winter, the head and nape is darker with a varied smudged or mottled pattern, and the bill colour becomes duller, often with dark markings near the tip. Young birds are brown or gray with black beaks, and take four years to reach adult plumage. Behaviour The glaucous-winged gull nests in the summer, and each pair produces two or three chicks which fledge at six weeks. It feeds along the coast, scavenging for dead or weak animals, fish, mussels and scraps. In urban areas it is well known for its tendency to accept food from people and peck open unprotected garbage bags in search of edibles. Its call is a low-pitched "kak-kak-kak" or "wow", or a more high-pitched wailing. Hybridization The Glaucous-winged Gull frequently hybridizes with western gull where their ranges overlap in Washington and Oregon. In the Puget Sound area, hybrids may outnumber pure birds and backcross with either parent species leading to further identification problems. Referred to as the Olympic gull, this hybrid is found along the Pacific coast from British Columbia south to California. Hybrids are variable and have characteristics of either parent species. Often they resemble Glaucous-winged gulls but have darker grey wingtips which strongly contrast from the mantle, and may have a heavier bill more typical of Western gulls. Hybrids also tend to exhibit dark head markings in nonbreeding plumage, compared to the virtually unstreaked white head of pure Western Gulls. The prevalence of 'Olympic gull' hybrids is an example of bounded hybrid superiority, where natural selection favours hybrids in areas of intermediate habitat. One study found that females paired with hybrid males have higher breeding success than pairs of the same species. In the central part of the hybrid zone, clutch size was larger among pairs with hybrid males, many of which established breeding grounds in more vegetative cover than pure western gull males, which preferred sand habitat resulting in heavier predation. In the northern section of the hybrid zone, there was no difference in clutch size, but breeding success is higher due to the hybrids being more similar to western gulls in foraging behaviour, feeding more on fish than glaucous-winged gulls. Little evidence of assortative mating was observed, except for weak assortative mating among hybrids in absence of mixed species pairs. This species also hybridizes regularly with the American herring gull in southern Alaska, sometimes referred to as the Cook Inlet Gull. This hybrid