Negative Interspecies Interactions in a Glaucous-winged Gull Colony on Protection Island, Washington

Protection Island, Washington, is one of the most important nesting sites for Glaucous-winged Gulls (Larus glaucescens) in the Puget Sound area. Changes in the numbers and distribution of nests in a Glaucous-winged Gull colony on Violet Point, Protection Island, were tracked from 1980 to 2010. The colony grew steadily until the early 1990s, then declined to about half its former level. The main nesting sites also shifted from the relatively vegetated central and eastern portions of the point to the sparsely vegetated land near the marina, which had previously been almost completely unused for nesting. This shift was correlated with steady expansion of tall grasses on the central and eastern point. The expansion of the tall grass appears to be displacing the gulls, which is an unusual situation. Further, in the early years of this study the edges of the tall grass areas were prime nesting habitats, but these locations were mostly avoided by 2010. We suggest that predation by Bald Eagles (Haliaeetus leucocephalus) may also be affecting the numbers and locations of gull nests and may be the prime reason that few gulls now nest in the edges of the tall grass areas. Together, the tall grass and eagle predation appear to be carrying out a pincer movement which may be forcing the gulls into suboptimal nesting habitat and driving the decline