Experimental measurement of nesting substrate preference in Caspian terns, Sterna caspia, and the successful colonisation of human constructed islands

Caspian terns, Sterna caspia, recently bred in Hamilton Harbour, at the western end of Lake Ontario, on private property that is likely to be developed in the next decade. To reduce this land-use conflict and to promote the current level of biodiversity of colonial nesters in the area, artificial islands were built in the winter of 1995–1996 with different areas designated for a variety of nesting waterbirds including Caspian terns. In 1994, prior to island construction, we tested three substrate types for tern nesting preferences so that an appropriate substrate could be placed on the Caspian tern designated portion of the new islands. We found a preference for sand over pea-gravel and crushed stone, and indirect evidence for a preference favouring the experimental substrates over the pre-existing substrate of hard-packed ground. Based on these results, the small area of the island designed for Caspian tern nesting was surfaced with sand and was subsequently colonised successfully. The colony established and reproduced successfully on the designated site in 1996 and grew in numbers of nesting pairs in 1997.