Evaluating Wader Habitats in Roebuck Bay (North-western Australia) as a Springboard for Northbound Migration in Waders, With a Focus on Great Knots

Abstract

We studied the diet and foraging behaviour of the Great Knot Calidris tenuirostris and to lesser extent of the Red Knot Calidris canutus rogersi and other waders at Roebuck Bay, Broome, Western Australia, in March-May 1991, the period of northward migration. Visual feeding observations, benthic fauna sampling and faecal analysis showed that the diet of Great Knots consisted mainly of bivalves. The total zoybenthic biomass in Roebuck Bay averaged 13.9 g ash free dry mass (AFDM) m^-2, of which 5.7 and 6.0 g AFDM m^-2 were available (sensu Zwarts et al. 1992) to Red Knots and Great Knots, respectively. The profitabilities of several possible prey types are discussed and compared with the recorded diet. Intake rates of Great Knots were estimated on the basis of recorded prey intake (0.24 mg AFDM s^-1) and defaecation rates (0.33 mg AFDM s^-1), and used to calculate mass gains prior to northward migration. The repercussions of the large tidal range on foraging possibilities are discussed. Changes in tidal regime may mean that Great Knots do not always have access to their preferred feeding sites. We indicate that the array of prey types available to, and taken by, Great Knots is much greater in Roebuck Bay than in the Wadden Sea. We discuss the repercussions of diet diversity on wader distribution. When compared with other springboard areas such as the Banc d'Arguin in Mauritania and the Wadden Sea in western Europe, the available Roebuck Bay benthic food stocks are larger.