There were about three-year cycles in the populations of arctic foxes, and the breeding productivities of brent geese and curlew sandpipers on the Taimyr Peninsula, Russia, The populations of arctic foxes and lemmings changed in synchrony. The breeding productivities of the birds tended to be good when the arctic foxes were increasing in numbers and poor when the arctic foxes were decreasing. There was a negative relationship between arctic fox numbers (or occupied lairs) and the breeding productivity of brent geese in the following year. Although there was evidence of wide-spread synchrony In the lemming cycle across the Taimyr Peninsula, some localities showed differences, However, such sites would still have been influenced by the general pattern of fox abundance in the typical tundra zone of the Taimyr Peninsula, where most of the arctic foxes breed and from which extensive movements of foxes occur after a decline in lemming numbers. The results support a prey-switching hypothesis (also known as the alternative prey hypothesis) whereby arctic foxes, and other predators, feed largely on lemmings when these are abundant or increasing, but switch to birds when the lemming population is small or declining. The relationships between arctic foxes, lemmings and brent geese may be further influenced by snowny owls which create fox-exclusion zones around their nests, thus providing safe nesting areas for the geese.