Colonisation of Helmsand Island by the black-headed gull (Larus ridibundus) and its intrusion into the breeding grounds of the common tern and Arctic tern (Sterna hirundo and S. paradisaea)
DOI:
https://doi.org/10.21248/corax.v3.390Abstract
During the expansion of its range from 1958 onwards, the black-headed gull colonised the island of Helmsand/Meldorfer Bay.
After initially establishing colonies on the edge of existing tern colonies, settlements and thus mixed colonies were formed within tern colonies.
No notable conflicts between gulls and terns were observed, nor were there any attacks by black-headed gulls on the eggs or young of the terns.
It is possible that common terns and Arctic terns avoid forming larger mixed colonies with black-headed gulls by relocating their breeding sites. The black-headed gulls then colonise the vacated areas.
For ten years (1958-1967), the breeding population of common terns and Arctic terns averaged 290 pairs and did not decline during this period. In 1968 and 1969, it was still above the ten-year average at 300 and 315 pairs respectively, but by 1971 it had fallen to just under 60% (170 pairs) of this figure.
After a steady increase in population on Helmsand (from 1 pair in 1958 to approx. 600 pairs in 1971), the black-headed gull colonised other sites on the nearby (foreshore) coast from 1968 onwards.
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