The impact of macroalgal mats on the feeding distribution and feeding behaviour of mussel feeding birds on the Ythan estuary, Aberdeenshire
The thesis describes the feeding distribution and feeding behaviour of two species of mussel feeding birds, eider Somateria mollissima and oystercatcher Haematopus ostralegus on intertidal mussel beds on the Ythan estuary, Aberdeenshire, in relation to the distribution of green macroalgae during different phases of growth. Two hypotheses were tested; (1) The presence of algae deters birds from feeding in a place. (2) The presence of algae negatively affects the feeding behaviour of birds. Five different growth phases of algae were identified; (1) Early re-establishment (Jan - Feb), (2) Late re-establishment (Mar - Apr), (3) Accelerated (May - Jun), (4) Peak (Jul - Aug) and (5) Declining (Sep - Oct). The greatest area of bare mud with scattered algal areas occurred during the early and late re-establishment phases and imposed minimal physical interference on the birds' feeding. A mosaic of patches of bare mud and algal-covered areas, which increased habitat heterogeneity, occurred during the accelerated and declining phases and imposed a substantial interference on the birds' feeding. A uniform layer of thick mat of about 0.5 cm and up to 2.6 kg FW.m-2, covering more than 75% of the studied mussel beds, occurred during the peak phase and imposed maximal physical interference on the bird's feeding. The effects of algae on the feeding distribution and behaviour of mussel feeding birds were species specific. The first hypothesis was supported only during the re-establishment phase and was apparent only for eiders. In contrast, both the species appeared to be attracted by algae during the late phase of algal growth; eiders during the accelerated and declining phases and oystercatchers during the peak and declining phases. It was evident that the presence of algae over the mussel beds did not deter the mussel feeding birds absolutely from feeding in a place. The second hypothesis was supported only during the re-establishment phase and was apparent only for eiders.