Prey versus substrate as determinants of habitat choice in a feeding shorebird

Abstract

Many shorebirds on their non-breeding grounds feed on macrobenthic fauna which become available at low tide in coastal intertidal flats. The Eastern Curlew Numenius madagascariensis in Moreton Bay Australia, varies greatly in density among different tidal flats. This study asks: how important is the abundance of intertidal prey as a predictor of this variation? We quantified feeding curlews' diet across 12 sites (different tidal flats, each re-visited at least eight times), through 970 focal observations. We also estimated the abundance of total macrobenthic fauna, potential prey taxa and crustacean prey on each tidal flat; measured as the number of individuals and a relative biomass index per unit substrate surface area obtained from substrate core samples. We estimated curlew density at each site using low-tide surveys from every site visit. Curlew density showed a strong positive association with both the density and biomass of fauna and of potential prey (r values all around 0.70) across the 12 flats. Associations with crustacean density and biomass were also statistically significant (r values both 0.60). However, these variables also showed a strong negative correlation with a measure of substrate resistance (based on the amount of hard material in the substrate core), which was the best predictor of curlew density (r = -0.82). Curlews were most abundant at sites with the least resistant substrate, and these sites also generally had the highest faunal density and biomass. When the effect of substrate resistance was statistically removed, curlew density was no longer significantly correlated with fauna density and biomass. This suggests that macro-scale habitat choice by Eastern Curlew 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.