Following catastrophic exploitation throughout the North Atlantic, breeding seabird populations have begun to recover thanks to regulatory protection and restoration and management efforts. As bird populations increase, new challenges emerge, including overgrowth of vegetation that limits the open nesting habitat favored by most tern species. Though managers have used a variety of measures to reduce vegetation cover, these techniques have rarely been quantified or compared experimentally.
During the summers of 2009 and 2010, I applied two different techniques, controlled burning and artificial weed barriers (muslin fabric and artificial turf) to experimental plots on Eastern Egg Rock and Outer Green Island, near-shore seabird nesting islands in mid-coastal Maine. I then monitored vegetation regrowth and use by nesting terns to assess the effectiveness of these techniques for opening and maintaining Common Tern nesting habitat during a full breeding season, comparing treated plots to vegetated control plots and existing tern nesting habitat. Burned areas did not remain open for the full nesting season, but regrew shortly after laying, leading to near-complete nest failure in these plots. Tern nest and fledging success was similar in weed barrier (1.37 chicks/pair) and untreated tern nesting habitat (1.38 chicks/pair) plots. Replacement of existing vegetation, tested at a limited scale on Outer Green Island, did not succeed.
These three techniques represent only a small fraction of vegetation management techniques used throughout the North Atlantic region. Through literature review and consultation with North Atlantic colony managers, I collected information on vegetation management on 34 tern nesting islands between 33 and 55° N latitude and developed a summary of different vegetation control techniques used. I identified 14 technique types suitable for use in nesting colonies: i.e., that can be applied before and after (but not during) the nesting period of May-July, that do not cause destructive impacts to the surrounding ecosystem, and that involve materials and labor that can be transported to inaccessible offshore islands. Of these techniques, 8 created usable tern nesting habitat for a full breeding season, and the most successful techniques required constructing habitat over existing vegetation. The success of different methods depended heavily on the plant communities and soil types involved. In general, vegetation management options were more limited and less successful for elevated, rocky islands than for low, sandy islands. Often, techniques that successfully removed one species or group of species (i.e., perennial grasses) failed due to rapid colonization by other species (i.e., herbaceous annuals). This review of past and ongoing vegetation management techniques used on seabird nesting islands, including their costs, methods for application, and effectiveness, provides seabird managers a reference when evaluating current and future vegetation management programs
