Status of the Peregrine Falcon, Falco peregrinus pealei, on Langara Island, Queen Charlotte Islands, British Columbia, 1968-1989

Abstract

From 1968 to 1989, Langara Island, British Columbia, held 5-7 territorial pairs of Peregrine Falcons (Falco peregrinus pealei). Annual production from the island varied greatly. In 1968-1979, it averaged 9.1 nestlings per year (range 5-13), and in the 1980s it averaged 14.1 (range 8-20). Disappearance (presumed mortality) of known territorial birds averaged 26% per year for males and 37% per year for females, higher than in five other studies of turnover in breeding Peregrines. Levels of DDE in falcon eggs in 1968-1972 were somewhat below the level that results in population decline. By 1986 the DDE had dropped to less than half of its former level. During the same period the PCB in eggs remained unchanged at moderate levels. The Langara Island nesting population of the Ancient Murrelet (Synthliboramphus antiquus), primary prey of the falcons, declined from very large numbers (probably well over 250 000 pairs) in the 1950s and earlier, to 80 000-90 000 pairs in 1971, to 22 000-24 000 pairs in 1981 and 1988 surveys. Several possible causes of the murrelet decline are outlined, including changes in the ocean and predation by introduced rats. Between 1957 and 1968 the falcon population declined from 20+ pairs to 5 pairs and then remained at 5-7 pairs through the 1970s and 1980s despite the further decline in the murrelet population. Although the number of pairs of falcons in earlier years appeared closely tied to the number of pairs of murrelets nesting on the island, from about 1968 onwards the Langara Island falcons apparently "relied" not on locally nesting murrelets but on murrelets commuting from distant nesting colonies. This falcon population in 1968-1989 was stable and reproductively healthy. A proposed rat eradication program might result in the recovery of the Langara Island murrelet population, and the falcon population, toward the very large numbers reported in the 1950s and earlier.