies, the Tasman Little Tern Sternula albifrons placens, is found in south-eastern Australia and New Zealand (Gochfeld et al. 2016; Mahon et al. 2021; Department of Agriculture Water and the Environment 2022). This subspecies is, however, not currently universally agreed, with all birds in Australian territory previously considered to belong to S. a. sinensis(Department of Agriculture Water and the Environment 2022).

In Western Australia, the first Indo-Pacific Little Tern (hereafter, Little Tern) colonies were recorded near Broome in 1995 and 1996 and represented an important breeding range extension on the western coast (Fig. 1, Collins and Jessop 1997; Johnstone and Storr 1998). Subsequently, breeding Little Terns were recorded on Eighty Mile Beach (~350 km south of Broome), Walsh Point and elsewhere in the Kimberley, where the local breeding population appeared to be increasing, and establishment in the Pilbara was predicted (Fig. 1, Johnstone and Storr 1998; Johnstone et al. 2013). These southern colonies most probably emanated from the spring–summer component of the breeding population that inhabits northern Australian waters (Chatto 2001).

This contribution documents a recent expansion of the breeding range of the Little Tern along the Western Australian coastline and records of Little Terns in nuptial plumage. It also discusses the overlap and relationship with the closely related Australian Fairy Tern Sternula nereis nereis, and associated conservation implications.

Since 2013, there have been ≥ 57 unique records (excluding breeding records) of Little Terns in breeding plumage, including adults feeding fledglings, on the Western Australian coastline between Dampier and Mandurah (Fig. 1, Table 1). Presumably, these records of nuptial birds made during the Austral summer (including April and May) are individuals from the Australian breeding population (Table 1). It is possible there have been further observations, however, records available from Birdata (BirdLife Australia 2022) and eBird (Cornell Lab of Ornithology 2022) often do provide detail on plumage that would allow sightings to be allocated to breeding or migratory populations.

Table 1.  Records of Indo-Pacific Little Tern Sternula albifrons sinensis in breeding plumage south from North West Cape (NW Cape), Western Australia.

Since 2016, there have been ≥ 25 known breeding attempts made by Little Terns between Dampier and North West Cape (Table 2), outside the previously reported breeding distribution. Most breeding consisted of a single pair, and several observations were made at the same location over multiple breeding seasons, suggesting a degree of site faithfulness among pairs (Table 2). Between 2019 and 2021, there were three instances where Little Terns nested in small colonies of 2–4 pairs, with nests spaced ~50 m apart (J. Greer, M. Panhuyzen, pers. comm.). In spring 2020, four pairs of Little Terns were observed nesting ~50 m north and south of a small Australian Fairy Tern colony at Jurabi Point, North West Cape (J. Greer, M. Panhuyzen, pers. comm.). While Little Terns and Fairy Terns are known to breed in mixed flocks in the eastern states of Australia, this is the first record of breeding sympatry between two Sternula species in Western Australia.

Table 2.  Breeding records of Indo-Pacific Little Tern Sternula albifrons sinensis in Dampier and North West Cape (NW Cape), Western Australia.

Seabirds colonised the Western Australian continental shelf as islands were isolated by the rising sea levels of the Holocene marine transgression (Serventy et al. 1971; Surman 2019). It is likely that the tropical species arrived early in the period as the Leeuwin Current strengthened (Surman 2019). Subsequently, during the historical period of the late Holocene and Anthropocene, a number of tropical terns and seabirds have established colonies south of the Abrolhos Islands. For example, Bridled Terns have now colonised islands off the entire western coast (Surman and Wooller 2000) and island groups off the north-west coast of Eyre Peninsula, South Australia (D. Harper, pers. comm.). The southward shift of Bridled Terns has been attributed to changes in the ENSO pattern and a general rise in the background temperature of the Eastern Indian Ocean (Dunlop 2009).

Since 1995, the known breeding distribution of the Little Tern in Western Australia has extended south-east along the coast to Dampier in the Pilbara and North West Cape in the Gascoyne, a distance of ~1000 km (Fig. 1). Nuptial-plumaged adults have been observed as far south as Wedge Island (~170 km north of Perth) during the autumn months and a single bird in nuptial plumage was observed in Mandurah in February 2020 (Table 2). A similar southward range shift of Little Terns from the tropical colonies in the north to the south-eastern Australian coastline appears to have preceded the expansion in Western Australia.

The Australian Fairy Tern in Western Australia occurs as a breeding species along the coast south from Dampier Archipelago to, at least, Israelite Bay, west of the Great Australian Bight. The core migratory population winters on the mid-west coast, with most of the birds stationed at the Houtman Abrolhos Islands during this time. During spring and summer, these birds move to breeding areas from around the tip of North West Cape to the western edge of the Great Australian Bight (Dunlop and Greenwell 2021). This two-way breeding migration to both tropical and temperate waters is curious and may reflect the evolutionary history of Sternula nereis.

During the previous period of global warming, the Eem interglacial (120 000–130 000 BP), the Houtman Abrolhos Islands area was a vibrant coral reef near the shelf-edge (Wyrwoll et al. 2009). Indeed, that reef (the Wallabi Limestone) provided the geological basement for the present day ‘high’ islands.

Determining the taxonomic boundaries within the S. a. sinensis and S. nereis superspecies and the emergence of S. nereis as a sibling species to S. a. sinensis will require genetic studies of both forms and may provide further insights into processes influencing speciation. However, during this warm period, it is hypothesised the Little Terns expanded south (as they are now) to colonise the coral cays in the reef system. As sea levels dropped during the Pleistocene these Little Terns were possibly isolated from the tropical population and evolved into the Australian Fairy Tern during the Pleistocene.

During the glacial maximum of the Würm Ice Age, the location of the Houtman Abrolhos Islands was still only about 18 km from the coastline. This assessment is based on a bathymetric cross-section on Marine Chart Aus 751 from Rat Island in the Easter Group and on Pleistocene sea levels and dating adapted by C. A. Surman (Surman 2019) from a variety of sources including Wyrwoll et al. (2009) and Veeh and France (1988). Apatite deposits derived from guano and dated from the Eem Interglacial indicate that seabirds were breeding among the Houtman Abrolhos coral reefs at this time (Veeh and France 1988). Unlike other terns, Sternula often nest on mainland shorelines and could have progressively shifted colony locations with the falling sea level.

Presumably, as sea levels rose during the early Holocene, Fairy Terns colonised the shallow waters of the continental shelf, north and south of their origin, as per their current distribution. From here, the Western Australian population purportedly colonised south-eastern Australia and New Zealand (Baling and Brunton 2022). Later, New Zealand birds founded the population in New Caledonia (Baling and Brunton 2022). Genetic investigations support this evolutionary history with the Western Australian birds being the most genetically diverse than the other populations on the south-east coast, New Zealand (subspecies Sternula nereis daviase) and New Caledonia (Sternula nereis exsul) (Baling and Brunton 2005, 2022; Baling 2008).

The former sibling species Little Tern and Fairy Tern have probably been allopatric for, at least, the last 7000–10 000 years. Bridge et al. (2005) suggest divergence between the two species may have occurred as long as 1–2 million years ago but note this may be an overestimate. However, now with the continental shelf largely submerged and a warming eastern Indian Ocean, breeding Little Terns on the western coastline are sympatric with Australian Fairy Terns for the first time, at least in the Pilbara and Gascoyne coastal regions.

The species remain very closely related and in eastern Australia are known to hybridise (Cox and Close 1977; Ross et al. 1999). Hybridisation may be a conservation threat due to genetic dilution or the potential for reduced fitness or fertility of offspring (Ross et al. 1999), and has been identified as threat under the National Recovery Plan for the Australian Fairy Tern (Commonwealth of Australia 2020). In New South Wales, interventions are required to actively discourage hybridisation (Commonwealth of Australia 2020).

The conservation status of populations within a superspecies complex are difficult to determine. Baling and Brunton (2022) recommended the retention of Fairy Tern subspecies S. n. exsul and S. n daviase because these populations are very small, genetically distinct, impoverished (probably resulting from founder effects), and isolated from the core Australian population (S. n. nereis, Baling and Brunton 2022).

Banding studies of the Western Australian migratory S. n. nereis population indicate that the regional natal philopatry of individuals produces a substructure of management units (Dunlop and Greenwell 2021). Maintaining small tern populations in local marine/estuarine ecosystems may be a better conservation objective than a pre-occupation in saving closely related taxa. The real conservation problem here is species/subspecies-level conservation legislation. Most jurisdictions require populations to be defined at the species (sometimes sub-species) level to be listed and managed as threatened, and species with a higher conservation status often attract greater resources and funding. However, relying on these taxonomic criteria obscures the necessity to manage the threats in each inter-breeding population unit.

The data that support this study are available in the article.

The authors declare no conflicts of interest.

This research did not receive any specific funding.