|dc.description.abstract||The wide, unstable, braided riverbeds of the eastern South Island, New Zealand, have been inhabited by a diverse avifauna. Several species including the wrybill breed only on these rivers. Use is restricted mainly to the breeding season from September to December, so this was the critical study period. Previous, behaviourally oriented studies have occurred mainly in the high country catchments where habitat modification is slight. However, substantial bird numbers also occur on lowland riverbeds. These areas are subject to existing or planned water developments which may have negative impacts on the conservation of these habitats and bird species. The principal goal of this study was to add to the information necessary to conserve lowland riverbed habitat within multiple use planning strategies. Specific objectives associated with assessing breeding and feeding requirements were therefore hydrologically oriented.
Study areas were chosen on the lower Rakaia and lower Ashley rivers. Both are braided but have markedly different flow regimes. The Rakaia is snowfed and has peak flows occurring during the breeding season. The Ashley is primarily rainfed and has a declining breeding season flow regime. Wrybills, black-fronted terns, banded dotterels, South Island pied oystercatchers, and pied stilts breed in these areas. The first two of these were selected as indicator species. It was assumed that because of their restricted habitat needs, flow requirements recommended to meet their needs would also meet those of most other species. Banded dotterels were also studied because of their close taxonomic relationship and overlapping distribution with wrybills. South Island pied oystercatchers and pied stilts were chosen to check the validity of the approach.
Breeding, nest site requirements, microhabitat preferences for foraging, diets, home ranges and food supply were studied. Wrybill time-budgets were examined, and the incremental approach to impact assessment was applied to wrybill foraging needs.
Breeding success for all species was dependent on the flow regime. In 1982 wrybill fledging success was moderate on the Rakaia, whereas in 1983 it was very low. Conversely, in both years fledging success was very high on the Ashley. This variability resulted from serious flooding on the Rakaia in 1983 which effected all species. Wrybills have nest site characteristics most closely approximating a habitat specialist, while the other species should be considered habitat generalists. Nests are generally close to water, near minor braids, and on non-vegetated shingle substrates. On average there is a higher chance of nests being flooded on the Rakaia than on the Ashley where predation is more likely to reduce nesting success. Vegetation encroachment threatens nest site provision and floods are presently the only natural regular of exotic plants such as lupin.
Banded dotterel home ranges were significantly smaller than wrybills, and on an intra-specific basis were smaller on the Ashley. A correlation existed between home range size and habitat quality, so that small wrybill home ranges were dominated by the occurrence of productive minor braids, and larger ones by less productive major channels.
Time-budgets provided further insights into the influence of hydrological factors on wrybill ecology. Wrybill time-budgets appear to be inflexible because a high proportion of time is devoted to foraging, with relatively little time available for other activities. Resource depression on the Rakaia in 1983 lessened the chances for wrybills to breed successfully because sufficient food could not be gathered to provide for breeding energy needs. This did not occur on the Ashley River where flows were generally more stable. From a management viewpoint, development strategies which lead to greater flow fluctuations will have a detrimental impact on wrybills.
The foraging patterns for all species except black-fronted terns were examined. There was a general preference for aquatic habitats, particularly those associated with minor braids and disconnected pools. Depth and substrate use were studied in detail for wrybills and banded dotterels. Wrybills displayed consistent use patterns between rivers, which reflected specialisation in habitat use. Banded dotterel habitat use varied considerably. Diet was studied by faecal analysis. Both wrybills and banded dotterels fed on invertebrates of aquatic and terrestrial origin. For wrybills it was noticeable that Ephemeroptera larvae did not dominate the diet as had previously been reported for high country catchments. Coleoptera, Hemiptera, and Ephemeroptera were important on the Rakaia with Trichoptera replacing Ephemeroptera larvae on the Ashley. Banded dotterels were more reliant on Coleoptera and Hemiptera on both rivers. Pitfall trapping results showed that terrestrial invertebrate availability was dependent on the proximity of water. Fewer floods on the Ashley led to a more consistent food supply on that river. This helps explain the greater bird density on this river. Severe flooding in 1983 appeared to seriously depress aquatic invertebrate densities on the Rakaia.
The incremental approach to impact assessment was applied to wrybill foraging requirements. Depth, substrate, and water velocity preferences were included within a weighted usable area model already developed for fisheries use on the Rakaia River. Over the range of median to low flows studied, weighted usable area increased with declining discharge. A simplified usable width approach was applied to a highly braided section of the Ashley River. Here, usable width declined with falling discharges. This inter-basin difference could be explained with reference to the braiding pattern of each river.
The main study objectives were achieved, but the indicator species management approach was of limited value in areas other than nest site requirements. Pied stilts and South Island pied oystercatchers fed at greater depths than wrybills, and often used different microhabitats. However, for foraging, wrybills need appeared adequate indicators of banded dotterel requirements.||en