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

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

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

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

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

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

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
Seabird Demography and Its Relationship with the Marine Environment 125
Procellariiformes are pelagic and stand at the slow extreme of the gradient. Examining the distri-
bution of inshore, offshore, or oceanic species within families does not lead to the clustering of
inshore or oceanic species at one or the other extreme of the gradient (Figure 5.7a). This suggests
that the assumption that pelagic species are more long-lived than inshore species only exists when
groups are compared at high taxonomic levels (for example, when comparing Procellariiformes
and Charadriiformes). But when the effect of size is controlled and each order examined separately,
the data available today do not allow us to conclude that pelagic species have a slower turnover
than inshore species.
Polar waters are generally more productive than tropical waters, which may have influenced
the evolution of demographic traits. Therefore, we might expect that tropical species might be
located at the slow end of the gradient compared to polar species. This tendency is not apparent
with the data available (Figure 5.7b). For most families, either some breed only over a narrow
range of climates (Alcidae or Fregatidae, for example) or data are not available (tropical Procel-
laridae, for example), limiting the possibility of making generalizations.
These first examinations indicate that the role of the marine environment in shaping demo-
graphic tactics is difficult to determine, and that the conjunction of several factors has probably
been involved in shaping the demographic traits of marine birds. Because data are lacking for many
groups, comparisons at lower taxonomic levels are impossible at this time.
5.6 INTRASPECIFIC VARIATIONS IN DEMOGRAPHIC TRAITS
Some species that are separated geographically show very homogenous demographic traits between

Extensive differences exist between three populations of Black-browed Albatross (Thalassarche
melanophris) for which fecundity, adult survival, and other life history characteristics are known
(Table 5.3). The Kerguelen population is characterized by high breeding success that does not vary
from year to year, a relative low minimum age at first breeding, and a relatively low adult survival
(see references in Table 5.3). On the other end, the South Georgia population has a low and very
variable breeding success, years with complete breeding failures, a later minimum age at first
breeding, and a fairly high survival. The Campbell population (the smallest birds) is intermediate
between the two others, similar in fecundity to Kerguelen and in survival to South Georgia. Birds
at three sites rely on similar diets with the same squid species, typical of the Polar frontal zone.
At Kerguelen birds forage close to colonies, at an average distance of 250 km, over offshore waters
(on the slope of the shelf) where the Polar front passes. At South Georgia, birds forage over the
shelf, or slope area, and feed on krill to a large extent during some years, but have to forage farther
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 days to raise
their chick. (Photo by H. Weimerskirch.)
© 2002 by CRC Press LLC
Seabird Demography and Its Relationship with the Marine Environment 127
TABLE 5.2
Fecundity and Survival of Black-Legged Kittiwake (Rissa tridactyla) in Different
Sites of the North Atlantic and Pacific Oceans
Fecundity Chick/Year
(Range) Adult Survival References
Atlantic
North Shield — England 1.25 (1–1.4) 0.801 Aebischer and Coulson 1990,
Coulson and Thomas 1985
Brittany — France 0.78 (0.23–1.48) 0.808 Danchin and Monnat 1992
Isle of May — Scotland 0.890 Harris and Calladine 1993
Shetlands — Scotland 1.1 (0–1.8) Monaghan 1996
Hornøya — Norway 0.55 0.922 Erikstad et al. 1995
Eastern Canada 0.74

Foraging trips incubation 12 days 4 days 11 days
Foraging trips chick rearing 2.1 (1–12) 2.1 (1–7) 2.0 (1–12)
Fledging period (days) 116 120 130
a
Before 1990, Prince et al. 1994, Croxall et al. 1998, Prince et al. 1998, Tickell and Pinder 1974.
b
References: Weimerskirch et al. 1997, Weimerskirch 1998.
c
References: Waugh et al. 1999a, 1999b, 1999c.
© 2002 by CRC Press LLC
128 Biology of Marine Birds
away when krill is rare (Veit and Prince 1997). They also exploit the Polar Frontal zone that is on
average 500 km north of the breeding site. At Campbell birds forage within a year alternately in
the Polar Frontal zone (2000 km south of the island), and on the shelf close to the island. On the
three sites, birds are using the same habitats, the Polar Frontal zone and shelf slopes, but the
geographic location and trophic situations of these favored habitats are different, leading to varying
demographic tactics. At Campbell the extensive distance to the Polar Front area makes provisioning
more difficult, with longer fledging periods and small fledglings, possibly the reason for the smaller
size of the adult birds. Yet fecundity, as well as survival, is high. The sizes of the populations are
also different between sites. Probably because the size of the shelf is related to the amount of
resources available to the population, the size of the Black-browed Albatross population for each
breeding locality varies directly with the size of the surrounding shelf (Figure 5.9).
Similar divergent evolution in demographic tactics probably exists within many other taxa of
seabirds where populations rely on different marine environments, or when the food resource is
more or less distant from the breeding grounds. The two examples presented here highlight the
importance of the marine environment in shaping demography of seabirds (Figure 5.1). Fecundity
is dependent on the amount of resources available in the environment, i.e., on oceanographic
processes, and their variability is influenced by climatic variability (see Chapter 6). Seabirds rely
on marine resources but have to breed on land. Colonies are often located in proximity to productive
ocean zones, but the distances between the colony and the resources put constraints on the amount

environmental conditions. The curve would be different for each species, as suggested by that for
Black-browed Albatrosses, and represents the possible demographic scenarios for a stable popula-
tion. Points with lower values (inside the convex curve) represent declining populations; those
outside, increasing populations. Hypothetical scenarios are shown as extremes for a very long-lived
species (Wandering Albatrosses with long life span and low fecundity, maximum one egg every
second year) and a shorter-lived species (shag with multiegg clutches, Figure 5.7). These different
scenarios are based on the assumption of density-dependent feedback. Although hypothetical, they
probably represent more accurately the possible variability in demographic traits found within a
species, and contrast, of course, with the single figure or average figure that is often proposed in
comparative studies. This representation does not integrate the survival of chicks from fledging to
recruitment, which plays a significant role in the population dynamics of long-lived species, but is
generally not considered, in part, because of the paucity of empirical data.
5.7 POPULATION REGULATION AND ENVIRONMENTAL
VARIABILITY
It has been suggested that populations of seabirds are mainly regulated by food availability, in a
density-dependent way (Ashmole 1963, Birkhead and Furness 1985). The sizes of populations in
relation to potential food availability around breeding grounds (Figure 5.6), or in relation to the
FIGURE 5.10 Relationship between fecundity and survival in kittiwakes (black dots) and Black-browed
Albatross (white dots) populations, suggesting a convex curve, representing the optimization between survival
and fecundity, specific to each species. Hypothetical relationships for a long-lived species (Wandering Alba-
tross) and a short-lived species (shag) are represented with the dotted lines.
Annual adult survival
Fecundity
© 2002 by CRC Press LLC
130 Biology of Marine Birds
location of other colonies of conspecifics (Furness and Birkhead 1984), are good examples. Also,
the degree of density dependence is likely to be different, especially during the breeding season,
between species relying on resources close or distant from breeding grounds (Birkhead and Furness
1985). Other factors such as predation or breeding site availability are likely to be important only
in isolated species, with the exception of predation by introduced species, such as cats or rats, on

variability is poorly known due to few field studies of marked birds. Only recently developed
techniques in modeling of survival should allow us in the future to relate environmental variability
and adult survival in seabirds. Using such techniques, it has been possible to relate the survival of
adult Emperor Penguins to oceanographic anomalies related to the Antarctic Circumpolar Wave.
During the warm events that occur every 4 to 5 years, adult survival drops to low values, some
years to 0.75, whereas in other years survival is 0.92–0.97 (Barbraud and Weimerskirch 2001).
Another aspect of the demography of long-lived seabirds that is still poorly known is the extent
to which nonbreeding by adult mature birds affects the dynamics of populations. In addition to the
absence of reproduction in populations strongly affected by ENSO (Schreiber and Schreiber 1984,
Duffy 1990), it appears that nonbreeding could be a general feature in other populations in response
to ENSO. In some petrels in the Southern Ocean, up to 70% of the population refrains from breeding
in some years (Chastel et al. 1995), but adult survival is not affected by these poor years when
breeding success is low, or when few birds are able to breed (unpublished data). Thus it is important
to be able to distinguish between absence due to nonbreeding and absence due to mortality.
Environmental variability has probably had a major influence on the evolution of life history
traits of seabirds. It is generally assumed that birds which live in a highly variable environment
© 2002 by CRC Press LLC
Seabird Demography and Its Relationship with the Marine Environment 131
have increased reproductive rate and therefore reduced survival (Schaffer 1974). However, in our
examples with kittiwakes, as well as in Black-browed Albatrosses, populations with highly variable
fecundity, i.e., probably living in the most variable environment, are those with the highest survival
and the lowest fecundity. This paradox shows that the degree to which environmental variability
influences the evolution of life history strategies is not clearly understood (Cooch and Ricklefs
1994).
The possible tendency for some seabirds to be longer-lived when living in a variable environ-
ment may be explained by taking into account several important factors specific to seabirds, and
especially the ability or inability of species to disperse when conditions are unfavorable. Kittiwakes
and Black-browed Albatrosses disperse from the vicinity of breeding grounds, and are thus able to
escape from poor environmental conditions, especially outside the breeding season. Low average
fecundity due to high variability in breeding success and especially to the occurrence of complete

accessible to study. Young birds, after they have fledged, remain at sea until they first breed, and
it is impossible to obtain information on the factors that affect their survival and maturation. Yet
immature birds represent a significant portion of the population, up to 40% in some species. Again
the only way to obtain information is to carry out long-term population studies and band large
numbers of fledglings. Doing this is vital to understanding the demography of a seabird population.
Another aspect that is also poorly known is the dispersal rates of a seabird population. Seabirds,
and especially Procellariiformes, are generally assumed to be highly philopatric but there is some
© 2002 by CRC Press LLC
132 Biology of Marine Birds
evidence that it is not always the rule. For example, the expansion of fulmars (Fulmarus) in the
Atlantic can only be explained by high emigration rates from large colonies. Most snow petrel
(Pagodroma nivea) chicks do not return to their birthplace (Chastel et al. 1993). The role of dispersal
in the dynamics of seabird populations is technically difficult to study, but it is likely that, in some
species at least, it plays a significant role.
There is a paucity in studies on tropical species for most families compared to the large number
of long-term studies on the demography of temperate or polar species. There is definitely a need
for studies on tropical species such as most Pelecaniformes (especially tropicbirds and frigatebirds),
tropical terns, or Procellariiformes. This would allow fruitful comparison and allow tests of hypoth-
esis such as, for example, that related to the lower productivity of tropical waters.
To conclude, it appears that much is still to be learned on the demography of seabirds, and
many exciting questions remain unanswered. Problems will only be solved by the development of
new ideas and modeling, but there is a striking need for empirical data. Comparative studies between
high taxonomic levels are probably not optimal to understand the role of the marine environment
in shaping demographic studies. Comparing populations of the same species, living in contrasted
environments is probably more promising.
ACKNOWLEDGMENTS
I would like to thank E. A. Schreiber and J. Burger for inviting me to write this chapter and for
extensive help in the preparation of the manuscript
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Wilson’s Storm-petrel Feeding on the Wing
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