VERKUIL YI, HOLTING M, BOTH C, DREEF C, WEEDA S & VAN DER WINDEN J (2023) Diet of Little Terns Sternula albifrons breeding in the freshwater archipelago Marker Wadden determined with DNA metabarcoding. LIMOSA 96 (3): 99-109.

In the Netherlands, Little Terns breed almost exclusively in the Wadden Sea and the southwestern Dutch Delta, in pioneer habitats with sparse or no vegetation. With the construction of the man-made archipelago Marker Wadden in the freshwater lake Markermeer, inland freshwater habitat became available again for breeding terns. Since 2017, a year after the first levees were built, Little Terns have successfully raised chicks on Marker Wadden's sand dikes and sand plains every year. It is unknown which food resources Little Terns exploit in this freshwater ecosystem. In this paper we describe the diets of three foraging adults in June and seven chicks in July and August, by DNA-metabarcoding their faeces. Since the DNA-metabarcoding method is under development, we explore whether fish species that occur around Marker Wadden and Markermeer can be detected in DNA extracted from faeces using either COI or 18S as barcoding gene. We discuss the performance of the two barcodes and make recommendations for future DNA-metabarcoding of tern diet. Our specific interest was whether the diet would reflect the species community of the newly created shallow waters of Marker Wadden. We also questioned whether the Little Terns perhaps eat prey other than fish. Indeed, all faecal samples contained both fish and invertebrate DNA.
Invertebrates were predominantly insects (mainly Chironomidae) and crustacean (Hexanauplia sp.). Chick faeces also commonly contained arachnids (both feather mites and true spiders) and molluscs (land lung snails). Fish species detected in the faeces were in decreasing importance, Common Roach Rutilus rutilus, European Perch Perca fluviatilis, Ide Leuciscus idus, European Smelt Osmerus eperlanus, Zander Sander lucioperca, Round Goby Neogobius melanostomus, Ruffe Gymnocephalus cernua, Carp Cyprinus carpio and Common Bleak Alburnus alburnus. Common Bream Abramis brama was only detected by 18S, but since species assignments turned out to be reliable for COI but not 18S, it remains uncertain whether bream was part of the diet. Adult faeces (collected in early summer) contained roaches, perches and carps. The variety of fish species in chick faeces from July and August was larger with nine species. The diet showed similarities with fish communities present in shallow waters of the archipelago. This suggests that the shallows of Marker Wadden contribute to the diet of breeding Little Terns.

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