Faecal samples reveal the diet of breeding adult Sandwich terns Thalasseus sandvicensis in Belgium and the southern part of the Netherlands

- A detailed protocol for the analysis of faecal samples is presented.
- The temporal and spatial variability of the diet of adult Sandwich terns is described.
- Sandwich terns are very specialistic, both in prey species and prey size.
- The adult diet proves to be a good indicator for the availability of small pelagic fish species.

The diet of terns is often considered indicative for the availability of prey or shifts in prey communities. So far, most studies focused on the chick diet. Here we study the diet of breeding adult Sandwich terns based on faecal samples collected in five different colonies in Belgium and the Netherlands in 2007-2015. Breeding adult Sandwich terns in the study area proved to be very specialistic with only three prey types frequently found in their diet. Clupeidae were the most abundant fraction by number, making up 40 to 70% of the diet in most years. The clupeid fraction almost exclusively consisted of relatively small (< 95 mm) Herring Clupea harengus and Sprat Sprattus sprattus. Ammodytidae accounted for another 20 to 60% of the diet composition, 80-140 mm being the most common length class. In all years jaws of Nereididae were found in the diet with up to 57% of the total diet consisting of Nereis-worms. We show that there is a clear link between the adult diet of Sandwich terns and the ecology of the involved prey species. Also, differences in both species composition and mean length of the prey species between colonies in the same year were much smaller than the differences between years. We therefore argue that the diet of adult Sandwich terns might prove a cheap and good indicator for the availability of small pelagic fish species.