Amanda Lilleyman, Charles Darwin University and Micha Jackson, The University of Queensland.
Migratory shorebirds are in serious decline globally. A recent analysis of data spanning 1973 to 2014 showed population decreases in twelve species of migratory shorebirds species in Australia. Further, the Threatened Bird Index for Australia (TSX) has shown an average decline of over 60% in the population sizes of 11 species of migratory shorebirds since 1980, the most concerning finding for any group of birds.
As part of a project by the Threatened Species Recovery Hub of the Australian Government’s National Environmental Science Program, we are working with the busy Port of Darwin and Larrakia Traditional Owners on research to benefit the Critically Endangered far eastern curlew and other threatened migratory shorebirds. This work has highlighted that many ports, commercial salt works and wastewater treatment plants are important custodians of these species.
A disturbing business
Around the world, multiple threats are impacting migratory shorebirds throughout their lifecycles. One of the most significant threats in Australia is disturbance – especially at high tide roosts. A common cause of disturbance is people and their dogs using beaches for recreation. Disturbance causes shorebirds to expend extra energy keeping watch and fleeing by walking or flying instead of just resting or feeding. The consequences are often underappreciated, but can be significant, resulting in reduced weight gain – weight that is vital to enable birds to complete long migrations back to their northern hemisphere breeding grounds.
Lee Point is a long stretch of natural beach and adjoining mudflats just north of Darwin. Expansive, backed by natural vegetation and free from any development, the area would be considered ideal high tide roosting habitat except for one factor: the area is a very popular place for people and dog walking – much of which is off-leash. A study on the impact of alarm flights on migratory shorebirds at Lee Point by Amanda Lilleyman found that the energy spent as a result of 10 alarm flights per day could have serious consequences, such as reducing reproductive success or even survival.
A lack of roosts that are safe from disturbance may even constrain the number of shorebirds that a given region can support. Another study led by Madeleine Stigner at the University of Queensland within a protected area in Brisbane’s Moreton Bay found that the recreational use of foreshores, particularly with dogs, consistently reduced the occupancy and abundance of shorebirds.
In Australia there are several very important migratory shorebird roosting sites that have been artificially, usually unintentionally, created. While they should not be considered a replacement for natural habitat, a number of studies have shown that artificial habitats can provide attractive high tide roosts, particularly when they are shielded from disturbance. They can also provide foraging opportunities for some species.
Sites are generally associated with commercial salt works, ports, wastewater treatment and, in some cases, specifically constructed roosts. Some notable examples of successful artificial roosting habitat in Australia include:
- Constructed ponds at a former commercial salt operation in the Gulf St Vincent, South Australia, which have consistently recorded over 15,000 shorebirds during counts over the last several decades.
- Constructed waste water treatment ponds at the Western Treatment Plant, Victoria, which consistently hold internationally significant numbers (more than 1% of the global population) of many shorebird species. The ponds are used throughout the tidal cycle for roosting and foraging by different species.
- Multiple ports including Gladstone Port, Port of Newcastle, Broome Port and Darwin Port.
- A common characteristic of the sites above is their relative inaccessibility to the public, so birds can roost with little disturbance.
Despite the natural habitat, frequent disturbance by people and dogs at Darwin’s Lee Point is a problem for migratory shorebirds. Credit: Amanda Lilleyman.
On the other side of Darwin from Lee Point are the East Arm Wharf settling ponds. Although artificially created, and part of the busy commercial Darwin Port, the ponds are a very important roosting site for the far eastern curlew, sometimes recording close to 1% of the global population. As such the port are important custodians of the species, a responsibility it takes pride in.
The East Arm Wharf ponds provide roosting habitat at all tide heights (high tide heights in Darwin can vary from about 5.5 meters to over 8) and surveys have found consistently large numbers of roosting far eastern curlews. The abundance of this species, and others at the site has increased over the last nine years despite global declines over the same period. Surveys by the project team have found 27 species of shorebird roosting at the site at high tide when surrounding mudflat feeding habitats are inundated.
In contrast to Lee Point, a large part of the attractiveness of East Arm Wharf to migratory shorebirds is believed to be the lack of disturbance from people and dogs. It is also close to suitable foraging habitat on intertidal mudflats. In combination, the short travel distances and lack of disturbance mean that birds can minimise their energy expenditure, so more of the energy they get from foraging can go into very important weight gains.
Lack of disturbance by people and dogs makes the East Arm Wharf settling points in Darwin Port a valuable roosting site for migratory shorebirds. Credit: Michael Lawrence-Taylor.