Impact of ringed-billed gull Larus delawarensis control on common terns Sterna hirundo nesting on Ice Island, upper St. Lawrence River, Ontario, Canada
Blokpoel H., Tessier G.D. & Andress R.A. (1997) Successful restoration of the Ice Island common tern colony requires on-going control of ring-billed gulls. Colonial Waterbirds, 20, 98-101.
Background
In 1976, 121 pairs of common terns Sterna hirundo nested on Ice Island (44°27'N, 75°50'W), a small (0.17 ha) island in the upper St. Lawrence River, Ontario, Canada, but by 1989 the island had been completely taken over by ring-billed gulls Larus delawarensis. This study investigated the effectiveness of various measures to control gull numbers and encourage terns to recolonise the island.
Action
During April 1990–1993, an exclosure made from 40-lb (c.18 kg) test monofilament lines was installed over approximately 60% of the island. The perimeter line was attached to reinforcement rods embedded in concrete blocks, with parallel lines spaced at 70 cm intervals to deter gulls (but not terns) from landing. During 1990 and 1991, 8–10 wooden tern decoys were placed within the exclosure.
Following the installation of the exclosure, 4–5 further visits were made each season to count tern nests, monitor nesting success, and destroy gull nests and eggs.
During 1994–1996, intensive control of gull nests was not carried out (although nests were destroyed during the initial visit in both 1994 and 1996), and the exclosure was not installed.
Consequences
The exclosure was effective at deterring ring-billed gulls, with number nesting on the island dropping from 181 pairs during 1989 to zero in 1990 and 1991. In 1992, 17 gull nests were found following the early removal of the exclosure on 8 May (rather than in August, as in the preceding two years), but these were destroyed, and the exclosure re-installed, on 19 May. In 1993, eight gulls nested, but their nests were destroyed on 30 June (when half of the exclosure was removed).
Two pairs of common terns nested (outside the exclosure) in 1990, with the number of nests increasing to 135 (52% of which were inside the exclosure) by 1993. However, although tern numbers remained high in 1994 (141 pairs) and 1995 (149 pairs), they declined dramatically to just three pairs in 1996, coincident with the recovery of the ring-billed gull population (to 200–300 nesting pairs in 1996) following the end of intensive control measures.
Note: If using or referring to this published study, please read and quote the original paper, the abstract of which can be viewed at: http://www.jstor.org/stable/info/1521769.