Seabird Colonies in Strait of Georgia IBAs

Important Bird Areas (IBAs), like all natural areas, are dynamic systems where changes in habitat conditions and other environmental factors can lead to shifts in bird abundance and distribution. While change is often caused by humans, natural fluctuations also occur. This ever-changing landscape creates interesting challenges when implementing conservation initiatives like the Important Bird Areas program, which designates specific sites of importance to birds based on the number of birds using that site.

One situation we are currently considering comes from the Strait of Georgia. Several IBAs designated for breeding colonies of Glaucous-winged Gull, Double-crested Cormorants and Pelagic Cormorants no longer support the species for which they were designated, or no longer support numbers that trigger IBA status. At the same time, new colonies, for cormorants at least, have emerged in other locations within the Strait of Georgia. Other colonial breeding species, like Great Blue Heron, are also known to shift their breeding location in response to changing conditions. As with the cormorants and gulls, herons no longer breed at several BC IBAs originally designated for their Great Blue Heron colonies.

Declines at colonial breeding sites can occur for a number of reasons, such as habitat alteration, increased disturbance or predation, redistribution to other sites where predation risk is less and/or food resources are greater, or a general population decline of that species. One of the authors of this article, Louise Blight, is completing her PhD at UBC studying declining Glaucous-winged Gull populations in the Strait of Georgia. As part of her research she visited gull breeding sites in 2009 and 2010 to census as many colonies as possible in the region. With the help of colleagues at Parks Canada and the BC Breeding Bird Atlas, she was able to survey 43 of the 76 colonies originally visited a quarter-century before by Canadian Wildlife Service scientists. Another monitoring program led by Trudy Chatwin with the BC Ministry of Environment is collecting valuable yearly data on cormorant breeding colonies in the Strait of Georgia. This data complements observations made by the Caretakers for IBAs in the Strait of Georgia.

According to Louise’s work, breeding Glaucous-winged Gulls in the Georgia Basin have dramatically decreased over the last 25 years. Since 1986, the year of the last Basin-wide surveys, no colonies have grown in size. In addition, Mitlenatch Island, one of the biggest colonies in British Columbia and an IBA, now supports only about 50% of its earlier gull population. Population declines near Nanaimo have been the most dramatic. In 1986, about 1600 pairs of Glaucous-winged gulls – 12% of Georgia Basin numbers, and over 2% of the national population – nested on islands in the approaches to Nanaimo Harbour. In 2010, only 32 nests were found at these sites. In this area, the Snake Island IBA was designated for its national significance to breeding Glaucous-winged gulls and Pelagic Cormorants. Louise’s surveys of the island, as well as those conducted by Caretaker Bill Merilees, detected very few nesting gulls in recent years and no nesting cormorants in 2010.

Trudy’s work shows similar declining trends for breeding cormorants in the Georgia Basin. Surveys of 34 Pelagic and 17 Double-crested Cormorant colonies in 2000 demonstrated that overall counts of Pelagic Cormorants were down by half, and Double-crested Cormorants had declined by two-thirds since 1987. While some of these declines may be due to redistribution, the main cause of decline appears to be Bald Eagle predation, and to a lesser degree, changes in prey availability and human disturbance. Interestingly, some cormorants have shifted their breeding sites to “artificial” habitats. For instance, two Pelagic Cormorant colonies have established under the Burrard and Granville Bridges in Vancouver, and Double-crested Cormorants are nesting on crane structures on Vancouver Island. Some gulls have also turned to urban nesting since about the mid-1900s, with colonies now occurring on rooftops in Vancouver, Victoria and Nanaimo.

For the IBA program, changes in breeding colonies present an important conservation and management challenge. If an area is set aside to protect significant concentrations of a species, but does not currently meet this goal, should the area’s IBA status be revoked, or is this a sign that restoration of site or species is long overdue? There are no simple answers.

In order to tackle this question we are seeking input from expert scientists like Louise and Trudy, and the National Technical Committee. It is very possible that an abandoned colony could be recolonised in the future provided the habitat and food supply remain suitable, or are restored. One idea being considered is clustering existing and former breeding sites, or sites with suitable breeding habitat for a certain species, into a single IBA composed of multiple small island components. This is an evolving concept, and our efforts in British Columbia are feeding into a larger-scale approach being developed by BirdLife International as part of a new global push to identify marine IBAs. Caretakers and local scientists can play an important role in this process by continuing to monitor birds during the breeding period so we can track how breeding habits are shifting. Our ultimate goal is to ensure that the IBA program is a strong and vibrant one that effectively identifies and conserves key sites for current and future bird use, in the face of natural and anthropogenic changes.