Importance of drainage channels for waders foraging on tidal flats: relevance for the management of estuarine wetlands.
Despite covering a relatively small area on a global scale, estuarine tidal flats are vital to many aquatic bird species during much of the yearly cycle. Small-scale features of tidal flats, such as water drainage channels, can influence the carrying capacity of estuarine wetlands, yet their ecological impact is poorly understood. In this study we evaluated the influence of drainage channels on the quality of feeding habitat for waders in the Tagus estuary, Portugal. We measured the abundance of seven wader species at five distance classes from drainage channels, and analysed the variation in peck rates, step rates, turning rates and success rates. In addition we measured the density of macroinvertebrate prey and the sediment physical-chemical characteristics at different distances from the channels. All wader species occurred at higher densities near the channels. In the study area 44% of the birds fed on just 12% of the available surface, less than 5 m away from drainage channels. Wader foraging behaviour also suggests a greater feeding effort near the channels. While the characteristics of the sediments did not change significantly with distance from channels, prey abundance corresponded closely with wader abundance, suggesting that the small scale distribution of birds on the tidal flats may be related to the presence of their prey. Synthesis and applications. Our study shows that the areas around drainage channels are particularly important feeding sites for waders foraging on tidal flats. Consequently, managers of estuarine wetlands should strive to preserve or improve channel networks. This can be achieved by (i) preserving saltmarshes and saltpans adjacent to tidal flats, (ii) minimizing the reclamation of upshore flats and (iii) avoiding embankments and canalizations of inland water flows that reduce the number of water entry points onto tidal flats.