Recently some mass mortalities have been associated with specific causes. Bailey and Davenport (1972) reported that starvation caused the die-off of common murres in the southern Bering Sea—Bristol Bay area. Foul weather, which apparently inhibited feeding between 19 and 23 April 1970, culminated in an intense storm. Similarly in late winter 1969 bad weather in the Irish Sea, combined with strains of molt and perhaps contamination with industrial chemicals, seems to have contributed to mass mortality of the same species (called common guillemot in Britain; Holdgate 1971). The seabird victims of this event had metabolized their body fat and as a result, polychlorinated biphenyls (PCB) and other industrial chemicals passed into livers, kidneys, and brains. Again, a storm at the end of a period of stress seems to have been more than the birds could tolerate.
A further example of a die-off of waterfowl apparently brought on by starvation was given by Barry (1968), who estimated that about 100,000 king eiders (Somateria spectabilis) died when they arrived before the ice broke up in the Beaufort Sea in spring 1964.
Diseases have produced massive die-offs in marine birds. Fowl cholera caused high mortality in nesting common eiders (Somateria mollissima) in the Gulf of St. Lawrence in Quebec (Reed and Cousineau 1967) and in Penobscot Bay, Maine, in the early 1960's (H. Mendall, personal communication). Poisoning from a "red tide" (a bloom of the dinoflagellate Gonyaulax tamerensis) caused a die-off of black ducks (Anas rubripes) and herring gulls on the coast of New England in 1972. Similarly a die-off of shags (Phalacrocorax aristotelis) on the east coast of England was caused by a "red tide" (Coulson et al. 1968). During a period of 1 week 90% of the shag nests on the Farne Islands in Northumberland were deserted and about 80% of the breeding population died.
Gradual Declines
When the new volcanic island of Bogoslov emerged in the western Aleutians, Preble and McAtee (1923) reported that it was colonized by large numbers of pigeon guillemots (Cepphus columba), but in the following decades the guillemots have steadily decreased (G. J. Divoky, personal communication). As a further example, the nesting population of Atlantic puffins (Fratercula arctica) in the Atlantic has declined over the past several years, especially those nesting on the Outer Hebrides (Flegg 1972; Harris 1976).
It is difficult to find seabird species whose nesting grounds have not been affected by humans but whose numbers have been censused. The best illustrations of secular changes in relatively constant habitats are probably those available in the British Trust for Ornithology's breeding censuses of songbirds. Songbirds are short-lived and their populations change on relatively short time scales. The northwestern European landscape has remained relatively constant for the last 75 years, yet there are observable decade-long trends—for example, of willow warblers (Phylloscopus trochilus) and dunnock (Prunella modularis). There are detailed data on population changes in great tits (Parus major) through the work of Kluyver (1951), Lack (1964), and Perrins (1965).
Effects Reflecting Environmental Change
Nelson (1966) argued that the increase of gannets in the North Atlantic during this century has been related to increasing temperatures rather than (as usually ascribed) to increased food from fish damaged or escaped during commercial fishing.
Ainley and Lewis (1974) described a particularly interesting example of the effects of environmental change on seabird populations. The events begin with the decrease of seabirds on the Farallon Islands off California as a result of human depredations. Even after fowling was made illegal, the populations of murres, double-crested cormorants (Phalacrocorax auritus), and especially of tufted puffins (Lunda cirrhata) and pigeon guillemots continued to decline as a result of oil pollution. During the last 3 decades the smaller species of seabirds nesting on the Farallons, such as rhinoceros auklets (Cerorhinca monocerata), have increased rapidly and the authors suggest that their increase was abetted by an increase in the small prey fish, northern anchovy (Engraulis mordax). One of course expects predators to be affected by changes in the abundance of their prey. During this same period, larger species of seabirds such as double-crested cormorants and tufted puffins have failed to recover their numbers, and the authors speculate that this failure is related to a decrease of the larger prey fish, Pacific sardine (Sardinops caerulea).
A widely publicized impact of environmental fluctuation upon seabird populations is that of the northeast wind, El Niño, off the Peruvian coast. This wind pushes the upwelling Humboldt Current water offshore and causes mass mortality in the Peruvian anchovies (Engraulis ringens) and, as a consequence, a die-off among the millions of seabirds such as Peruvian guanay cormorants (Phalacrocorax bougainvillii) and Peruvian boobies or piquero (Sula variegata) which feed upon them (Murphy 1936).