Exclosure study on the exploitation of macrophytes by summering and moulting waterbirds at Lower Lake Constance

Abstract

Owing to synchronous moult, most waterbird species are constrained by flightlessness and limited mobility for several weeks. As new feather production is energy demanding, these birds need to choose a safe moulting site with appropriate food supply. Up to 20,000 waterbirds carry out moult at Lake Constance, gathering at sites where they find food close to safe hiding places from predators and human-caused disturbance. In this study, we focused on the food supply at one prominent moulting site, Mettnau Sudbucht, at Lower Lake Constance. We aimed to determine the food items and quantity as well as their utilization by summering and moulting waterbirds. We conducted experiments with exclosure cages which protected macrophytes from bird grazing and compared these sites with unprotected grazed sites. In these experiments, we found that the summering and moulting waterbird community, dominated by Eurasian Coots (Fulica atra Linnaeus), caused a significant decline of the macrophyte biomass at 1.5-m depth (MWL), where they were responsible for a loss of over 40% of the total charophyte biomass. No grazing effect was found at a greater depth (2-m MWL). The available food consisted mostly of Chara spp. with a biomass density of about 350 g m−2. Animal food items were negligible: Macroinvertebrates, mainly Asellus aquaticus Linnaeus, that were associated with the macrophytes, made up only 2% of total biomass, and were very unevenly distributed.