Declines in Populations of Peregrine Falcons and Their Seabird Prey at Langara Island, British Columbia

Publisher Website
Google Scholar
Abstract
The resident population of peregrine falcons [Falco peregrinus pealei] at Langara Island, British Columbia [Canada] has diminished. Between 1968 and 1973 the population fluctuated between 5-6.5 territorial pairs. Biocide levels in addled eggs were substantial. Productivity was low; of the eggs laid, 15.6% vanished during incubation, 20.0% failed to hatch and at least 9.6% gave rise to young that died as nestlings. Compared to the 1950''s, the falcons now experience more nest failures and produce somewhat fewer nestlings and fledglings per successful pair and per occupied site. The productivity appears to be adequate to maintain (perhaps even increase) the population. The ancient murrelet [Synthliborhamphus antiquus], the principal prey species of the falcons, has declined greatly at Langara Island in the last 20 yr. This decline appears to be linked to the murrelet''s food supply. Either biocides have affected plankton (probably via the Davidson Current flowing north from California, USA), or an anomalous intrusion of warm water near the British Columbia coast for most of 1957-71 has reduced the supply of plankton. The murrelet decline is considered to be the immediate cause of the falcon decline. If the murrelet decline continues, the Langara Island falcon population probably will dwindle further and, perhaps, vanish.