ABSTRACT
Seabirds comprise about 328 species in four orders, the Spenisciformes (penguins; 17 species in one family), Procellariiformes (albatrosses, shearwaters, petrels, diving petrels, storm-petrels; 125 species in four families, here termed petrels), Pelecaniformes (pelicans, tropicbirds, frigatebirds, gannets, and cormorants; 61 species in five families), and Charadriiformes (gulls, terns, skuas, skimmers, and auks; 128 species in four families: see Appendix 1 for a complete list of species). Seabirds range in size from the Least Storm-petrel (
Halocyptena microsoma
; body mass = 20 g) to the Emperor Penguin (
Aptenodytes forsteri
; body mass = 30 kg). They exploit a broad spectrum of marine habitats, from littoral to pelagic and from tropical to polar, breeding at higher latitudes and in colder environments than any other vertebrate on earth. The general characteristics of the different families of seabirds are summarized in Table 8.1 (Family Sternidae is included in the Family Laridae following Croxall et al. 1984, Nelson 1979, Croxall 1991). Seabirds can all be broadly categorized as long-lived species with delayed sexual maturation and breeding and low annual reproductive rates. Many species have a lifespan well in excess of 30 years with fewer than 10% of adults dying each year, and most do not commence breeding until age 3 years or older (over 10 years in some albatrosses; see Appendix 2). Most species lay only one to three eggs per clutch and in some cases rearing offspring takes so long (e.g., 380 days in Wandering Albatrosses,
Diomedea exulans
) that successful parents breed only every second year. These life history traits are adaptive evolutionary responses to conditions of living in the marine
and maritime environment, both at sea and on land. They have been generally assumed to reflect the patchy and unpredictable distribution of marine food resources, although there are additional explanations that have not received sufficient recognition (see Chapter 1 and conclusions below). Some confusion has arisen in the literature because of a failure to distinguish between life histories (comprising sets of evolved traits) and life-table variables such as age-specific fecundity and mortality (that indicate an individual’s performance and are the consequence of how life history traits interact with the environment; Charnov 1993, Ricklefs 2000). For instance, all petrels are constrained by their life history evolution to lay a single-egg clutch, no matter how favorable the environment (Figure 8.1). Some other species have the potential to lay larger clutches, with the number of eggs laid varying from one individual to another, and between years within individuals. Life history adaptations determine the potential limits to this variation within each population, whereas variation within individuals is better expressed in life-table variables.
