Heavy Metal Levels in Gulls
Gulls have often been used as bioindicators of environmental conditions and contaminant expo sure because they are colonial, abundant, and have diverse foods and feeding methods. Some spe cies have a wide distribution, allowing broad geographic comparisons. Many also exhibit colony site stability; samples can be collected over many years, providing temporal trends in reproductive success, colony numbers, and contaminant levels. As they are abundant and usually lay three eggs (but often raise fewer than three young), collecting an egg from a clutch or a hatchling from a brood does not greatly affect reproductive success.