Extreme nomadism in desert waterbirds: flights of the banded stilt

1. Introduction

In well-studied avian species in Northern Hemisphere temperate environments, migration is usually spatially and temporally predictable and linked to seasonal cycles [1,2]. By contrast, desert waterbirds survive and reproduce in highly variable environments, where they must exploit ephemeral resource pulses that are stochastic and widely separated in space and time [3–5]. The nomadism that evolves is perhaps nowhere more extreme than in the banded stilt (Cladorhynchus leucocephalus) [6,7], exquisitely adapted to the stochastic ‘boom–bust’ cycles of Australian deserts, which are characterized by low and highly variable rainfall [8] (figure 1). Banded stilt have a unique life-history strategy in which they somehow detect and rapidly move towards infrequent large rainfall events that occur hundreds of kilometres away in the arid inland [9]. These flooding events transform vast and normally dry salt lakes into highly productive shallow water bodies [10], which banded stilt exploit by arriving within just days of filling; they then synchronously breed in nesting colonies of thousands of pairs [9]. However, understanding of this behaviour is limited owing to its rarity and remoteness. In particular, the mechanisms used by banded stilt to detect and navigate to these waterbodies are completely unknown [9], as are the speeds, distances and coordination of flights. Exploiting satellite telemetry, and working at some of the most arid and variable rainfall regions of Australia (figure 1), we reveal some of the strategies that allow banded stilt to be desert nomads.

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2. Material and methods

Twenty-one adult banded stilt (17 male, 3 female, one unknown, weighing 250–300 g) were fitted with 5 g PTT-100 solar satellite transmitters (Microwave Telemetry Inc, Columbia, MD, USA), at three sites in South Australia, two of which were in the most arid parts of continent (figure 1). Ten birds were captured from their nests at an inland breeding colony of 9500 pairs on Kati Thanda-Lake Eyre South (South Australia; 29.36° S, 137.27° E) in March 2012. Four birds were captured at Lake Harry, a drying ephemeral arid-zone wetland (South Australia; 29.40° S, 138.30° E) in April 2013. Seven birds were captured at a coastal saline refuge wetland, Morella Basin (South Australia; 36.15° S, 139.70° E), near the Coorong, in April 2013. Deployments were timed with breeding and resource pulses at stochastic arid inland sites, and during an extended dry period at the predictable coastal wetland (figure 1). Breeding birds were caught in walk-in traps at nests; non-breeding birds were caught while foraging using the Super Talon Ultimate net gun (www.humanecapture.com) [11]. Transmitters were mounted on 10 mm neoprene to prevent feathers obscuring solar panels and attached using a leg-loop harness made from 4.75 mm tubular Teflon ribbon (Bally Ribbon Mills, Bally, PA, USA) and fixed in place with less than 1 mm resized sections of Jinkai aluminium fishing crimps. This harness design was the result of testing and refinement during a 3-month aviary trial on black-winged stilt (Himantopus himantopus). Transmitters were programmed to operate on a duty cycle of 16 h off and 10 h on. Locations in latitude and longitude decimal degrees and date/time stamp were estimated by the ARGOS system (www.argos-system.org), using a Doppler shift in signal frequency. All data were uploaded to Movebank (www.movebank.org) and then filtered using the Douglas Argos filter [12] to remove low accuracy fixes and other implausible data points, based on rate and angle of movement between fixes. Movement distances and durations were determined from the filtered dataset using R [13]. The distance and duration of long-distance flights (more than 350 km) were determined, using the last fix from the starting wetland and the first fix from a wetland site that was occupied for more than 72 h.

3. Results

A total of 13 192 filtered locations were obtained over a combined 4120 transmission days, with birds tracked for a mean of 196.2 days (deployment 1: mean 229.7 days (range 90–398), deployment 2: mean 128.5 days (range 9–436 (as per 4 July 2014, but ongoing)), deployment 3: mean 187 days (range 33–500, as per 4 July 2014, but ongoing)).

Movements of birds were not seasonal; they were irregular and non-seasonal, with birds moving to the inland following rainfall and leaving inland sites as conditions deteriorated. Two birds moved inland following flooding (table 1, figure 2a): 14 days after a widespread inland rainfall event of 40–100 mm in April 2014, subject E7 moved 375 km inland leaving a coastal wetland refugia where it had remained for 11 months. Over the next 59 days, it visited five major inland salt lakes, travelling 2025 km before returning to the same coastal wetland. Similarly, subject D7 flew more than 400 km from coastal refugia, which it had occupied for two months, to Lake Torrens in August 2013, although nearly nine weeks after rainfall had inundated that lake and ca 11 000 pairs of banded stilt had already begun nesting there.

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Twelve of the 14 birds tagged at inland deployments 1 and 2 made long-distance flights (mean 683.9 km, range 357–1298 km) to either the coast or distant inland sites (figure 2b). The two remaining birds were predated within 150 km of the catch site. Birds tagged at the Lake Eyre South breeding colony departed over a period of six weeks as salinity increased from 66 000 µS cm⁻¹ at tagging to 127 000 µS cm⁻¹ seven weeks later (table 1, figure 2b). Some individuals moved south over a period of days, stopping at other ephemeral wetlands en route, while others rapidly relocated, flying 400–800 km to coastal wetlands in a single day or less. Two individuals that departed from Lake Eyre moved even further inland to desert lakes in Western Australia that had been filled by cyclonic rainfall six weeks earlier (figure 2c). Subject A0 travelled north-west over the Gibson Desert to reach an ephemeral lake, flying a minimum of 2263 km in 55.9 h, while subject A8 left the following day and travelled a minimum of 1572 km in 144.9 h, crossing the Great Victoria Desert to reach the same area, where local observers reported the presence of tens of thousands of banded stilt. Three other instances were recorded in which birds made long-distance movements from inland sites to southern coastal wetlands, arriving within days of each other at the same destination wetland hundreds of kilometres away, yet departing at different times and following different paths.

4. Discussion

These first tracking data from this extreme desert nomad reveal that banded stilt make rapid continent-wide movements in response to unpredictable ephemeral resource pulses that are widely separated in space and time. Flights of more than 2000 km following stochastic non-seasonal environmental changes are longer and more rapid than flights documented in other desert waterbirds [4,5,14,15], and despite the banded stilt's much smaller body size (e.g. less than 300 g compared with grey teal (Anas gracilis) less than 600 g [5] and lesser flamingo (Phoenicopterus minor) ca 2000 g [14]).

Banded stilt appear to use a range of strategies to detect and exploit ephemeral opportunities between inland and coastal wetlands. Movement behaviours include rapid and seemingly directed long-distance flights between locations, contrasting ranging movements by the same individuals, involving journeys of longer duration, with stopovers at multiple wetland sites, or shorter flights returning back to the starting location following visits to other sites. Some of these movement behaviours broadly match those recorded from other desert waterbird species [5,16,17], with infrequent long-distance flights punctuating the extended periods of occupation at suitable sites [4] and evidence of individuals using previous or stored knowledge of a range of locations [5].

Some flights by banded stilt followed large rainfall/flooding events at distant sites, with birds possibly detecting the passage of these distant weather systems via low-frequency sound, temperature or pressure gradients [4]. However, several flights occurred many weeks after major rainfall had occurred and so presumably were detected by other means, perhaps even olfactory signatures [18] specific to flooded salt lakes or food sources.

The longest and most rapid flights that we found involved two satellite-tagged individuals moving longitudinally across the continent to reach the same destination, yet departing separately and travelling in independent flocks. These flights demonstrate definite linkages between populations in south-eastern and western Australia, which were previously assumed separate [9] due to non-continuous distribution. Additionally, it seems that these birds (and tens of thousands of others gathered at the destination wetlands) had moved there in response to common cues. However, as arrival occurred nearly six weeks after the site had been flooded, rainfall or other recent weather-related stimuli were unlikely to have prompted movement. This and other instances of similar movements from the same or nearby starting points to common locations hundreds of kilometres distant, contrast with other studies of nomadic desert waterbirds, which show little coherence among individuals [5,14].

These first empirical satellite tracking data from banded stilt demonstrate that this species indeed epitomizes extreme nomadism in stochastic desert environments. The suite of behaviours revealed give an insight into the complex movement strategies needed for survival and reproduction. However, there is still much to learn regarding the sensory capabilities and specific cues that guide movements, and the physiological challenges undertaken by individuals in doing so.

Ethics statement

Animal Ethics licencing was provided by South Australian Department for Environment, Water and Natural Resources permit U-25774-4 and Wildlife Ethics Committee approval 43-2009-M5.

Funding statement

Funding was from BHP Billiton and Deakin University's Centre for Integrative Ecology.