Population status and patterns of distribution and productivity of kittiwakes on St. George Island, Alaska

Abstract

I studied populations, distributions, and reproductive performance of red-legged and black-legged kittiwakes on St. George Island in the summers of 1993-1995, where populations of both species have experienced generally poor reproductive performance and population declines of ca. 40% over the past 20 years. In 1995, I conducted a whole-island census of kittiwakes on St. George Island and found estimated breeding populations of 193,930 red-legged kittiwakes (81% of their global population), and 62,568 black-legged kittiwakes. In addition, I analyzed census trends on 51 land-based census plots on St. George Island and found that numbers of both species have stabilized in recent years. I experimentally evaluated the hypothesis that nesting red-legged kittiwakes on St. George Island are competitively displaced by larger-bodied black-legged kittiwakes to narrower rock ledges and higher elevations. I determined nest-site preferences of both species by attaching narrow and wide artificial nesting ledges within high-and low-elevation areas of St. George Island and found no evidence of competitive displacement: red-legged kittiwakes preferred narrow ledges, black-legged kittiwakes preferred wide ledges, and both species preferred ledges in areas where conspecifics nested at high density. Multiple regression analyses suggested that kittiwakes breed earlier and more successfully in summers preceded by cold winters and that inter-annual variability in kittiwake breeding success was unrelated to weather conditions during the breeding season itself. These results suggest that winter weather has indirect effects on breeding kittiwakes by influencing prey abundance several months later. Furthermore, strong winds impaired growth rates of kittiwake chicks in exposed nest sites and the growth of black-legged kittiwake chicks relative to red-legged kittiwake chicks. I identified two prominent patterns of within-colony spatial variability in kittiwake productivity and suggest that patchy "bird quality" or localized "information neighborhoods" may be responsible because traditional explanations do not apply. The "information neighborhood" is a new hypothesis which proposes that individuals are influenced by the breeding status of neighbors because their status represents an additional source of information about current breeding conditions that can be used to better tailor parental investment.