SUCCESS AND FAILURE OF RING-BILLED GULL DETERRENTS AT COMMON TERN ANDPIPING PLOVER COLONIES IN MINNESOTA

Increasing populations of Ring-billed Gulls (Larus delawarensis) in th e Great Lakes region and early arrival of gulls at breeding sites resu lts in gradual usurpation of Common Tern (Sterna hirundo) colony sites were the two species are sympatric. Ring-billed Gulls also may compet e for breeding space with Piping plovers (Charadrius melodus). We test ed several gull deterrent designs consisting of elevated lines of nylo n string, wire, or monofilament at the traditional Common Tern and Pip ing Plover breeding sites in Minnesota to determine whether Ring-bille d Gulls could be excluded from the sites while minimizing entanglement s of other species. Our results were mixed. Use of brightly colored ny lon strings was very effective at preventing Ring-billed Gulls from oc cupying their breeding sites at small or new colonies, but not at larg e, dense colonies having a prior history of successful breeding. Visib le wires were ineffective at deterring gull use at a large colony site . Monofilament was an effective Ring-billed Gull deterrent at a small colony site although it was slightly less effective than nylon string. However, where Caspian Terns (S. caspia) were also present, they were quite vulnerable to entanglement in monofilament. No birds became ent angled in wires and relatively few were entangled in nylon strings spa ced at least 1.2 m apart. No piping Plovers became entangled. Common T erns and Piping Plovers were not visibly deterred from nesting and chi ck rearing by the elevated lines. Of the three materials tested, we be lieve that brightly colored nylon string shows the most promise as a R ing-billed Gull deterrent, particularly at smaller colonies and/or whe re potential bird entanglements are a concern.