Waterbirds are highly mobile, with many species conducting long-distance movements of hundreds or thousands of kilometres in days or weeks [1–3]. Understanding these movements is key to effective management of waterbird populations [4], particularly since rates of wetland habitat loss are increasing due to water harvesting and climate drying [4–7]. Maintaining waterbird diversity and habitats are often important goals for water and wetland managers worldwide [8–10]. However, there are knowledge gaps that affect our ability to manage habitats and to predict waterbird responses at local, national and global scales. One of the most critical gaps is knowledge of waterbird movement behaviour and variability, particularly for inland species [4, 11–13].
Waterbird movements can be complex, occurring along a continuum of variation where the behaviour of any one species or individual may be classified variously as: (1) Residency, including ‘sedentary movements’, ‘central-place movements’, and ‘commuting’, where individuals remain in one area or home range, usually returning to favoured locations between relatively short-distance foraging trips, and sometimes displaying territoriality (including during breeding periods); (2) Nomadism, including ‘facultative migration’ and ‘facultative movements’ where individuals move different distances and directions irregularly and seem to generally be responding opportunistically to resource availability, including ‘fugitive movements’ in response to disruption or disturbance of resources, individuals or flocks; or (3) Migration, sometimes called ‘obligatory migration’ or ‘seasonal migration’, where individuals regularly move relatively long distances predictably between particular locations and in consistent directions, usually seasonally or annually [14–17]. Adding to this complexity, individuals can show plasticity in their movement strategies over time [18].
Advances in telemetry are revealing complexities in movement patterns within and among individuals and species, including ‘partial migration’, where residency, nomadism or migration strategies may each be employed within a species by different individuals or by an individual at different times [19, 20]. It has been suggested that there is greater variation in movement strategies in the southern hemisphere compared to the northern hemisphere due to greater variability in climatic factors [21, 22]. However, there are relatively few satellite-telemetry studies tracking southern hemisphere species, especially inland waterbirds, and those few have usually involved relatively small numbers of individuals [12, 23–25]. Moreover, understanding of nomadism is relatively limited compared to that for migration and residency [17, 20, 26].
Movement ecology is often poorly understood even for common and conspicuous taxa that are the focus of significant management investment. Examples include ibis, spoonbills, egrets, and herons that nest in large aggregations, often in Ramsar sites, in response to specific environmental conditions such as flooding [11, 27, 28]. Despite extensive leg-banding and other marking programs, resighting or recovery data are usually limited for these taxa. For example, < 0.8% of ibis and spoonbills banded in Australia have been resighted or had their bands found > 3 months after banding, post-dispersal from natal or nest sites [29]. Taxa such as these are of particular interest for wetland and waterbird management and policy makers because they often nest in areas that are susceptible to adverse effects of environmental change; consequently, their habitats and populations are the subject of intensive conservation management [30–34]. For species dependent on surface water, management can include the allocation of environmental water or ‘environmental flows’, to provide the quantity, timing, and quality of freshwater flows and levels necessary to sustain aquatic ecosystems [35]. Wetlands and other areas that receive environmental water (actively or passively), or that may be inundated by natural flooding if key constraints are overcome, are colloquially termed the ‘managed floodplain’ [36]. Understanding waterbird movements at fine spatial and temporal scales can maximise the efficacy of environmental water application, by guiding where and when to provide water and for how long.
Australia has breeding populations of three ibis species and two spoonbill species in the Threskiornithidae family: Straw-necked ibis (Threskiornis spinicollis); Australian white ibis (Threskiornis molucca); glossy ibis (Plegadis falcinellus); royal spoonbill (Platalea regia); and yellow-billed spoonbill (Platalea flavipes). These species nest in aggregations of up to hundreds of thousands of birds when conditions are good, while in poor conditions they may not nest at all. Inland, these species are experiencing reduced breeding event frequency, size, and success, including mass nest abandonment events due to prematurely falling water levels [37–39]. The birds are dependent on wetland inundation for breeding, but the degree of breeding-site fidelity among individuals or groups is not well understood, even within the relatively well-studied breeding sites of the Murray-Darling Basin in south-eastern Australia, which is thought to be the core breeding area for these species, with 46% of aggregate-nesting wetlands in Australia [40]. Some breeding sites are used every year by the same species; but it is not known if it is the same individuals revisiting each time.
There are major knowledge gaps about movements outside of breeding events. While these birds are known to be capable of moving at continental scales (many 100s – 1000s km) within a few months [28, 29, 40, 41], some authors have suggested there may be regional sub-populations and seasonal migrations with site fidelity [42, 43]. Leg-banding data suggest individuals that breed in eastern Australia mostly remain in eastern Australia and rarely move west; most recorded displacements of leg-banded birds have been from the south-east to the north-east [29, 40, 41, 44]. Seasonal fluctuations in local abundance along with observations of flocks in flight suggest that east coast areas are important winter and drought refuge locations, while inland wetlands of the Murray-Darling Basin are important breeding sites [29, 40, 42]. However banding recoveries do not suggest any relationships among bird age, season, and movement distances or directions [29, 40, 41].
There has been very limited direct movement tracking using telemetry for ibis and spoonbill species in Australia. No royal spoonbills, yellow-billed spoonbills, or glossy ibis have been tracked prior to this study. In February 2000, two straw-necked ibis were fitted with satellite transmitters in the Macquarie Marshes in New South Wales (NSW) and tracked for c. two months [28]. Of these two, one bird flew 1,438 km north of the capture site within a month, while the second bird remained within 200 km of the capture site [28]. Tracking of Australian white ibis movements has largely focused on breeding sites in urban and suburban coastal environments, rather than natural inland wetlands [45–47].
For conservation and management, important knowledge gaps remain at the continental scale about movement patterns, movement variability, movement rates and movement timing for these species. At local to regional scales, questions such as how far birds travel to find food are also relevant for managers of waterbird populations and habitats. Answering these questions would assist water and wetland managers to identify key habitats associated with movements and to understand better the places, scales and times at which resources are required. This should in turn improve capacity to target land and water management actions (such as strategic watering and drying of wetlands), evaluate progress, and predict future outcomes for waterbirds.
Over seven years, we used GPS (global positioning system) satellite telemetry to track the movements of three species in the Threskiornithidae family – straw-necked ibis, Australian white ibis, and royal spoonbills. We aimed to address knowledge gaps regarding intra and interspecific variation in movement strategies of these species post-dispersal from breeding sites and consider consequent implications for water, wetland and waterbird management.