Rhinoceros Auklet (Cerorhinca monocerata) Burrow Counts, Burrow Density, Occupancy Rates, and Associated Habitat Variables on Protection Island, Washington: 2008 Research Progress Report

Unlike many species of seabirds that use the Salish Sea (Puget Sound, Georgia Basin and Strait of Juan de Fuca) during migration or for over-wintering, rhinoceros auklets (Cerorhinca monocerata) depend on this region for reproduction. As a result, population trends are more likely to be tied to events occurring locally. In addition, as a top-level piscivorous predator, the species is particularly susceptible to fluctuations in forage fish populations. These characteristics make the rhinoceros auklet an ideal candidate for assessing the health of the Salish Sea ecosystem. An additional advantage to using this species is the availability of historic data from the 1970s for assessing population trends. Finally, greater than 95% of the North American population of the rhinoceros auklet occurs in Washington, British Columbia, and southeast Alaska. Nearly all of these birds breed on eight large colonies, of which Protection Island in Washington is one of the largest. The importance of this island to the species as a whole inspired us to assess changes in its population.

Using a stratified random sampling scheme, we estimated that there are 54,113 ± 9,390 (95% CI) burrows on Protection Island. This estimate is approximately 51% greater than any of the previous estimates. Using infra-red camera probes to assess burrow occupancy, we estimated occupancy to be 66% ± 5% (95%CI) which is very similar to previous occupancy estimates. Using these two values, we estimate that there were 71,430 ± 13,514 (95%CI) birds breeding on the island in 2008. These results make Protection Island the third largest rhinoceros auklet nesting colony in North America. In general, there was a positive correlation between burrow density and percentage of unvegetated area (bare ground) and percent slope. There is a negative correlation with perennial grasses, shrubs, and forbs. These results confirm the importance of steep, grass-dominated slopes located relatively close to the water for auklet nesting.

The concentration of such a large portion of the North American rhinoceros auklet population on Protection Island suggests that population trends and success on this island has significant implications to the species. Our results suggest that the Salish Sea rhinoceros auklet population is healthy; however apparent methodological differences with past estimates lead us to caution against directly comparing our results to those previously reported. Management actions should focus on maintaining suitable nesting habitat and address issues that inhibit successful nesting. Finally, we provide detailed information on a statistically robust and relatively inexpensive sampling scheme that will allow future and comparable estimates for assessing population trends.

Future goals include providing similar estimates and measurements of the same habitat variables on Smith Island in Puget Sound and Destruction Island on the outer Washington coast. Once these surveys are complete, we will submit to a peer-reviewed journal a manuscript examining changes in colony size between the 1970s and today for Protection and Destruction islands and will compare population changes between Sound (Salish Sea) and the outer coast. Such comparisons are critical to determining if events occurring in the Puget Sound are unique to the Sound or are part of larger-scale phenomena. Finally, the development of a relatively sophisticated habitat model will allow us to identify habitat features important to Rhinoceros Auklets when they select locations for their burrows. These types of analyses are critical to helping land managers understand how to restore and manage existing habitat, especially in the face of invasion by non-native grasses (e.g., Ammophila spp. and Bromis techtorum) and animals (e.g., European rabbit Oryctolagus cuniculus) that these islands have experienced.