Drivers of colony failure in a vulnerable coastal seabird, the Australian Fairy Tern (Sternula nereis nereis)

Colony outcomes

From the 77 colonies monitored, 27% (n = 21) had high breeding success, 18% (n = 14) were moderately successful, 7% (n = 5) had low breeding success and 48% (n = 37) failed (Fig. 2, Table 1). Among those that failed, predation (32%, n = 12) and inundation (27% n = 10) were, purportedly, the biggest causes of colony failure, followed by human disturbance, i.e. fishing, walking, camping, kite surfing (8%, n = 3, Fig. 2). The sources of failure were not able to be determined at 22% (n = 8) of the monitored colonies (Fig. 2).

Fig. 2.

Sources of colony failure (percentage of occurrence) at 77 monitored Australian Fairy Tern (Sternula nereis nereis) colonies in Western Australia between 2017/18 and 2021/22. Numbers shown inside bars indicate the number (n) of occurrences the source of failure was observed.

Non-native species were recorded at 29 colonies during the study period (Fig. 3). Red foxes, black rats (Rattus rattus) and cats (Felis catus) were believed to be implicated in the failure of at least nine colonies, and in all cases, predation was evidenced by tracks in the sand and broken eggs (inferred predation), direct observations and/or wildlife cameras (Table 1). Red foxes appear to have contributed to the failure of at least six colonies (Table 1) and the partial failure of a colony at Point Walter in 2018/19 (Greenwell et al. 2021b). Predation by a single, free-roaming cat purportedly led to the failure of one colony of 111 pairs in Mandurah in 2018 (Greenwell et al. 2019a), and black rats are believed to have predated eggs (inferred predation) at two colonies at Garden Island (Table 1, C. Greenwell, pers. obs; G. Davies, pers. comm.). Two colonies purportedly failed following predation by native species – one by ruddy turnstone (Arenaria interpres) at Rottnest Island (Greenwell 2021) and the other by silver gulls (Chroicocephalus novaehollandiae) at Wellstead Estuary (S. Elson, pers. comm.).

Fig. 3.

Threats observed (percentage of occurrence) at 77 monitored Australian Fairy Tern (Sternula nereis nereis) colonies in Western Australia between 2017/18 and 2021/22. Numbers shown inside bars indicate the number (n) of occurrences the threat was observed.

Natural weather events, i.e. inundation, flooding (heavy rainfall) and egg burial from strong winds, contributed to the failure of at least 33% (n = 12) of colonies and were recorded in all years (Table 1). Tidal inundation (27%, n = 10) was the most persistent source of ‘natural’ failure, recorded in 4 of the 5 years of observation (Fig. 2, Table 1). In 2017/18 on Rottnest Island, a 1/500-year rainfall event caused the flooding and abandonment of an entire colony of 176 pairs. In 2018, beach erosion and predation by a nankeen kestrel (Falco cenchroides) contributed to the failure of a colony in Mandurah, following predation by a cat at a nearby colony (Table 1, Greenwell et al. 2019a).

Human disturbance, including off-road vehicles, caused the failure of 8% (n = 3) of colonies (Fig. 2). On two occasions, off-road vehicles were implicated, evidenced by tyre tracks through the nesting area (Table 1). At the Leschenault Estuary mouth, near Bunbury, a colony in the early stages of development consisting of ~18 nests was destroyed by a vehicle(s) that drove, in circles, within the sign-posted nesting area (Table 1, C. Taylor, P. Morris, pers. comm.). At Rous Head in 2020/21, the first nest to be established was abandoned between Christmas Day and Boxing Day due to human disturbance. A beach towel, empty beer bottles and a beach ball were found in the fenced breeding area, and these activities were detected on wildlife cameras (M. Pryor, pers. comm.). On a beach in Cowaramup, a single nest was abandoned before it could be fenced off, presumably due to disturbance (Table 1, M. Singor, pers. comm.).

Individual nest failure, predation and colony disturbance

Human disturbance was observed at many monitored sites, including Mandurah Marina, Dawesville, Penguin Island, Point Walter, Rous Head, Carnac Island, Wedge Island, Hangover Bay and North West Cape (Table 1). For example, at Point Walter, fishers and other recreational users intermittently breached the fenced exclusion zone and disturbed nesting Fairy Terns as they walked along the narrow sandbar in all years (C. Greenwell, pers. obs.). At Rous Head, fishers and beachgoers were often observed or detected on wildlife cameras, walking through the fenced site (C. Greenwell, pers. obs.). At Woodman Point, numerous disturbances were documented, including off-leash dogs, recreational fishers that walked through the nesting site and kite surfers (S. Herath, W. Garrard, pers. comm.). At Point Walter in 2018/19, predation of eggs and chicks by a red fox contributed to the partial failure of the colony, and on Penguin Island in 2019/20, predation by silver gull(s) and a juvenile crested tern (Thalasseus bergii) contributed to a high incidence of individual nest failure (≥70%; Fig. 4, Table 1).

Fig. 4.

Examples of disturbance, predation and colony failure at Australian Fairy Tern (Sternula nereis nereis) colonies in Western Australia between 2017/18 and 2021/22. (a) Red fox (Vulpes vulpes) at Point Peron, (b) adult silver gull (Chroicocephalus novaehollandiae) predating a chick at Penguin Island, (c) juvenile greater crested tern (Thalasseus bergii) predating a chick at Penguin Island, (d) Australian hobby (Falco longipennis) predating an adult tern at Dawesville, (e and f) dead Fairy Tern runner and fledgling following suspected predation and trapping of a cat at Dawesville, (g) incursion by an off-leash dog at Dawesville and (h) Fairy Tern chick with an injured wing from a suspected dog attack at Dawesville. Image credits: (a) Peter Moore; (b and c) Claire Greenwell; (d) Amy Loffler; (e–g) Julie White; (h) Paul Fenton.

Numerous native species were observed predating Fairy Tern eggs, chicks or adults (Fig. 4, Table 1). Predators included Australian pied oystercatcher (Haematopus longirostris; C. Greenwell, pers. obs.), ruddy turnstone (Greenwell 2021), silver gull (Greenwell et al. 2021b; S. Elson, C. Corker, pers. comm.); juvenile greater crested tern (Greenwell et al. 2021b), nankeen kestrel (C. Corker, C. Greenwell, pers. obs.), Australian hobby (C. Greenwell, pers. obs., A. Loffler, pers. comm.), dugite (Pseudonaja affinis; D. Martin, pers. comm.).

Arctic jaeger (Stercorarius parasiticus; S. Goodlich, pers. comm.), pacific gull (Larus pacificus; K & P Norris, pers. comm.), peregrine falcon (Falco peregrinus; C. Greenwell, pers. obs.; A. Smith, S. Kalab, pers. comm.), swamp harrier (Circus approximans), white-bellied sea-eagle (Haliaeetus leucogaster; C. Greenwell, pers. obs., P. Moore, pers. comm.) were observed near or harassing colonies, and despite obvious disturbance to terns, no cases of predation by these species were confirmed (Fig. 4, Table 1). In addition, Australian sea lions (Neophoca cinerea) purportedly crushed several nests on Carnac Island in 2018/19 and Fisherman’s Island in 2020/21 (Table 1, S. Goodlich, A. Smith, pers. comm.).

In 2020/21, domestic dogs were a common source of disturbance at a colony in Dawesville, and are suspected of having injured or killed at least two Fairy Tern chicks (Fig. 4g, f, P. Fenton, pers. comm.). Subsequently, the beach was rezoned from an on-leash area to a dog prohibited area. At the same site in January 2022, a deceased 5-day-old chick with punctures in the body and four wing sets belonging to fledgling Fairy Terns were found (Fig. 4e, f, J. White, pers. comm.). On further investigation, animal tracks suspected of belonging to a cat were found on the colony’s outskirts. The following evening, a cat was trapped by the City of Mandurah (B. Beal-Richardson, pers. comm.). The cat was not collared, desexed, nor microchipped, and was believed to be semi-feral/unowned due to its poor body condition (i.e. an animal that is partially provisioned by people, deliberately or incidentally through garbage; Cove et al. 2018) (B. Beal-Richardson, pers. comm.). No further mammal tracks or instances of terrestrial predation were found following the trapping and removal of the cat in the 2021/22 season.

Predation

At least 14 predators (10 native and 4 invasive/domestic) were observed at Fairy Tern colonies during the 5 years of monitoring and contributed to egg, chick and/or adult mortality or complete colony failure. The high incidence of colony failure associated with invasive and domestic predators reinforces the need for monitoring and targeted predator removal or deterrence, both in the lead-up to and during the breeding season, i.e. pre-emptive and reactive controls. This is particularly important at frequently used sites, which may be subject to a build-up of predators or where the locations of colonies become predictable to predators over time (Ward et al. 2011; Dunlop 2018; Greenwell et al. 2019b). Red foxes, black rats and cats have been identified as key invasive predators impacting the breeding success of Fairy Terns elsewhere on mainland Australia (Trees and Natural Resources 1997; Maguire 2008; Paton and Rogers 2009; Lacey and O’Brien 2015; Commonwealth of Australia 2020). The development of local/regional protocols by land management agencies may be useful for informing predator control efforts, particularly at short notice, to prevent swift colony decline (Greenwell et al. 2019a; Commonwealth of Australia 2020). Improved regulation of pet animals, i.e. dog prohibition and/or strict leashing requirements and cat containment, in areas adjacent to colony sites would also likely improve outcomes for nesting terns.

Inundation

Inundation was a major source of colony failure due to the tern’s preference for breeding sites in low-laying sandy beach habitat, often just above the intertidal zone. Inundation frequency is predicted to increase with climate change, driven by sea-level rise and higher sea surface levels (Indian Ocean Climate Initiative 2012; Garnett et al. 2013), engineering impacts (e.g. the Dawesville Cut influencing water levels on the Peel Inlet), and more severe and frequent summer storm events. Flood mitigation strategies, such as raising the height of colony areas or encouraging settlement in low flood risk areas, should be developed and implemented as a priority to provide a safe habitat for beach-nesting species, including Fairy Terns, and to improve breeding productivity (Garnett et al. 2013).

In New Zealand, options used to reduce inundation risk for Fairy Terns (Sternula nereis davisae) have included sand-bagging nests from tidal surges, ditch and dyke systems to reduce flooding risk of low laying nests and oyster shell banks to increase substrate height and reduce the impacts of tides and storm surges (Ferreira et al. 2005; Department of Environment and Conservation New Zealand 2019). Alternatively, managed and/or engineered sites (generally on dredge-spoil) dedicated specifically to breeding may provide an effective long-term solution, with elevated sites removing the potential for inundation (Krogh and Schweitzer 1999; Jenniges and Plettner 2008; Pakanen et al. 2014; Greenwell et al. 2019b, 2021a).

Human disturbance

Although human disturbance only accounted for 8% (n = 3) of colony failures, disturbance was recorded at ≥30% (n = 23) of colonies. The incidence of disturbance may be underestimated, particularly during the settlement and early egg-laying periods before the colony site has been identified or at remote sites that are difficult to monitor regularly. In these cases, colony abandonment is more likely to occur before protective measures are introduced, such as the installation of temporary fencing and signage. During the early settling period, site attachment is relatively low due to the low investment of time and energy into egg laying or incubation and moving to an alternative site may outweigh the risk of remaining at a disturbed site (Nisbet 1981; Safina and Burger 1983; Burger and Gochfeld 1991; Nisbet 2000; Greenwell et al. 2021b).

In the early stages of colony formation, the first eggs to be laid are often intermittently incubated or deserted at night, potentially as a mechanism for assessing potential nest predators or increasing egg synchrony and reducing the time the colony is detectable by predators (Gochfeld 1980; Atwood 1986; Nisbet 2000; Jovani and Grimm 2008; Greenwell et al. 2019b). During this period, fewer terns may be present at the breeding site, thus group defence strategies, including dive-bombing intruders, are likely to be lower (Brunton 1999).

Fairy Terns often form colonies on spits and wide sandy beaches located near estuary mouths, on nearshore islands or salt lakes, or within sheltered coastal embayments where small baitfish are abundant (Higgins and Davies 1996; Paton et al. 2009; Lacey and O’Brien 2015; Greenwell et al. 2021b, 2021d). The protection of these important coastal landforms should be prioritised to improve the long-term conservation needs of Fairy Terns and other beach-nesting species, such as Australian pied oystercatchers and red-capped plovers (Charadrius ruficapillus), and to preserve their high natural value. Reducing the impact of recreational activities, particularly from off-leash dogs and unregulated, highly destructive off-road vehicles in these sensitive habitats, is critical for maintaining ecosystem function (Williams et al. 2009; Schlacher et al. 2015; Maguire 2018).

When breeding behaviour is anticipated or identified, pre-emptive or proactive management interventions may be used to reduce human disturbance. Interventions such as seasonal site closures and restricting human access by erecting temporary fencing and signage near nesting areas are often effective in reducing disturbance, facilitating an increase in reproductive success (Burger and Leonard 2000; Lafferty et al. 2006; Medeiros et al. 2007; Maguire 2008).

Unidentified failures

Outcomes could not be documented at several colonies due to their remoteness or a lack of resources to undertake regular monitoring. Documenting the outcomes of breeding success is a key objective under the Recovery Plan to better target management actions (Commonwealth of Australia 2020). Engaging local observers, including commercial operators, and/or expanding the opportunities for citizen science programs may effectively support monitoring and management efforts (Tulloch et al. 2013; Greenwell et al. 2021c).