Diet and prey size preference in Razorbills Alca torda breeding at Stora Karlsö, Sweden

Abstract

The Razorbill Alca torda is one of three auk species breeding in Sweden, often occurring in colonies with Common Guillemot Uria aalge. The largest colony in the Baltic Sea is situated on the island Stora Karlsö west of Gotland. During the chick-rearing phase, Razorbills forage at sea and return to feed the young with single or multiple prey carried sideways in the bill, unlike Common Guillemots that carry a single fish in the middle of the bill. Clupeids, especially sprat Sprattus sprattus, are the main prey of Baltic Sea Razorbills, but studies are scarce. Here, we investigate the diet preference of Razorbills at Stora Karlsö with regard to prey taxonomy and size, analysing photographs and applying morphometric equations. We show that most feeding attempts involved a single clupeid (88% of known fish taxonomy), in most cases likely sprat. The average prey size (115 mm) and prey species was similar to that of Common Guillemots in the same colony. The variation in size was much smaller in the Razorbill diet than the size distribution of clupeids at sea, indicating a strong prey size selectivity. The Razorbills’ high specialisation underlines the importance of maintaining strong populations of clupeids in their foraging area, as these fish are also targeted by industrial fisheries.

First data on the diet of Razorbill Alca torda wintering in the Mediterranean Sea: insights from social media

“…(e.g., Atlantic Ocean), probably due to differences in dietary habits during winter. However, since Razorbills are known to usually target prey with an average body length of 115 mm (Engvall et al, 2022) and generally smaller than 250 mm (Bradstreet & Brown, 1985), the authors suggested that some fish species of that size, such as Sardina pilchardus, present both in the Atlantic and in the Mediterranean, could constitute potential Razorbill prey in the Mediterranean as well. The Razorbill is a specialized but highly flexible top predator capable of adapting rapidly to climate change and able to track changes of preferred prey even well outside the boundary of their normal range (e.g., Gaston & Woo, 2008), and can shift dietary trophic levels under certain circumstances (e.g., Glew et al, 2019).…”