Tiendoornige stekelbaarsjes Pungitius pungitius als prooi van Zwarte Ruiters Tringa erythropus

Spotted Redshanks were observed in the province of Drenthe from 6-15 October 1995. The birds were foraging in dense, small flocks in pools on heathland in 20-45 cm deep fresh water. Principally, two methods of foraging were employed while swimming: (1) fast jabbing (>60 times/min) with slightly opened bill, and (2) fast up-ending (on average once per 1.1 sec, n=122 sec), the latter invariably in deeper water. The transparency of the water was poor because of the presence of a submerged layer of dead organic material, and it is thought that prey was caught by using tactile clues, rather than visually. Prey consisted of ten-spined sticklebacks of 2-5 cm length (estimated in comparison to bill length). This accords with sticklebacks caught with a net (n=29, length on average 4.5 cm, SD=0.5, range 3.5-S.3 cm). After having caught a stickleback, the bird usually swam towards shallow water or the shore, and started to "massage" the fish with fast movements of the lower mandible. Handling time (time between catching and swallowing) averaged 27.1 sec (n=71, SD=12.1, range 7-61 sec). Mean hunting success was 1.5 stickleback per group per minute (based on 31 foraging bouts, 553 minutes of foraging time and 741 captured sticklebacks), and on average 2.5 sticklebacks per bird per foraging bout (a foraging bout lasted on average 19 min, SD=13, range 4-54 min)(basic data in Appendix 1). Foraging took 41% of total observation time. It is estimated that a single Spotted Redshank captured 35 ten-spined sticklebacks per day, i.e. 84 g/day based on an average weight of 2.4 g/stickleback (n=8, range 1.6-3.0 g). Profitibility of a stickleback as prey of Spotted Redshanks was probably determined by its width (in 29 specimens caught with a net, on average 0.8 cm, SD=0.09, range 0.62-0.96 cm), rather than its length. Kleptoparasitic attempts were recorded three times, twice by a Black-headed Gull and once by a Herring Gull; none was successful.