Habitat selection, foraging ecology and conservation of Eastern Curlews on their non-breeding grounds

Abstract

Migratory shorebirds generally utilise very different habitats, great distances apart, during breeding and non-breeding seasons. The coastal intertidal flats in their non-breeding range provide a wide variety of benthic prey and shorebirds are primarily utilising these habitats for there foraging potential. Feeding strategies among shorebirds are largely determined by morphology and broadly range from visual pecking at the substrate surface to tactile probing deep within the sediment. Some shorebirds are capable of very deep (>15 cm) and complex probing of the sediment and this feeding guild is represented globally by three very large curlew species: Eastern Curlews (Numenius madagascariensis), Eurasian Curlews (N. arquata) and Long-billed Curlews (N. americanus). These deep-probing shorebirds are threatened globally due to recent declines in population sizes, largely resulting from habitat loss and hunting pressure. To prevent further loss and possible extinction of shorebirds at risk, better knowledge of their feeding ecology outside the breeding grounds is required. Moreton Bay on Australia’s subtropical east coast supports over 15% of the global population of Eastern Curlews during the austral summer months and is therefore of worldwide importance for this species during its non-breeding season. The present study investigates the feeding ecology of the Eastern Curlew in Moreton Bay and what drives their habitat choice on these nonbreeding grounds. Initially the broadscale distribution of Eastern Curlews among tidal flats within Moreton Bay was assessed. Low-tide surveys were conducted with the help of 60 skilled volunteers who made repeated counts at defined sections of tidal flat (sites) totalling approximately 41.3% (9,500 ha) of the intertidal habitat within Moreton Bay. Sites varied in size (0.68–474 ha), substrate, topography, and other features. There was a very high consistency between counts on different days within a month and different months within a season, across all sites. Winter numbers of Eastern Curlews totalled about 25% of those in summer, and the proportional distribution of birds among sites was generally similar across both seasons. The within-season constancy of curlew numbers across sites suggests that short surveys can give reliable results. Curlew densities (birds/ha) also showed a strong correlation between summer and winter counts. Curlew density varied greatly among tidal flats and important intertidal feeding habitat was identified. Numbers of Eastern Curlews on low-tide feeding grounds were then compared with those at high-tide roosts in order to understand their habitat use more fully. Counting shorebirds on feeding grounds provided an accurate population census tool but a relatively large area had to be covered compared with counting birds at roost sites. The correspondence between numbers of curlews at high-tide roosts was compared with those on low-tide feeding grounds at several spatial scales, leading to the conclusion that the typical distance birds travelled between roosts and feeding grounds would be in the order of 5–10 km, with high mobility between alternative roosts and/or feeding grounds occurring at or below this distance. The habitat preferences of curlews were next investigated at large (betweenflat) and small (within-flat) spatial scales in order to determine factors underlying habitat selection. Using data from the low-tide surveys of feeding grounds, 32 defined sections of intertidal flat (sites; 200–400 ha in area) which varied greatly in their curlew density and other environmental characteristics were analysed. Of ten environmental characteristics measured for each site, substrate resistance was the best predictor of curlew density at the broad scale. Sites with the least resistant substrates had curlew densities three times those with the most resistant substrates. Characteristics that were poor predictors included distance to the nearest roost, level of human disturbance and intertidal area and width. For a finer-scale assessment, microhabitat use and feeding behaviour were recorded during low tide within 12 intertidal flats, which varied in size (23–97 ha), curlew density, substrate and other features. Across all flats, curlews strongly preferred to feed relatively close (0–50 m) to the moving low water line. They fed on a variety of substrates (including sand, sandy-mud, mud and seagrass) in broadly similar proportions to their occurrence in the habitat. There was a statistically significant preference for sand, although its magnitude was not strong. These results indicated that curlews selected habitat more strongly at the between-flat rather than the within-flat scale. The curlews' diet was next quantified across the same 12 intertidal flats described above, though 970 focal observations (each of 3 minutes duration). Food resources were also estimated from substrate core samples. Curlew density, estimated at each site using low-tide surveys from every site visit, showed a strong positive association with both the density and biomass of prey across the 12 flats. However, the density and biomass of prey also showed a strong negative correlation with substrate resistance, which was the best predictor of curlew density. Curlews were most abundant at sites with the least resistant substrate, and these sites also generally had the highest prey density and biomass. When the effect of substrate resistance was statistically removed, curlew density was no longer significantly correlated with prey density and biomass. This suggests that broadscale habitat choice by Eastern Curlews on their non-breeding grounds is more strongly influenced by prey availability (which is higher when substrate resistance is lower) than by prey density or biomass, although in Moreton Bay a positive correlation across sites meant that these factors were synergistic. Finally, the present study tested whether foraging success could explain choice of feeding sites by curlews. Foraging success was assessed at 11 of the 12 intertidal flats described above. Biomass intake rate explained 57% of the variation in curlew density across sites, and was a far better predictor of curlew density than either the feeding or success rate. Both the biomass intake rate and curlew density were highest on substrates with higher penetrability. There were some differences in feeding behaviour between sexes, which were generally consistent with expectations given the females’ greater body size and bill length, although neither sex had a clear energetic advantage. The positive relationship between curlew density and biomass intake rate was stronger than the previously-established positive relationship between curlew density and substrate penetrability, suggesting that the curlews in Moreton Bay choose feeding habitat directly on the basis of the food supply. Low aggression rates suggest relatively little constraint from intraspecific competition during most of the overwintering period. These results also suggest that the population of Eastern Curlews in Moreton Bay is not limited by the availability of food resources but perhaps by factors either on the breeding grounds or at stop-over sites during migration. These findings indicate that, for assessing and monitoring the quality of feeding grounds for deep-probing shorebirds, a time-efficient measure of the physical properties of the substrate (such as penetrability) would be useful. Any structural modification of the curlews’ soft-sediment feeding flats that reduces substrate penetrability may inhibit successful foraging and be detrimental to them. Direct effects on the structure of soft sediments could come from activities including intertidal oyster farming, the compaction of sediments by vehicles and the dumping of rubbish. In the present study, curlews foraged in a broad variety of habitats andconsumed a variety of different types and sizes of prey. Even impenetrable flats, delivering a low foraging success, were used for feeding by some birds. The use of their long bills to obtain many different types of prey...