Migratory shorebirds of Moreton Bay

Abstract

Tens of thousands of migratory shorebirds return to Moreton Bay each year from their breeding grounds in the Arctic. The Bay’s extensive tidal flats provide a rich feeding resource for the birds while they recuperate from their long migration flight and prepare for their next one. The abundance of many migratory shorebird species has declined dramatically in Moreton Bay, and while some of the causes are located elsewhere along the birds’ migration routes, there are significant threats to the birds and their habitats within the Bay, ranging from habitat loss to disturbance. New partnerships between conservation management agencies and NGOs have led to exciting examples of conservation action to reduce some of these threats, including collecting high quality monitoring data, careful zoning of recreational and commercial uses to avoid important areas for shorebirds, and extensive awareness-raising activities. Migratory shorebird conservation will become more and more critical as the human population using the Bay continues to increase over the coming decades.

Keywords: birds, population declines, monitoring, zoning, marine park

Introduction

Migratory shorebirds undertake some of the longest regular migrations of any animal group, with many species breeding in the high Arctic tundra and migrating all the way to the Southern Hemisphere to spend the non-breeding season, often stopping along the way to refuel in the vast tidal flats of East Asia (Fig. 1). Small tags fitted to the birds have revealed the magnitude of the journeys they undertake, and the bar-tailed godwit, Limosa lapponica, is one of the best studied species (1–3). The subspecies baueri flies from eastern Australia north to the eastern coast of the Yellow Sea to refuel (2, 3), then on to Alaska to breed. Following breeding, it flies non-stop across the Pacific Ocean in a flight of almost 12,000 km to its non-breeding grounds in eastern Australia and New Zealand (4).

What does all this mean for Moreton Bay? Moreton Bay is the crucial end point of the journey of the bar-tailed godwit and a number of other species. Moreton Bay has supported up to 33,900 migratory shorebirds annually in the period since 2011 (Table 1). It is one of the most numerically important non-breeding sites for shorebirds in Australia, and supports internationally significant numbers (>1% of the total flyway population) of at least nine migratory shorebird species (Table 1) (5, 6). In 1993, it was declared both an internationally important wetland under the Ramsar Convention on Wetlands and a marine park, partly on the basis of the bird populations. A zoning plan providing for the ecologically sustainable use of the park was produced in 1997 (7).

Ecologically sustainable use is of course critical for the long-term health of migratory shorebird populations in Moreton Bay, where birds are continually affected by habitat loss and disturbance (8, 9). Coastal ecosystems in Moreton Bay are critical for providing food and shelter for the shorebirds to recover from their long migration, conduct a feather moult, and gain condition again before the next long migration back to the breeding grounds. Such migrations may seem extreme, but the birds are uniquely adapted to undertake the journeys (10), which allow them to exploit a summer flush of resources in the Arctic, and spend the non-breeding season feeding on the rich benthic infauna of sediments in estuaries such as Moreton Bay. Yet many migratory shorebird species are in rapid decline in the East Asian–Australasian Flyway (11–13). In Moreton Bay, at least six species of migratory shorebird were identified as in rapid decline in an analysis of monitoring data collected by the Queensland Wader Study Group (QWSG) between 1992 and 2008 (red knot, bar-tailed godwit, ruddy turnstone, common greenshank, great knot and whimbrel), and a further two were possibly in decline (greater sand plover, far eastern curlew) (14).

The declines are thought to be mostly driven by habitat loss in the East-Asian stopover areas where, for example, more than two-thirds of intertidal habitat has been lost in the Yellow Sea in the last 50 years, primarily as a result of land reclamation for infrastructure development (15). Indeed, recent studies have shown that the Australian species declining most quickly are those that are highly dependent on the Yellow Sea while on migration (13), and that survival rates are declining for migratory shorebirds that depend on the Yellow Sea (16). Yet migratory species depend on a complete chain of intact habitats along their migration routes (17), and habitat degradation anywhere along the chain can impact the birds (18). Thus, the proper management of important sites such as Moreton Bay is crucial in the context of the birds’ lengthy migration journeys.

Table 1. Species and highest count of migratory shorebirds estimated in Moreton Bay regularly since 2008 (61). Data are extracted from the Queensland Wader Study Group monitoring database. Note that all migratory shorebirds are listed as migratory/marine under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) — the conservation listings shown here relate to listings as threatened species only.

Status and ecology of migratory shorebirds in Moreton Bay

About 23 species of migratory shorebird regularly occur in Moreton Bay, with another five migratory species recorded irregularly along with another 13 locally breeding, non-migratory species of shorebird. The most abundant migratory shorebird is the bar-tailed godwit, followed by the red-necked stint. Moreton Bay also supports all the threatened species of migratory shorebird listed under the federal Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) (Table 1). It has one of the largest populations of the ‘Critically Endangered’ far eastern curlew in Australia, and is now one of the last major strongholds in the world for this rapidly declining species.

Non-breeding shorebirds typically feed on invertebrates that live in or on intertidal habitats, mostly within soft sand and mud in Moreton Bay (19, 20). As the tide comes in and covers feeding habitats, birds move to high-tide roost areas, where they often gather together in large numbers to rest, preen and sleep, and supplement their feeding if the opportunity arises. High-tide roosts are often in claypans, rocky headlands, mangroves, or a range of artificial sites where they can roost away from disturbance and where they have good visibility of the surrounding area to scan for approaching predators (21, 22). This dependence on two markedly different kinds of habitat every day is a crucial factor for understanding the ecology and conservation management needs of shorebirds. To occupy an area, shorebirds need access to high-quality feeding sites, but also nearby suitable roosting sites (21, 23). Energy reserves can be conserved by minimising the flight distance between roosting and feeding areas. For far eastern curlew in Moreton Bay, the typical distance travelled between roosts and feeding grounds is 5 to 10 km (23). As the mainland coast of Moreton Bay has become increasingly developed, there are now few places where significant numbers of birds can gather to roost free of disturbance, and disturbance of roosting birds is a significant management challenge (Fig. 2) (24).

Migratory shorebird monitoring in Moreton Bay

Migratory shorebirds in Moreton Bay have been systematically counted by volunteers at up to 180 coastal sites from 1992 onward (25, Box 1). Monthly counts are conducted around high tide (80% of visits made within 2 hours of the time of high tide), when birds are concentrated at roost sites (22). The number of sites visited per year increased between 1992 and 1995 and has remained relatively stable thereafter. The spatial extent of survey effort was also greater in summer months (January–March) when non-breeding migrants are most abundant.

Habitats for migratory shorebirds in Moreton Bay

(i) Intertidal soft sediment and hard substrates

Moreton Bay contains a complex system of intertidal flats totalling some 23,000 ha at low tide (26), providing a range of feeding habitats for shorebirds. Substrate types within the Bay are diverse and have been broadly categorised into sand, coral, sandy-mud and mud (27). Sand is more prevalent in the eastern side of the Bay and tends to be more penetrable than mud or sandy-mud. This is because the latter frequently contains higher proportions of resistant material such as rocks, coral or shells just below the surface (19). Therefore, there is a need to look below the substrate surface to assess the suitability of feeding habitat for deep-probing shorebirds. Substrate penetrability has been shown to be a good predictor of far eastern curlew feeding density at the broad scale within Moreton Bay, with lower densities of far eastern curlew in areas where the substrate has a low penetrability (19). This is hardly surprising when one considers that the bird can rapidly thrust its whole head into the substrate, reaching a depth of over 20 cm, to capture large, deep-burrowing crustaceans. Pressure-sensitive receptors in the bill of some probing shorebirds allow them to detect solid objects embedded in the wet substrate (28), but inanimate objects buried within the substrate could also interfere with prey detection and capture, and even damage the birds’ bills. Several shorebird species have been shown to switch between tactile hunting on soft substrates and visual hunting on hard substrates (e.g. 29, 30). In terms of assessing and monitoring the quality of feeding grounds for deep-probing shorebirds, a simple measure of substrate penetrability would be the most efficient method, and it could be used to map their probability of use across landscapes.

(ii) Supra-tidal, mangroves, saltmarshes, artificial habitats as roosting and supplementary feeding sites

A subset of the shorebird species that spend their non-breeding season in Moreton Bay has a strong affinity with mangroves for roosting and sometimes feeding. There are three main species that associate regularly with mangroves for roosting. These are grey-tailed tattler, Terek sandpiper and whimbrel. Whimbrel also feed close to and among mangroves in many parts of Moreton Bay, such as Pumicestone Passage and the southern Bay islands.

The main artificial habitats in Moreton Bay used by shorebirds are in the Port of Brisbane reclamation area at the mouth of the Brisbane River. Bunded ponds in various stages of partial reclamation provide extensive roosting habitat adjacent to the rich intertidal feeding grounds at the mouth of the river. These reclamation ponds also provide non-tidal feeding habitat for the smaller migratory shorebirds such as curlew sandpiper and red-necked stint. During the early stages of reclamation, sediment from dredging of the main shipping channel is pumped into the ponds. These sediments contain small invertebrates, including bivalves and crustaceans, that are prey of small shorebirds. The provision of these additional feeding opportunities has led to an increase in the overall Moreton Bay population of red-necked stint and their concentration within the Port of Brisbane reclamation area. It is unclear how these shorebirds will respond when the reclamation is complete and the additional artificial feeding habitat is lost.

Threats to migratory shorebirds in Moreton Bay

There are numerous threats to migratory shorebird populations in Moreton Bay, and more generally in the flyway, including climate change, which may affect wetland breeding habitat in the Arctic (31); loss of stopover sites in mainland Asia (15, 32, 33), and reduction in the area and quality of non-breeding grounds, primarily in Australia (34).

In Moreton Bay, far eastern curlews require deep, soft sediment to be able to use their extremely long bill to its full potential and achieve their greatest foraging success (20). Any structural modification of soft-sediment feeding flats that reduces substrate penetrability may inhibit successful foraging and be detrimental to deep-probing shorebirds (20). Direct and indirect effects on the structure of soft sediments could come from activities including intertidal oyster farming, bait harvesting, the compaction of sediments by vehicles, beach nourishment, nutrient enrichment and the dumping of rubbish or debris (35).

Additional threats include loss of habitat through development, changes in benthic food availability, changes in mangrove and seagrass distribution, and human disturbance.

Management of migratory shorebirds in Moreton Bay

The commitment of Australian governments to protect shorebirds in Australia is reflected in federal and state legislation such as the EPBC Act 1999, Nature Conservation Act 1992 and Marine Parks Act 2004. Such legislation provides for the listing of shorebird species, declaration of marine and terrestrial protected areas, development of recovery plans, and assessment of actions that may impact shorebirds or their habitat.

Development and land-use planning in the coastal zone managed under the Coastal Protection and Management Act 1995 and state planning policies provide protection and management of coastal resources and link to matters of state interest such as marine park highly protected areas, ecologically significant wetlands and wildlife habitat. Numerous other pieces of legislation exist to protect marine resources and habitat to the benefit of shorebirds such as the Environmental Protection Act 1994 and Fisheries Act 1994 and declared fish habitat areas.

The Moreton Bay Marine Park aims to conserve the unique values of Moreton Bay whilst allowing activities such as commercial and recreational use to occur. This balancing act is achieved through zoning that protects representative habitat types and regulates entry and use via the Marine Park (Moreton Bay) Zoning Plan 2008. Protecting sensitive habitats and species is a key consideration in administering the marine park and the zoning plan contains specific provisions intended to protect shorebirds and their habitat from unreasonable disturbance. Disturbance levels at shorebird roost sites in Moreton Bay are strongly related to marine park zones, with marine national park zones, the most highly protected zone, showing the lowest frequency of disturbance to shorebirds (36).

The Migratory Shorebird Conservation Action Plan and Shorebird Management Strategy of Moreton Bay provide guidance to cooperatively manage shorebirds. The Shorebird Management Strategy of Moreton Bay adopts a multifaceted approach to shorebird management, including protecting critical shorebird habitat, protecting shorebirds from disturbance and conducting research and monitoring. Practical mechanisms for achieving this include assessment and the placement of conditions on activities, compliance enforcement, education and awareness, regulation of access or activities, and cooperative management with local councils.

Active shorebird management in accordance with the above statutory and non-statutory tools depends on the responsibility and jurisdiction of relevant authorities. While there is a solid legislative basis on which to base and guide shorebird management, resources and funding allocation is a matter of competing priority within governments. Therefore, effective management is best achieved in collaboration with natural resource management bodies and non-profit organisations that are eligible to apply for grant funding.

Managing migratory shorebirds is challenging for three interlinked reasons:

(i) complex multi-uses of the landscape

Moreton Bay Marine Park is a multi-use marine reserve with areas designated as general use, habitat protection, conservation park and marine national park. Recreational and commercial access is allowed to most areas under the provision that shorebirds are not disturbed. Despite regulations against disturbance, the reality of allowing recreational activities on beaches means that birds are regularly in contact with kite surfers, horse riders, fishers, four-wheel-drive vehicles and dog walkers.

Human recreational use of natural areas can incur immediate behavioural costs to birds, including increased energy expenditure and loss of foraging time as a result of increased time spent being vigilant. In some cases, temporary or permanent avoidance of suitable habitat can occur, ultimately reflected in lower local abundance, poorer physiological condition or impaired reproductive success (8, 37). An analysis of QWSG data showed dogs, walkers and fishing to be the most frequent anthropogenic causes of disturbance to roosting shorebirds in Moreton Bay (36). Notwithstanding education, signage and enforcement of shorebird disturbance laws in the marine park, a high level of disturbance is still occurring at the majority of shorebird sites in the Moreton Bay Marine Park (Fig. 3).

Dog walking in particular can have a large impact on shorebirds, as many dogs are not kept on leashes on the beach (38). Furthermore, dogs actively and repeatedly chase shorebirds, forcing birds to either repeatedly take flight, to increase their vigilance, or even to leave an area. Disturbance by dogs is also a major issue in the terrestrial environment, and substantial reductions in woodland bird abundance have been documented as a result of dog disturbance, suggesting a need to restrict access by dogs in sensitive conservation areas (39).

Dogs must be kept under close control across the intertidal areas of Moreton Bay, yet the reality of living in a city means that beaches are one of the few places where dog owners can let their dogs run freely. Though common, disturbance by dogs is also one of the most easily manageable threats to shorebirds in Moreton Bay. There are two primary methods for managing disturbances to shorebirds: one is to manage public access to important shorebird areas; the other is to allow access to beaches while managing activities. The reality of restricting access to beaches in populated areas, however, means that large nature reserves such as Moreton Bay are difficult to enforce, particularly when management boundaries differ at the council, state and national levels, as is the case for intertidal habitats. Furthermore, restricting access creates conflict in highly populated areas.

In fact research shows that increasing access for people walking their dogs off-leash, and ensuring a smaller more restricted area important for shorebirds is better enforced (8, 24), can produce a win–win situation whereby important shorebird habitat is protected, and dog walkers have the area needed to exercise their dogs.

(ii) energetic consequences of disturbance

Long-distance migrations are energetically demanding, and shorebirds have developed a range of physiological adaptations enabling them to achieve such journeys. Prior to migration, shorebirds are able to increase their energy stores over very short periods through rapid weight gain of 50 to 80% of their body mass (40). To allow for this weight gain and for an increase in the size of flight muscles, birds must compensate through drastic shrinkage of certain organs (41, 42). During flight, energy consumption can remain relatively low and energy is burned straight from organs if needed (42). Refuelling prior, during and after migration is therefore essential in ensuring survival of the species. Repeated disturbance to shorebirds can prevent individuals from gaining the necessary weight to complete migration.

Shorebirds feed in the intertidal zone and roost during high tide, when large numbers concentrate into a small area. Disturbance during feeding can interrupt foraging, and disturbance during roosting can cause birds to take flight, wasting energy reserves. Indeed, shorebirds are highly responsive to anthropogenic stimuli and thus are readily disturbed (38). In the short-term, disturbance can result in increased levels of stress and behavioural changes (43). In the long-term, disturbance can result in chronic avoidance of disturbed habitat and abandonment of otherwise suitable habitat as individuals move to less-disturbed areas (44), increasing density and therefore competition between individuals at undisturbed sites (45).

Shorebirds can better conserve their energy at sites where there is little disturbance (46). The worst case is when birds are forced to stop feeding altogether or have to leave for a lower quality feeding area. Faster human movements (i.e. running as opposed to walking, jetskiing as opposed to canoeing) cause greater disturbance to shorebirds (47, 48). It is possible for shorebirds to adapt to human disturbance, by either extending their feeding period or by becoming habituated to the disturbance (49). It has been suggested that larger shorebirds may be less tolerant of human disturbance than smaller shorebirds (50, 51).

(iii) both local and remote drivers of change could be impacting the health of shorebird populations in Moreton Bay

Because many shorebird species are migratory, the number of birds we see in Moreton Bay can be influenced not only by conditions in Moreton Bay, but also by conditions hundreds, if not thousands, of kilometres away. Moreton Bay like much of Australia suffers from extreme rainfall and drought events, both of which can negatively impact shorebirds. The first by changing sediment structure and therefore food availability (52, 53), the latter by reducing inland habitat availability, thus forcing large numbers of birds to concentrate into a small number of coastal roosting sites and increasing their susceptibility to disturbance events. Repeated disturbances force birds to repeatedly take flight with nowhere else to go.

Though these impacts can be small in isolation, they can have a cumulative effect on birds by preventing them from gaining the necessary weight to start migration in an optimal body condition. As a result, migratory fitness can be reduced, and likewise migratory endurance and speed, meaning some birds will not be able to fly the required distances to reach their stopover sites spread across East Asia. Stopover sites are few and far between, making each individual site important for replenishing depleted fat reserves.

Within the East Asian–Australasian Flyway, the most strategically situated and nutritious stopover sites are located in the Yellow Sea. Most shorebirds funnel through this region on their way to the breeding grounds, creating a large migratory bottleneck (18). However, the Yellow Sea is one of the most populated regions in the world, and hunting, pollution, habitat loss and disturbance there are potentially impacting on the number of birds able to return to Australia each year (54). Loss of even small habitat patches in this location can result in disproportionately large contributions to shorebird declines (55).

Poor resting and refueling conditions in stopover sites mean birds continue their journey in poor physiological condition, and may not reach the breeding grounds in good enough condition to establish a good quality breeding territory. Shorebirds are primarily income breeders, meaning they gain some resources for breeding while in the breeding grounds (56). Conditions upon arrival are therefore important in determining the number of eggs and clutches produced, as well as chick survival. The Arctic is however one of the habitats most sensitive to climate change. Not only is the amount of available habitat predicted to decrease for shorebirds (57), but so is the timing of arctic green-up and peak abundance of arthropod prey, on which shorebirds depend while breeding. Indeed, warmer summers are predicted to cause an increase in mosquito growth rate, resulting in a shorter period where mosquitoes are available as a food source for shorebirds. A shorter season also means birds grow less and there is already evidence of red knots developing shorter bills (58). Once back in their tropical intertidal habitats, these birds with shorter bills have difficulties accessing their usual prey items buried in the mudflats, and suffer higher rates of mortality as a result.

Emerging issues

With the human population in the Greater Brisbane projected to grow rapidly in the coming decade (59), pressure on the natural environment of Moreton Bay looks set to intensify both in terms of recreational and commercial use, and major development projects. From the perspective of migratory shorebirds, we urge careful thought about the following five issues:

1. Continued generation of high-quality monitoring data, ideally through monthly shorebird monitoring at the major sites throughout Moreton Bay, with comprehensive bay-wide counts at least quarterly.
2. Integration of shorebird monitoring data into all relevant planning and decision-making tools within local and state government, together with continued updating of information.
3. Use of decision-support tools to plan and enforce recreational zoning across Moreton Bay, ideally as a partnership between state government and all local government areas adjacent to Moreton Bay.
4. Provision of data and availability of expertise to assist interpretation, supporting investigations of the impacts of proposed development projects.
5. Periodic expert analysis of shorebird monitoring data to assess any impacts of Moreton Bay’s changing environment on bird numbers, and to identify success or failure of conservation efforts.

Moreton Bay is a critically important site for migratory shorebirds in the East Asian–Australasian Flyway. Enormous efforts have been made, and are being made, to protect its ecological integrity in the face of strong demand for recreational and commercial use. Continued proper management and protection of key habitats are paramount for its long-term future as a non-breeding destination for migratory shorebirds.

References