Feeding behaviours of the black winged stilt at Bicentennial park
Katherine Forsythe (Supervisors: Professor Gee Chapman, Associate Professor Ross Coleman and Dr Trevor Tolhurst)
Animals must consume food to survive. An animals feeding behaviour can not only have consequences for the success of the individual animal but also for the prey species and even the ecosystem. Studying the feeding behaviours of animals can lead not only to a greater understanding of the study species, but also of the ecosystem in which it feeds.
The black-winged stilt (Himantopus himantopus) is a large wader found worldwide. The black-winged stilt feeds in a variety of habitats ranging from coastal to inland wetlands of either fresh or salt water. Due to their extremely long legs they can feed in a variety of water depths that are not available to many other shorebirds. These birds exhibit a large repertoire of feeding behaviours including pecking at prey on water surface, in water column or on sediment surface as well as probing and scything through the sediment. Their diet has been reported to include aquatic insects, crustaceans, small fish, molluscs and worms. The black-winged stilt displays great plasticity in terms of how they feed, what they feed on and where they feed, making them an ideal species to study factors influencing different feeding behaviours.
There is a large (~200) resident black-winged stilt population at the waterbird refuge at bicentennial park (WBR). A manually operated tidal gate provides us with the potential to manipulate water-depths within the lagoon. This refuge provides us with a testable system in which to study black-winged stilt feeding behaviour.
This study aims to:
1. Characterise differences in feeding behaviours of black-winged stilts between different areas and water depths within the waterbird refuge at Bicentennial park
2. Determine if differences are due to the individual birds, the area of mud of the water depth
3. Determine what it is about the birds, area or water depth that influence the feeding behaviour of the black-winged stilts
Shorebirds as well as the wetlands habitats in which they feed and reproduce are under threat globally. Although this species in not currently threatened, populations of black-winged stilts in eastern Australia have been estimated to have declined by 80% over the past 20 years. With increasing numbers of shorebirds utilising man-made and/or managed wetlands, understanding how water-level, as well as other factors influence feeding behaviours is important if the managers wish to provide feeding habitats for black-winged stilts and other shorebirds.
Initial testing in February showed that black winged stilts at the WBR feeding in deeper water pecked more often below the surface than at the surface of the water, where as stilts feeding in shallower water did not peck more often below the surface or at the surface of the water. This pattern was observed again at the WBR in June. The same pattern was also observed in another population of black-winged stilts at Kooragang Wetlands near Newcastle in April.
A water-level manipulation experiment carried out at WBR in June-August showed that stilts pecking in “shallow” areas responded to increases in water-level (making them deep) by pecking more often below the surface than at the surface, so that their behaviour became like that in original “deep” areas. So it seems that stilts are responding to some factor of the water-depth rather than the area. Black-winged stilts pecking in “deep” areas did not respond to decreases in water-level (making them shallow) by behaving in a similar way to stilts in original “shallow” areas, however the behaviour was not similar to stilts in original “deep” areas either. It seems that stilts in “deep” areas are not responding to either the water-depth or the area alone nor is there an effect of the water-depth on the area over the time the experiment lasted.
Differences in potential prey between “deep” and “shallow” areas that change with water-depth in “shallow” areas could explain the observed patterns. Differences in turbidity, affecting ability to detect prey below the surface, could also explain the differences in behaviour seen. Results of invertebrate sampling and turbidity measurements are still being analysed and will be posted as they become available.