It seems only reasonable to assume that populations of marine birds fluctuate even when not disturbed by man; such fluctuations would result both from the secondary effects of species adaptive tactics and from changes in the marine environment. I briefly review some human activities and some other natural processes that have resulted in changes in numbers and distribution of seabirds and present a short discussion of theoretical models which emphasizes that conclusions drawn or predictions made from models of the dynamics of populations depend upon the assumptions about stability that were used in preparing the models. I then review those special characteristics of seabirds which are directly relevant to planning programs intended to protect seabirds or encourage their increase and identify several goals for improving our understanding of the population dynamics and biology of marine birds. My general conclusion is that enough is already known to undertake effective conservation programs, and that time is pressing.
Seabirds have been categorized as renewable resources in only a few places, although their symbolic value has been recognized for centuries (for example, the medieval poem "The Seafarer" and the designs on Saint Cuthbert's tunic). With the exception of the Russians (Belopol'skii 1961; Uspenski 1956), the Australians (Serventy 1967), and the Icelanders, industrialized peoples have not considered seabirds to be salable and therefore worth managing. Yet during many centuries the seabirds of the northern seas were a major food for coastal and island villages (Bent 1919, 1921, 1922; Fisher and Lockley 1954).
Some biological principles that affect the dynamics of seabird populations are identified in this paper. I believe these principles must form the basis of plans to maintain and increase seabird numbers.
I describe some observations of population changes, review briefly the conflicting theoretical frameworks for population dynamics, and identify some of the biological characteristics of marine birds that affect the way in which population changes occur. The terms "seabirds" and "marine birds" are used interchangeably for those bird species which depend upon salt water for some part of their annual cycle (c.f., the Pacific Seabird Group).
Population Fluctuations
Broadly stated, the populations of northern seabirds have shown marked short-and long-term fluctuations. Most authors have assumed that all such fluctuations reflect human disturbance of the natural system, because of the obvious effects of human predation during the last 200 years.
Human Impact
In the centuries before people traveled extensively between islands, seabirds were taken in ways that we judge must have allowed the survival of the colonies (e.g., those at the Faroes or Saint Kilda, those in Iceland and Greenland, or those in the Aleutian Islands and the Bering Strait). We presume either that the populations of island peoples were regulated by shortage of resources other than seabirds or that those who overcropped and eliminated the seabirds suffered the consequences.
Negative Effects
When a sea-going, commodity-oriented way of life evolved, seabirds were killed in huge numbers for such uses as the plumage trade, fish bait, or rendering into oil (Tuck 1960; Fisher and Lockley 1954). Even the elimination of several colonies—e.g., Funk Island, Newfoundland (Tuck 1960); Seal Island, Eastern Egg Rock, Maine (Norton 1921); Muskeget, Massachusetts (Forbush 1929)—may have had little effect on the rate of cropping because those who killed off one source could probably seek out another. As the colonial seabirds became scarce they became more valuable, which stimulated more intensive pursuit of the remnants (Dutcher 1901, 1904).