115 Seabird Demography and Its Relationship with the Marine Environment Henri Weimerskirch CONTENTS 5.1 Demography and Life History Strategies 115 5.2 Seabirds and Other Birds 117 5.3 Demographic Parameters of Seabirds 118 5.4 Comparing the Demography of the Four Orders of Seabirds 120 5.5 Factors Responsible for Differences in Demographic Tactics 124 5.6 Intraspecific Variations in Demographic Traits 125 5.7 Population Regulation and Environmental Variability 129 5.8 Perspectives 131 Acknowledgments 132 Literature Cited 132 5.1 DEMOGRAPHY AND LIFE HISTORY STRATEGIES Demography is the study of the size and structure of populations and of the process of replacing individuals constituting the population. The study of demography was developed to forecast pop- ulation growth. The rate at which a population increases or decreases depends basically on the fecundity (number of eggs laid) and survivorship of the individuals that belong to the population (Figure 5.1, bottom), but also to a lesser extent (especially for seabirds) on migration. Because many organisms, and especially seabirds, breed several times in their lives, a population consists of cohorts of individuals of different ages, born in different years. Moreover, mortality and fecundity rates are generally age-specific; life tables represent these birth and death probabilities. The rela- tionship between the rate of increase or decrease and demographic parameters can be translated into more or less complex equations. The basic equation is the Euler–Lotka equation (Euler 1760, Lotka 1907) that specifies the relationships of age at maturity, age at last reproduction, probability of survival to age classes, and number of offspring produced for each age class, to the rate of growth of the population (r). The demography of organisms is a key to the evolution of life histories because it allows us to examine the strength of selection on life history traits. Although they can achieve similar population growth rates, i.e., being stable, increasing, or declining, each population living in a particular habitat has specific dynamics, with specific age-related survivals and fecundities. The particular values of the demographic traits depend upon the adaptation of individuals and the attributes of the environment in which they live. Therefore, comparing demographic traits of populations allows us to elucidate the ecological and evolutionary responses of populations to their 5 © 2002 by CRC Press LLC 116 Biology of Marine Birds environments. The comparison of demographic traits among taxa shows that demographic “tactics” exist; the concept of demographic tactic describes a complex co-adaptation of demographic param- eters (Stearns 1976). Basically these co-adaptations result in the existence of a gradient from taxa with high fecundity and a low survival, to species with a high survival and a low fecundity. This fast–slow gradient (fast meaning fast turnover, and slow, slow turnover) or r/K gradient (Pianka 1970) provides a convenient (although not perfect) summary of the patterns linking life histories and habitats. However, caution must be taken when life histories are compared. First it is possible to compare taxa from an ecological point of view as long as the allometric relationship linking them at a higher taxonomic level is known (Clutton-Brock and Harvey 1979). For example, within a taxa (a genus, for example), individuals of a particular species may live longer or produce fewer offspring than another species, not because they rely on a different habitat, but only because they are larger. Because they are larger they have a lower metabolism and therefore could live longer; they may produce fewer offspring because their offspring are larger and therefore require more energy (Calder 1984). The second constraint is phylogenetic (Harvey and Pagel 1991). Species are prisoners of their evolutionary past and can evolve to only a limited number of options. The single egg clutch of all Procellariiformes, and many other seabird species, has often been taken as an example for this (Stearns 1992). The life histories and habitats of two albatrosses can reasonably be compared, but care has to be taken when an albatross is compared to a species belonging to a different order. Phylogeny sets limits on an organism’s life history and habitat but the ecological task of relating life histories to habitats is a fundamental challenge in ecology (Begon et al. 1996). Comparing demographic tactics within taxonomic levels that are closely related (ideally within the same species, see Lack 1947) to habitats or ecology remains a powerful tool to understand the influence of the environment on the evolution of life histories (Figure 5.1). The aim of this chapter is first to describe the demographic traits of seabirds and compare these traits between taxa to examine whether demographic tactics can be found between and within the four orders of seabirds. Second, the variation in demographic traits will be examined to see whether it can be related to differences in the marine environment or the way seabirds FIGURE 5.1 Schematic representation of the relationships between demographic traits and the marine envi- ronment. © 2002 by CRC Press LLC Seabird Demography and Its Relationship with the Marine Environment 117 exploit it, when comparing species within the same order, but also by comparing populations within the same species. 5.2 SEABIRDS AND OTHER BIRDS In this study, a seabird is considered the species breeding along the seashore and relying on marine resources during the breeding season. Therefore several species of Pelecanidae, Laridae, Sternidae, and Phlacrocoracidae breeding inland or relying on freshwater resources are excluded, although they often winter in marine habitats. The data set used here includes 177 species of seabirds, with information on fecundity for 103 species, on age at first breeding for 111 species, and on survival/life expectancy for 76 species. All three parameters were simultaneously available for 62 species, and fecundity and age at first breeding for 84 species. Data were taken from Cramp (1978), Jouventin and Mougin (1981), Cramp and Simmons (1983), Marchant and Higgins (1990), Del Hoyo et al. (1992, 1996), Gaston and Jones (1998), unpublished data from a long-term data base for southern seabirds (CEBC-IFRTP), and unpublished data provided by E. A. Schreiber for tropical Pelecaniformes. When compared with other birds, seabirds have lower fecundity; they breed at an older age and have higher adult survival. Since age at first breeding, survival, and to a lesser extent clutch size, are explained in part by mass (relationship between log body mass and log of demographic parameters: clutch y = –0.081x – 1.33, r 2 = 0.028, p < 0.01, n = 362 species of seabirds and other birds; age at first breeding y = 0.215x – 0.545, r 2 = 0.313, p < 0.001, n = 261, survival y = 0.249x + 0.4859, r 2 = 0.394, n = 127, p < 0.001), it is important to remove the effect of size. Indeed it could be argued that on average, seabirds are larger than land birds. To remove the variation of demographic traits related to body mass, they were transformed as log (parameter) – 0.25 log (mass) (Stearns 1983, Gaillard et al. 1989). Once the effect of body mass has been removed for clutch size and age at first breeding, seabirds still appear to stand at the extreme slow end of the fast–slow gradient that exists for bird species (Figure 5.2), underlying the low reproductive rate of seabirds. FIGURE 5.2 Relationship between the clutch size and minimum age at first breeding (both corrected for body mass) in 175 species of seabirds (black dots, y = 0.571x + 0.72, r 2 = 0.541, p < 0.001) and 187 species of other birds (white dots, y = 0.306x – 0.118, r 2 = 0.216, p < 0.001) belonging to all the existing orders of birds for which data are available. © 2002 by CRC Press LLC 118 Biology of Marine Birds 5.3 DEMOGRAPHIC PARAMETERS OF SEABIRDS When examining the demographic parameters of seabirds, extensive differences exist between and within orders, families, and species (Table 5.1). Fecundity is the product of clutch size, breeding frequency, and breeding success. Fecundity of seabirds is generally low, with all Procellariiformes, Phaethontidae, Fregatidae, and several species of Sulidae, Alcidae, Sternidae, Spheniscidae, and even some Laridae having a clutch of one (Table 5.1; see also Appendix 2). Several species of Diomedeidae, one of Procellaridae (the White-headed petrel Pterodroma lessonii; Figure 5.3), and probably most Fregatidae (at least females) breed only every second year when successful. On the other hand, some species of Phalacrocoracidae can have clutch sizes reaching five to seven eggs and many species of Laridae have clutch sizes of three and are able to lay a replacement clutch when failing early in the season (Figure 5.4). The reasons for the low fecundity of seabirds have been much debated, and David Lack used seabirds, especially pelagic seabirds with a very low fecundity, to illustrate his general theory on clutch size (Lack 1948, 1968). Basically, Lack suggested that altricial birds should lay the clutch that fledges the most offspring. The ability to provide enough food to offspring would therefore be the main reason for the low reproductive rate of some seabirds. The development of life history theory and especially the concept of cost of reproduction and residual reproductive value (Williams 1966) later sophisticated this view. The basic idea is that, because resources are assumed to be limited, reproduction can have a negative influence on the probability of survival to the next reproduction, and therefore individuals should balance present and future reproduction (allocation; see Figure 5.1). For a long-lived species, the risk taken, especially during the first years of life, should be limited in order to enhance future reproductive success. Long-lived animals would therefore behave as “prudent parents,” trying to limit risks of increased mortality when reproducing. Therefore the single clutch of albatrosses and many other seabirds may have evolved as the result of the low provisioning rate of chick due to distant foraging zones (Lack 1968), but also of the “prudent” behavior of the parents that would limit energetic investment because of their high reproductive value. However, whether a clutch of one is the best option for other seabirds with a different ecological specialization is not clear (Ricklefs 1990). Indeed the low fecundity of seabirds is generally attributed to the marine environment on which they rely, an environment that is assumed to be poor, patchy, and unpredictable (Ashmole 1971). However, obviously the marine environment is very diverse and heterogeneous, with localized rich feeding areas or areas of low productivity. Therefore we might expect differences in demographic tactics within taxa according to the envi- ronment exploited, or to the foraging technique used, or diet. Conversely, convergence might be expected between taxa exploiting the same resources or environment, and divergence within taxa when environments exploited are different. The minimum age at first breeding ranges from 2 to 4–5 years in most species of seabirds, except for Diomedeidae and Fregatidae and some species of Procellaridae that start breeding later (Table 5.1). Late age at first breeding is generally assumed to be necessary for long-lived species to attain similar foraging skills to those of adults, either because skills are complex to attain (e.g., Orians 1969, Burger 1987) and/or because of the high reproductive value of young birds. Like age at first breeding, but even more importantly, survival is a parameter that is difficult to estimate accurately because it requires the marking of birds and their recapture over several years. Estimates of adult survival are available for a limited number of species (Table 5.1) and have to be treated with caution. Indeed, the statistical methods to estimate survival are in constant refinement, resulting in an overall increase of the estimates of survival rates within a species as techniques improve (Clobert and Lebreton 1991). Therefore, comparisons of survival are often difficult to perform unless the same method has been used. Average longevity is generally used to illustrate survival but cannot be compared to longevity records that only give maximum age based on isolated recaptures. Most Procellariidae and Diomedeidae have high survival and life expectancy, but also several species within the other orders, for example, several species of Alcidae and one © 2002 by CRC Press LLC Seabird Demography and Its Relationship with the Marine Environment 119 TABLE 5.1 Range of Demographic Parameters Observed in the Families of Seabirds Order Family Symbol Used in Figures Number of Species with at Least One Parameter Average Clutch Frequency of Breeding Age at First Breeding Adult Life Expectancy (number of species with an estimate of survival) Relationship between Age at First Breeding – 1/Fecundity (both corrected for body mass) Relationship between 1/Fecundity – Life Expectancy (both corrected for body mass) Sphenisciformes Spheniscidae Cross 15 1–2 0.7–1 2–5 6.4–20.5 (10) y = 0.219x – 0.648, r 2 = 0.085, p > 0.1 y = 0.251x – 1.11, r 2 = 0.0262, p > 0.1 Procellariiformes Diomedeidae Circle 14 1 0.5–1 5–9 11.6–33.8 (12) Procellariidae Square 22 1 0.5–1 2–8 6.9–25.5 (20) y = 0.165x + 0.087, y = 0.97x – 1.8, Hydrobatidae Diamond 5 1 1 2–3 7.6–17.2 (4) r 2 = 0.112, p < 0.05 r 2 = 0.359, p < 0.01 Pelecanoididae Triangle 2 1 1 2 5.7 (1) Pelecaniformes Phaethontidae Circle 3 1 1 2–5 25.5 (1) Pelecanidae 1 4 1 2 ? y = 0.322x – 0.299, y = 1.032x – 2.16, Sulidae Diamond 9 1–3 1 2–5 17.2–20.5 (4) r 2 = 0.360, p < 0.05 r 2 = 0.495, p < 0.01 Phalacrocoracidae Triangle 8 2–4 1 2–4 6.7–10.4 (3) Fregatidae Square 4 1 0.5 5–8 ? Charadriiformes Stercoraridae Diamond 4 2 1 3–5 6.7–11.6 (4) Laridae Square 12 1–3 1 2–4 8.8–19 (4) y = 0.07x – 0.268, y = 0.543x – 1.702, Sternidae Triangle 14 1–2.1 1 2–4 5.7–9.6 (3) r 2 = 0.0226, p > 0.1 r 2 = 0.1236, p > 0.1 Alcidae Circle 19 1–2 1 2–5 4.7–20.5 (11) © 2002 by CRC Press LLC 120 Biology of Marine Birds Laridae. Unfortunately, no estimate is available for Fregatidae, nor for most tropical Procellariidae, Laridae, and Sternidae, limiting the scope of a general comparison. The low fecundity and late age at first breeding of Fregatidae suggest high survival rate (maximum age recorded 34 years [E.A. Schreiber personal communication]), probably similar to Diomedeidae. One reason for the high survival of seabirds, especially those breeding on oceanic islands, is the absence of terrestrial predators; this is probably true for most large species, but not for the smaller species that can suffer heavy mortality from avian predators. Estimates of survival between fledging and recruitment into the breeding population are more difficult to obtain logistically because of the delayed age at first breeding, and are rare in the literature, limiting the scope for meaningful comparisons between groups. 5.4 COMPARING THE DEMOGRAPHY OF THE FOUR ORDERS OF SEABIRDS Within seabirds, minimum age at first breeding and life expectancy (log transformed) are somewhat related to the log of mass (y = 0.092x + 0.666, r 2 = 0.0788, p < 0.01 and y = 0.1148x + 1.675, r 2 = 0.1532, p < 0.001). These relationships express the allometric component of demographic pattern and indicate that body mass is a significant, but not fundamental, determinant of the variation in demographic traits in seabirds. They represent a first-order tactic which expresses the biomechanical constraints of body mass (Western 1979, Gaillard et al. 1989). When parameters are corrected for the effect of body mass, the relationships between demographic traits are still very significant (Figure 5.5), representing a second-order tactic (Western 1979). It indicates that demographic parameters of seabirds covary after correction for the effect of body mass, which suggests the existence of demographic tactics among seabirds. The relationship between fecundity and life expectancy is very significant (Figure 5.5) and highlights the classical balance between clutch size and survival rates. The relationship between fecundity and age at first breeding, and that between age at first breeding and life expectancy, are also highly significant (Figure 5.5). The regression lines for the three relationships each describe a similar gradient within seabirds going from species with a fast turnover (high fecundity, early age at first breeding, and short life expectancy) to species with a slow turnover. When examining the species within each order, they appear not to be distributed evenly along this fast–slow gradient. Spheniciformes appear to be distributed at the left-hand size of the gradient FIGURE 5.3 A White-headed Petrel. They breed only every other year, incubating their egg for 60 days and spending 112 days raising their single chick. (Photo by H. Weimerskirch.) © 2002 by CRC Press LLC Seabird Demography and Its Relationship with the Marine Environment 121 or fast turnover end of the gradient: penguins breed relatively early, have a short life expectancy, and a high fecundity relative to their size. Conversely, many Procellariiformes species are found at the slow turnover extreme (Figure 5.5). Since the relationship considers all seabirds, i.e., four different orders, it is important to examine whether the relationships are a result of taxonomic differences in demography. Controlling for phylogeny (Harvey and Pagel 1991) was not possible because of the lack of a complete phylogeny covering all species of seabirds, and was out of the scope of this study. When investigating the existence of a gradient within orders, it appears that significant relationships persist within Procellariiformes and Pelecaniformes, whereas there is a tendency, yet nonsignificant for Charadriiformes, and no relationship for Spheniciformes (Table 5.1). This suggests the existence of different demographic tactics within Procellariiformes and Pelecaniformes, and perhaps Charadriiformes. We will now examine whether these tactics among taxa tending to show a fast or a slow turnover can be related to different environmental conditions or foraging strategies. (a) (b) FIGURE 5.4 Seabird species exhibit a range of fecundities. (a) Some gulls, such as this Herring Gull, may raise three chicks in a year, spending 45 to 50 days feeding them before they fledge. (b) Giant Petrels raise one chick a year and spend 100 to 120 days feeding it before it fledges. (Photos by J. Burger.) © 2002 by CRC Press LLC 122 Biology of Marine Birds FIGURE 5.5 Relationships between 1/fecundity, age at first breeding, and life expectancy (corrected by body mass) in the four orders of seabirds (Sphenisciformes, crosses; Procellariiformes, symbols filled in black; Pelecaniformes in gray; and Charadriiformes in white). The inverse of fecundity is used for clarity, so that the three variables are positively linked. Correspondences of symbols for families are given in Table 5.1. Fecundity is estimated as the number of young produced per female per year. It is the product of the average clutch size per year by the overall breeding success. Because data on the average age at first breeding are scarce, minimum age at first breeding is used. Adult life expectancy is directly derived from adult survival and is measured as (0.5 + 1/(1 – s)) (Seber 1973). When parameters are available for several populations, average values are used. © 2002 by CRC Press LLC Seabird Demography and Its Relationship with the Marine Environment 123 To allow an easier representation of the ranking of species along this gradient, the species have been plotted along the first component of a principal component analysis (PCA) performed on the demographic parameters. When the three parameters are used, the first principal component explains 71.1% of the total variance (Figure 5.6a). One extreme, the left-hand side, is characterized by a high fecundity, short life expectancy, and early age at first breeding, while the other extreme presents the opposite characteristics. Because of the low number of species for which life expectancy is known, with an absence of data for some families like Fregatidae (see Table 5.1), a PCA was also performed on the fecundity and age at first breeding only, to be able to plot a larger number of species. The first principle component then explains 74.3% of the total variance (Figure 5.6b). Because the two analyses provide very similar ranking (compare Figure 5.6a and 5.6b, Factor 1 (2 parameters) = 0.924 × Factor 1 (3 parameters) + 0.043, r = 0.956, p < 0.001). We use the ranking obtained from the PCA performed on fecundity and age at first breeding only, with the larger number of species (Figure 5.6b). (a) (b) FIGURE 5.6 Ranking of the four orders of seabirds along a slow–fast gradient described by the first principal component of the PCA analyses (see symbols for families in Table 5.1): (a) PCA performed on 1/fecundity, life expectancy, and age at first reproduction, all corrected for body weight (eigenvalues 2.133, 0.546, and 0.321); and (b) PCA performed on 1/fecundity and age at first reproduction, both corrected for body weight (eigenvalues 1.487 and 0.513). © 2002 by CRC Press LLC 124 Biology of Marine Birds Spheniciformes and Procellariiformes almost do not overlap on the gradient, whereas Pelecan- iformes extend throughout the gradient, and Charadriiformes are intermediate (Figure 5.6b). Whereas the species within the four families of Procellariiformes are scattered throughout the gradient, in Pelecaniformes the four families appear to be clearly separated from one another: Phalacrocoracidae, Sulidae, Phaethontidae, and Fregatidae ranking separately on the fast–slow gradient. This ranking probably reflects a strong phylogenetic effect on demographic tactics within this order, with each family having a distinct morphology and feeding specialization. Conversely, within Procellariiformes, Diomedeidae and Procellaridae are very similar in terms of morphology and feeding technique and are ranked similarly. Similarities in demographic traits between some families belonging to different orders suggest convergence. Phalacrocoracidae appear to have equivalent demographic tactics to those of Spheniciformes, having a fast turnover. Diving petrels, Pelecanoididae, also appear have faster turnover than most other petrels. This tendency to be at the fast extreme of the gradient in these three families could be associated with the constraints of diving that make birds poor fliers and therefore reduce foraging range. Convergence in demographic tactics may also be found between Fregatidae and the longest lived albatrosses and petrels. These birds have in common a pelagic life but especially economic flight. In Charadriiformes, a ranking of demographic tactics by families is also apparent, although less clear-cut than in Pelecaniformes, with Laridae (with the exception of one species, the Swallow-tailed Gull Creagrus furcatus) and Stercoraridae toward the fast extreme. Conversely, Sternidae and Alcidae are distributed over a wider range, rather at the slow end, suggesting convergence in demographic tactics with species that are well known to be long-lived like Procellariiformes. 5.5 FACTORS RESPONSIBLE FOR DIFFERENCES IN DEMOGRAPHIC TACTICS Some demographic traits are phylogenetically conservative and fixed at high taxonomic levels. For example, all Procellariiformes have a clutch size of one. Others, like minimum age at first breeding and maximum life expectancy, probably do not vary within populations of a species because they are likely not to be adapted to local environmental conditions. Maximum life expectancy is probably mainly related to allometric pressures or phylogeny. Small birds have a higher energy expenditure and therefore shorter life span than larger birds (Lindstedt and Calder 1976; see Chapter 11). There are negative correlations between survival and vigorous, energy-expensive activity such as flight (Bryant 1999); consequently, birds with a low-energy flight such as albatrosses may live longer compared to birds with a highly expensive flight such as shags. On the other hand, breeding success, breeding frequency, average age at first breeding, and adult and juvenile survival express the interactions between phenotype and environment and are influenced by the environment (Figure 5.1). These demographic traits are likely to be different between closely related species exploiting different marine environments, or even within the same species exploiting different environments. Therefore, families covering a wide range over the fast–slow gradient suggest a broad range of demographic tactics due, for example, to a group of species exploiting a diversity of habitats. Conversely, families with a restricted range along the fast–slow gradient suggest that all species belonging to this group probably face similar environmental conditions. For example, Sulidae rank over a relatively restricted range, but they breed from tropical to sub-Arctic waters. Seabirds have been classically separated into inshore, offshore, and oceanic or pelagic (Ashmole 1971), and it is generally assumed that pelagic species are the most long-lived, whereas inshore species are shorter lived (Lack 1968). Therefore, we might expect that pelagic species should be found at the slow turnover extreme of the fast–slow gradient. When considering the four orders simultaneously, there is indeed a tendency for oceanic families to stand at the slow end of the gradient (e.g., most Procellaridae, Hydrobatidae, Diomedeidae, or Fregatidae), whereas more inshore families are found at the other extreme. However, this is mainly due to the fact that many © 2002 by CRC Press LLC [...]... Kerguelenb Campbellc Atlantic 55 °S 650 00 356 0 ± 396 119.0 ± 2.4 34.2 ± 24.0 0.27 8 10 0. 95 ± 0.006 0.240 0 Shelf, Polar Front 100–600 km Fish, Krill 12 days 2.1 (1–12) 116 Indian 50 °S 3300 3 655 ± 353 118.4 ± 3.9 63.0 ± 10 0 .58 0 6 9.7 0.906 ± 0.0 05 0.21 0 Shelf, Polar Front 250 km Fish 4 days 2.1 (1–7) 120 Pacific 52 °S 10– 150 00 2 750 ± 161 112 .5 ± 2.9 66.3 ± 12.9 0 .54 3 6 10 0.9 45 ± 0.007 0.186 +1.1 Shelf,... 1997, DeLamare and Kerry 1992; Figure 5. 8), suggesting that each population is relying on similar resources in the three regions Indeed both in the Atlantic © 2002 by CRC Press LLC 126 Biology of Marine Birds FIGURE 5. 8 A pair of Wandering Albatrosses at their nest They are one of the most long-lived seabirds and have one of the longest breeding periods, incubating for 75 to 83 days and taking about 280... 1999c © 2002 by CRC Press LLC 128 Biology of Marine Birds FIGURE 5. 9 Relationship between population size and the surface of shelf as an index of food resource in different populations of Black-browed Albatrosses away when krill is rare (Veit and Prince 1997) They also exploit the Polar Frontal zone that is on average 50 0 km north of the breeding site At Campbell birds forage within a year alternately... density-dependent feedbacks: higher fecundity is balanced by lower survival, or alternatively lower fecundity by high survival 5. 8 PERSPECTIVES Understanding the demography of seabirds requires studying them over long periods having populations of marked birds Short-term studies of seabirds are inadequate to characterize the demographic pattern of seabird populations because seabirds are long-lived... with the Marine Environment 133 CODY, M L 1966 A general theory of clutch size Evolution 20: 174–184 COOCH, E G., AND R E RICKLEFS 1994 Do variable environments significantly influence optimal reproductive effort in birds? 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Comparative Method in Evolutionary Biology Oxford University Press, Oxford HATCH, S A., B D ROBERTS, AND B S FADELY 1993 Adult survival of black-legged kittiwakes Rissa tridactyla in a Pacific colony Ibis 1 35: 247– 254 JOUVENTIN, P., AND J L MOUGIN 1981 Les stratégies adaptatives des oiseaux de mer Terre et Vie 35: 217–272 © 2002 by CRC Press LLC 134 Biology of Marine Birds KARR, J R., J N NICHOLS, M K . Indian Pacific Latitude 55 °S 50 °S 52 °S Size of population 650 00 3300 10– 150 00 Mass of adult (g) 356 0 ± 396 3 655 ± 353 2 750 ± 161 Culmen length (mm) 119.0 ± 2.4 118.4 ± 3.9 112 .5 ± 2.9 Breeding success. orders of birds for which data are available. © 2002 by CRC Press LLC 118 Biology of Marine Birds 5. 3 DEMOGRAPHIC PARAMETERS OF SEABIRDS When examining the demographic parameters of seabirds,. periods having pop- ulations of marked birds. Short-term studies of seabirds are inadequate to characterize the demo- graphic pattern of seabird populations because seabirds are long-lived and live