Like knights in ancient castles, some birds avoid predators by surrounding themselves with water. Common eiders, arctic terns, and glaucous gulls place their nests on small, coastal islands accessible only by swimming or flying. Because these islands are small, eider nests are often packed very close to one another in order to fit. One drawback of these islands is that they are only safe havens once the sea ice has melted. Eiders and gulls must therefore wait to breed until the weather has warmed up and predators can no longer reach their nests. In cold years, spring breakup may arrive too late for the birds to breed at all.