Formerly one of the world's largest colonies, the population of black-legged kittiwakes (Rissa tridactyla) on Middleton Island has declined about 80% since 1981. Breeding has been a total or near-total failure in 14 of the last 16 years (1983-1998). The observed rate of population decline is equal to the rate of removal of breeding adults through overwinter mortality (Hatch et al. 1993; Ibis 135:247-254), i.e., this colony has experienced little or no recruitment for the better part of two decades. The situation on Middleton is a particularly severe example of poor breeding performance in kittiwakes, but many other colonies in Alaska have exhibited low productivity since at least the mid 1970s. Circumstantial evidence points to an inadequate food supply as the cause of poor productivity. In contrast to kittiwakes, glaucous-winged gulls have increased exponentially on Middleton, from fewer than 1,000 birds in the late 1970s to more than 15,000 by the early 1990s.

Gulls are the principal predators on the eggs and young of other species, including kittiwakes. It is difficult to argue conclusively that food supply is the ultimate determinant of kittiwake productivity when a population is faced with intense predation pressure such as currently occurs on Middleton.

Other species also show definite trends on Middleton. Common murres (Uria aalge) have declined markedly in the last 10 years, whereas rhinoceros auklets (Cerorhinca monocerata) have increased dramatically since the late 1970s. To account for contrasting population trends in different species, it is important that seabird monitoring include measures of three parameters--populations, productivity, and survival--not just the first, or first and second of these, as is typically the case (Baille 1990; Ibis 132:151-166).

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