SPAANS B, LEOPOLD M & LOOS B (2021) Estimating the number of terns visiting a breeding colony of Sandwich Terns Thalasseus sandvicensis on Texel, the Netherlands . LIMOSA 94 (4): 137-145.

The small nature reserve Wagejot, near Oosterend on the island of Texel, harbours a large colony of Sandwich Terns. By daily scanning for individually marked birds (colour-ring with inscription) during the breeding season of 2020, we obtained 5366 sightings from 689 different individuals. We also determined the density of marked birds in the colony. On average, one marked bird was seen among 70 terns. The total number of birds we estimated had visited the colony in 2020 at least once, was 48 230 (70 x 689). Drone pictures were used to count the number of breeding pairs: 6449 pairs or 12898 individuals. We conclude that the colony was visited by large numbers of terns that do not breed at Wagejot, about 35 600 individuals (48 230 – 12 898). These visitors, which could be non-breeders or individuals breeding elsewhere, were seen throughout the whole 110-day long breeding season.
We argue that marked birds that were seen at least 10 times during the breeding season were likely local breeders, whereas marked terns that were observed less often than 10 times were most likely visitors. The estimated number of visitors to the colony should be considered a minimum. Especially as 243 marked birds were only seen once, it is likely that more colour-ringed individuals would have been seen with a larger observation effort.

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