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Leucocyte Profiles and Family Size in Fledgling Greylag Geese ( Anser Anser )

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In precocial species, large brood sizes are usually considered as beneficial and individuals in larger broods grow faster and are more dominant compared to individuals in small broods. However, little is known whether family size also beneficially affects the offspring's physiology. In the present study, we investigated whether leucocyte profiles in fledgling Greylag Geese (Anser anser) are affected by (1) family size, (2) individual characteristics, i.e. age, body condition or sex, or (3) characteristics of the parents, i.e. previous reproductive success. From spring 2013 to autumn 2015, we collected blood samples from 100 juvenile Greylag Geese from 20 different pairs. From these samples we determined the absolute leucocyte number, an individual's differential blood cells count and an individual's haematocrit (HCT). The number of fledglings in a family and therefore the number of siblings a focal individual had, was positively related to the percentage of basophils, negatively to the heterophils/lymphocytes ratio (H/L), and tended to be negatively related to the percentage of monocytes and eosinophils in a sample. H/L ratio was negatively related to age in days and tended to be negatively related to body condition, whereas the percentage of basophils tended to be positively related to it. Absolute leucocyte number did not differ between individuals depending on family size. However, composition of different leucocyte types (basophils, eosinophils, H/L ratio) was modulated mostly by the social environment (family size) and not by the characteristics of the individual or the parents. In conclusion, even though we did not find clear evidence of a positive health effect, i.e. a better immune system, in fledgling Greylag Geese of large versus small families, our results suggest that family size modulates different components of the immune system hinting at its stress-reducing effect.
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AVIAN BIOLOGY RESEARCH 10 (4), 2017 246–252
Paper 1700780 https://doi.org/10.3184/175815617X15036738758871
www.avianbiologyresearch.co.uk
Leucocyte profiles and family size in fledgling Greylag Geese
(Anser anser)
Claudia A.F. Waschera,b*, Josef Hemetsbergera,c, Kurt Kotrschala,c and
Didone Frigerioa,c
aCore Facility Konrad Lorenz Forschungsstelle for Behaviour and Cognition, University of Vienna,
Fischerau 11, A-4645 Grünau im Almtal, Austria
bAnimal and Environment Research Group, Department of Life Sciences, Anglia Ruskin University,
Cambridge, UK
cDepartment of Behavioural Biology, University of Vienna, Althanstrasse 14, A-1090 Vienna,
Austria
*E-mail: claudia.wascher@anglia.ac.uk
ABSTRACT
In precocial species, large brood sizes are usually considered as beneficial and individuals in larger broods grow
faster and are more dominant compared to individuals in small broods. However, little is known whether family
size also beneficially affects the offspring’s physiology. In the present study, we investigated whether leucocyte
profiles in fledgling Greylag Geese (Anser anser) are affected by (1) family size, (2) individual characteristics,
i.e. age, body condition or sex, or (3) characteristics of the parents, i.e. previous reproductive success. From
spring 2013 to autumn 2015, we collected blood samples from 100 juvenile Greylag Geese from 20 different
pairs. From these samples we determined the absolute leucocyte number, an individual’s differential blood
cells count and an individual’s haematocrit (HCT). The number of fledglings in a family and therefore the
number of siblings a focal individual had, was positively related to the percentage of basophils, negatively to
the heterophils/lymphocytes ratio (H/L), and tended to be negatively related to the percentage of monocytes
and eosinophils in a sample. H/L ratio was negatively related to age in days and tended to be negatively
related to body condition, whereas the percentage of basophils tended to be positively related to it. Absolute
leucocyte number did not differ between individuals depending on family size. However, composition of
different leucocyte types (basophils, eosinophils, H/L ratio) was modulated mostly by the social environment
(family size) and not by the characteristics of the individual or the parents. In conclusion, even though we did
not find clear evidence of a positive health effect, i.e. a better immune system, in fledgling Greylag Geese of
large versus small families, our results suggest that family size modulates different components of the immune
system hinting at its stress-reducing effect.
Keywords: Greylag Geese, Anser anser, differential leucocyte count, haematology, social behaviour, family size
1. INTRODUCTION
In precocial waterfowl species, where the young feed
themselves, large brood sizes are usually considered as
beneficial for both parents and offspring. Benefits of large
broods include faster growth (Cooch et al., 1991; Williams,
1994) and higher dominance within the flock (Black and
Owen, 1989; Lepage et al., 1998; Loonen et al., 1999)
compared to individuals in smaller families. However,
large clutch sizes are costly to produce and therefore
strongly depend on the female’s egg formation ability
(Winkler and Walters, 1983). Later on costs in relation to
larger broods are increased vigilance behaviour (Forslund,
1993) and decreased feeding duration (Williams et al.,
1994). In altricial birds, immunosuppression in large
broods is a major cost of increased offspring numbers
(Saino et al., 1997; Hõrak et al., 1998; Ilmonen et al.,
2003), yet, effects of brood size on physiology are not
known in precocial species.
Haematological parameters are important diagnostic
tools in gaining information about an animal’s condition
and indicators of individual responses to environmental
and social contexts (reviewed in Fair et al., 2007; Davis
et al., 2008). The differential leucocyte count provides
information on the relative occurrence of different
leucocyte types (Dufva and Allander, 1995; Zuk et al.,
1995). Absolute leucocyte numbers generally scale
negatively with body condition (Verhulst et al., 2002).
In birds, heterophils are indicative of changes in the
environment (Gross and Siegel, 1983), whereas the
ratio of heterophils/lymphocytes (H/L ratio) is used as an
indicator of physiological stress, as lymphocyte numbers
decrease while heterophil numbers increase in response
to stressful conditions (Maxwell and Robertson, 1995;
Leucocyte profiles and family size in Greylag Geese 247
Vleck et al., 2000; Lebigre et al., 2011). Eosinophils play
a role in inflammation processes and are associated with
defence against parasites, especially helminth infection
(Hogan et al., 2008) and elevated levels of basophils have
been observed in birds with initial stages of inflammatory
disease and under stress (Maxwell and Robertson, 1995).
Finally, monocytes are long-lived phagocytic cells
associated with defence against infections and bacteria
(Davis et al., 2008).
Haematocrit (HCT), which is the relative volume of
red blood cells compared to the total blood volume, is
an indicator of an animal’s physical condition (Hõrak et
al., 1998; Fair et al., 2007) and is known to decrease in
response to stressful conditions (Dickens et al., 2009;
Ludwig et al., 2017). However, HCT also varies with sex,
age, reproductive status, as well as geographic distribution
(Dawson and Bortolotti, 1997; Fair et al., 2007) and
therefore it has to be mentioned that haematological
parameters as indicators of stress, physical condition and
health are controversial (O’Brien et al., 2001; Romero,
2004; Davis et al., 2008; Kaliński et al., 2011; Lill et al.,
2013; Minias, 2015).
In adult Greylag Geese (Anser anser), haematological
parameters are significantly affected by environmental
and social factors. During the mating season, unpaired
individuals had higher HCT compared to paired and
family individuals and this pattern reversed in the autumn.
Similarly, H/L ratio was positively related to pair-bond
status in a seasonally dependent way, with highest values
during mating and successful pairs had higher H/L ratio
than unsuccessful ones (Frigerio et al., 2017).
Furthermore, in Greylag Geese, a long-lived bird
species living in a socially complex system, agonistic
interactions have been shown to be among the strongest
modulators of the physiological stress response (Scheiber
et al., 2005; Wascher et al., 2008, 2009; Wascher and
Kotrschal, 2013). Emotionally supportive social contexts,
such as active support in aggressive interactions (Scheiber
et al., 2005; Scheiber et al., 2009) and ‘passive support’
due to the mere presence of a social partner (Weiß
and Kotrschal, 2004) are known to strongly modulate
hypothalamic-pituitary and the sympathico-adrenergic
stress responses (e.g. Frigerio et al., 2003; Scheiber et al.,
2005; Wascher et al., 2012a). Juvenile Greylag Geese stay
with their parents until the next breeding season in the
following spring (‘primary families’: Lorenz, 1979) and,
if the parents fail to fledge offspring, the juveniles might
re-join the parents, forming so called ‘secondary families’
(Scheiber et al., 2009). The effect of emotional support
seems to depend on family size, as in parental Greylag
Geese the excretion of immunoreactive corticosterone
metabolites decreases with offspring number (Scheiber
et al., 2005) but excretion of parasite products, more
specifically nematode eggs, increases with offspring
number (Wascher et al., 2012b).
In the present study, we investigate leucocyte profiles
in fledgling Greylag Geese and its relationship with (1)
family size, (2) individual characteristics, such as age,
body condition or sex, and (3) characteristics of the
parents, i.e. previous reproductive success. Individuals in
larger families are expected to be in a better condition
and less stressed compared to individuals in smaller
families and this is expected to be reflected in differences
in haematological parameters. Further, we do expect
a stronger effect of the social environment on Greylag
Geese fledglings’ leucocyte profile compared to the
characteristics of the individual as well as those of the
parents.
2. MATERIALS AND METHODS
2.1 Ethical statement
This study complies with all current Austrian laws and
regulations concerning the work with wildlife. Catching
and blood sampling were performed under Animal
Experiment License No. 6606/26-II/3b/2013 by the
Austrian Federal Ministry for Science and Research.
2.2 Study population
A non-migratory flock of Greylag Geese was introduced in
the valley of the Alm (Upper Austria) by late Konrad Lorenz
in 1973 and since then social behaviour and individual
life-histories have been monitored (Hemetsberger et al.,
2013). The birds are unrestrained and roam the valley
between the Konrad Lorenz Forschungsstelle (KLF;
47°48’N, 13°56’E) and a lake approximately 10 km
to the south (Almsee; 47°44’N 13°57’E), where they
roost at night. During the breeding season the flock
disaggregates and families roam between different areas
(i.e. Oberganslbach: 47°79’N, 13°94’E; Cumberland
Gamepark: 47°8’N, 13°94’E; and the meadows around
the KLF) to raise their offspring. The three locations do
not differ much with respect to their characteristics, e.g.
vegetation structure. At the time of the study (spring 2013
till autumn 2015) the flock consisted of approximately
160 individuals, all individually marked with coloured leg
bands for identification. The birds are supplemented with
pellets and grain twice daily at 08:00 and 17:00 hours
during the summer months.
In the present study, individual blood samples were
collected from 100 juvenile Greylag Geese from 20
different pairs. Age, pair bond duration and previous
breeding success of parents was known. Data were
collected in three consecutive years from spring 2013 to
autumn 2015 and every gosling that reached fledgling
age during this time was included in the presented study.
We aimed to catch and sample every gosling shortly
before fledging, however for families which were less
habituated to human presence, this was not always
possible. Furthermore, captures were avoided on days
248 Claudia A.F. Wascher, Josef Hemetsberger, Kurt Kotrschal and Didone Frigerio
with unfavourable weather conditions, for instance heavy
rain or particularly cold mornings. Therefore, at the
time of sampling mean age was 103.98 days ± 43.39
standard deviation (SD). Field constraints did not allow
for planning the time of catching and therefore fledglings
were sampled randomly. Focal individuals were 48 males
and 52 females, and the number of fledglings in each
family ranged from one to seven individuals (11 families
had one gosling, 10 families had two, three families had
three, eight families had four, three families had five, one
family had six and one family had seven; average ± SD
= 3.63 ± 1.679). Highest mortality happened within the
first 10 days of life and in the first winter, therefore we can
consider the family size at the time of fledging as stable
(Hemetsberger, J., unpublished data).
2.3 Data collection
To avoid potential bias due to diurnal variation all geese
were caught in the morning between 07:30 and 09:30,
either by a familiar human observer who approached
the geese from the back and picked them up when being
close enough, or in a ‘trapping enclosure’, which the
geese entered voluntarily during feeding. Chasing of the
individuals was avoided. A total number of 100 individual
blood samples were collected in order to determine (1) an
individual’s differential blood cell count (Prinzinger et al.,
2012) and (2) an individual’s HCT. Samples were collected
by puncturing the tarsal vein with a sterile needle (24 µm
diameter) and collecting blood in two heparinised micro-
haematocrit capillaries (75 mm). Furthermore, the geese
were weighed and tarsus measured. The whole procedure
lasted less than 7 min per individual.
In order to determine an individual’s blood cell count,
one drop of blood was smeared onto a microscope slide,
air-dried and stored until later identification of leucocytes at
the University of Veterinary Medicine in Vienna. Differential
blood cell count provided information on absolute
leucocyte number/μL (LEUCO) and the relative occurrence
of different leucocyte types (heterophils, lymphocytes,
monocytes, basophils and eosinophils). Blood smears
were Romanowsky-stained (Haemaquick; E. Lehmann
GmbH, Salzburg Austria) and microscopically evaluated.
Thereafter, 100 white blood cells were differentiated into
heterophilic, eosinophilic or basophilic granulocytes,
monocytes, and lymphocytes at 1,000× magnification
using oil immersion. Results were provided in percentages.
Directly after blood taking, haematocrit capillaries were
sealed with plasticine at the bottom and centrifuged at 8,000
rpm for 5 min in order to determine the HCT. Volumes of
red blood cell and plasma respectively were measured on
the capillaries to the nearest 0.5 mm with calipers. HCT was
then calculated as ratio as follows (Prinzinger et al., 2012):
red blood cell volume/(red blood cell volume + plasma
volume). The individual arithmetic mean of the two HCT
values was then taken into further analyses.
2.4 Statistical analysis
We ran linear mixed effect models (LMMs) fitted with
maximum likelihood using the nlme package (Pinheiro
et al., 2017) in R (R Team, 2015). Haematocrit, leucocyte
count, percentage of basophils, percentage of eosinophils,
percentage of monocytes and H/L ratio were considered as
response variable and number of fledglings, sex, juveniles’
age in days, parents’ previous breeding success, calculated
as ratio ‘breeding attempts/successful breeding attempts’,
the body size index (BSI), calculated as ratio ‘weight/tarsus’
(Green, 2001) served as fixed factors. In order to account
for repeated measures for each family, as siblings have been
sampled, the family identity was included as random factor.
We based our model selection on second-order Akaike’s
information criterion values (AICc; Hurvich and Tsai, 1989).
We calculated the difference between the best model and
each other possible model (ΔAICc) and ranked the model
combinations according to their ΔAICc, which provides
an evaluation of the overall strength of each model in the
candidate set. Multiple models qualified as similarly good
models, i.e. ΔAICc ≤ 2 (Burnham and Anderson, 2002;
Burnham, 2004) and therefore we applied a model averaging
approach, which calculates model averaged parameters
using the MuMIn package (v. 1.15.6; Barton, 2016).
3. RESULTS
Parameter estimates and standard error of all fixed factors
present in the final model are shown in Table 1. The number
of fledglings in a family, the previous breeding success of
the parents, age and sex of the individuals was not related to
the number of leucocytes (Table 1). Haematocrit increased
with age and BSI, but was not affected by the other factors
(Table 1). The number of fledglings in a family and therefore
the number of siblings a focal individual had, was positively
related to the percentage of basophils (Figure 1a), negatively
related to the H/L ratio, and tended to be negatively related
to the percentage of monocytes and eosinophils in a sample
(Figure 1b). H/L ratio was negatively related to age in days
and tended to be negatively related to the BSI, whereas the
percentage of basophils tended to be positively related to
BSI. All statistical results are presented in Table 1.
4. DISCUSSION
In the present study, we investigated the relationship between
family size, individual and parental characteristics and
different haematological parameters (haematocrit, differential
leucocyte count) in fledgling Greylag Geese, a precocial
waterfowl species. We predicted that number of fledglings
in a family would strongly modulate haematological
parameters, however the total number of leucocytes was
not related to family size, suggesting that this factor does
Leucocyte profiles and family size in Greylag Geese 249
not actually affect the overall immune function (Verhulst
et al., 2002). In altricial birds, number of fledglings, i.e.
clutch size, has been shown to negatively affect the immune
system, including the total number of leucocytes (Hõrak et
Table 1 Results of the best linear mixed models for each response variable. Model parameters for the averaged models are presented
Response variable Estimate ± SE z value P
Number of leucocytes
Number of fledglings 0.042±0.1 0.414 0.678
Breeding success of parents –0.089±0.1 0.872 0.382
Age in days –0.05±0.1 0.488 0.625
Sex –0.044±0.1 0.429 0.667
Haematocrit
Age in days 0.274±0.088 3.062 0.002*
BSI 0.185±0.081 2.241 0.025*
Sex –0.086±0.068 1.239 0.215
H/L ratio
Number of fledglings –0.2±0.09 1.984 0.047*
Age in days –0.242±0.121 1.971 0.048*
BSI –0.216±0.113 1.187 0.06
Sex –0.067±0.1 0.66 0.508
Monocytes
Number of fledglings –0.178±0.1 1.751 0.079
BSI 0.164±0.103 1.57 0.116
Breeding success of parents 0.156±0.099 1.539 0.123
Sex –0.112±0.099 1.109 0.267
Age –0.096±0.116 0.811 0.416
Basophils
Number of fledglings 0.256±0.098 2.558 0.01*
BSI 0.179±0.097 1.817 0.069
Sex –0.083±0.097 0.839 0.401
Eosinophils
Number of fledglings –0.177±0.099 1.753 0.079
BSI 0.159±0.098 1.596 0.11
Breeding success of parents 0.16±0.099 1.588 0.112
Figure 1 Percentage of basophils (a) and eosinophils (b) depending on the number of fledglings in a family. Circles indicate median
per group, error bars are based on interquartile ranges (lower: 25th and upper: 75th percentile).
al., 1998), however this comes as no surprise as feeding the
nestlings is a major energetic constraint for altricial birds,
which is different in precocial birds, such as Greylag Geese.
We did find differences in percentage of monocytes,
basophils and eosinophils depending on the number of
250 Claudia A.F. Wascher, Josef Hemetsberger, Kurt Kotrschal and Didone Frigerio
fledglings, indicating that family size modulates different
components of the immune system. Eosinophils play a
role in inflammation processes and are associated with
defence against parasites, especially helminthic infection
(Hogan et al., 2008) and are not affected by heat stress in
birds (Altan et al., 2003). This suggests that the immune
system of individuals living in larger families is less
prepared to deal with parasite infection. In fact, parents of
larger families excreted more nematode eggs compared
to smaller families (Wascher et al., 2012b). However,
at present we do not have data about parasite excretion
patterns in young geese available, and therefore do not
know if parasite load in goslings differs depending on
family size.
In contrast, the percentage of basophils was positively
related to family size. Increased numbers of basophils have
been observed in birds with initial stages of inflammatory
disease and under stress (Maxwell and Robertson, 1995).
There are conflicting results regarding the question
whether or not basophil numbers are modulated by stress
and it is suggested that modulation of basophils depends
on the type of stressor. Studies on food restriction and
heat stress revealed a significant increase in basophils
(Maxwell, 1993; Altan et al., 2003), whereas other studies
report no effect of stressors, such as handling (Newman
et al., 2005; Bedáàová et al., 2007). Our results suggest
that social factors might modulate basophil numbers in
fledgling Greylag Geese. Larger families had a lower
percentage of monocytes, a leucocyte type associated
with defence against infections and bacteria. The H/L
ratio, a blood parameter which increases in energetically
stressful conditions (Owen and Moore, 2006), was
negatively related to the number of fledglings in a family.
Therefore, when it comes to specific leucocyte types, our
results are in line with the expectation that family size
affects haematological parameters. A likely mechanism
underlying this difference is emotional support, defined as
the stress-reducing effect of the presence of a close social
partner (Sachser et al., 1998), which is likely to have a
buffering effect onto the physiological stress response
in large families (Scheiber et al., 2005). Social buffering
of both the hypothalamic-pituitary and the sympathico-
adrenergic stress axis is a well known mechanism in
mammals (Arnold and Dittami, 1997; Rault, 2012; Young
et al., 2014) and birds (Edgar et al., 2015).
Juvenile Greylag Geese stay with their parents until the
next breeding season in the following spring (‘primary
families’: Lorenz, 1979) and, if the parents fail to fledge
offspring, the juveniles might re-join the parents, forming
so called ‘secondary families’ (Scheiber et al., 2009).
The effect of emotional support seems to depend on
family size, as in parental Greylag Geese, the excretion
of immunoreactive corticosterone metabolites decreases
with offspring number (Scheiber et al., 2005).
H/L ratio and haematocrit generally suggest that older
individuals in a better condition were less stressed. An
effect of age on haematological parameters has previously
been shown in Little Auks (Alle alle) (Jakubas et al., 2015),
Thin-billed Prions (Pachyptila belcheri) (Quillfeldt et al.,
2008), and Southern Rockhopper Penguins (Eudyptes
chrysocome chrysocome) (Dehnhard et al., 2011) and
similar effects of body condition onto leucocyte counts
have been shown in Rufous-collared Sparrow (Zonotrichia
capensis) (Ruiz et al., 2002). However, a study in Burrowing
Parrots (Cyanoliseus patagonus) described no effect of
age but number of heterophils increased in individuals of
better body condition (Masello et al., 2009). We expected
and found that sex would not significantly affect any of
the haematological parameters we measured, because the
sampled individuals were not yet reproductively active.
This result is also in line with a previous study in adult
Greylag Geese, which does not find any differences in
haematological parameters between the sexes (Frigerio et
al., 2017). Differences between the sexes in physiological
stress response and immunological parameters are likely
triggered by differences in androgen hormones (Casto et
al., 2001), which are not present in non-reproductively
active Greylag Geese (Hirschenhauser et al., 1999).
Previous breeding success of the parents remained in
the final model of absolute leucocyte count, percentage
of monocytes and eosinophils, but did not explain a
significant amount of variation in the data. Therefore,
there would be no base to argue that fledglings stress
levels or health is significantly modulated by their parents’
previous experience in raising young. However, we do
have to interpret this result with caution, as we are dealing
with a low sample size and therefore cannot exclude that
there could be an effect of previous breeding success of
the parents in a larger sample.
In conclusion, we found evidence for a stress reducing
effect of family size in haematological parameters of fledgling
Greylag Geese. According to some of our parameters,
juveniles in larger families seemed less stressed compared
to individuals in smaller families. Future research on the
stress reducing effect of family size for juvenile Greylag
Geese and potential health benefits would be desirable.
5. ACKNOWLEDGEMENTS
We gratefully acknowledge Klara Fuereder and Nathalie
Rodenwald for helping in catching the geese in summer
2014. Blood samples were analysed at the Clinical
Pathology Platform of the University of Veterinary
Medicine in Vienna (Head Prof. Dr. I. Schwendenwein).
Financial support was provided by the FWF-Project
P21489-B17, the University of Vienna, the Verein der
Förderer der Konrad Lorenz Forschungsstelle’ and the
Herzog von Cumberland Stiftung’.
Published online: 27 September 2017
Leucocyte profiles and family size in Greylag Geese 251
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... pair-bond status, parental experience) and environmental factors (i.e. season) 39,40 . Finally, emotional social support, enjoyed by individuals in long-term social bonds, contributes to reducing long-term glucocorticoid levels and thereby may help avoid gastrointestinal diseases related to chronic stress 41 . ...
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Chapter
Over many years, greylag geese (Anser anser) have inspired much long-term scientific research. Thanks to this continuing work, our knowledge of social organisation in birds has greatly improved. Before presenting the latest findings in greylag goose research over the remainder of this book, we therefore introduce the reader to the species – its taxonomic affiliation and geographical distribution – as well as providing information about goose biology in general, and greylag goose biology in particular. This book focuses on a remarkable greylag goose flock at the Konrad Lorenz Research Station (Konrad Lorenz Forschungsstelle, abbreviated as KLF) in Grünau (Upper Austria), where much of our understanding of greylag goose biology has come together over the last 40 years. The origins of this flock, at that location, date back to the time of the late Konrad Lorenz (1903–89). We describe the KLF and also summarise the kind of research questions that can be addressed using this semi-tame goose flock, pointing out its distinctive features relative to wild goose populations, which may be relevant in the interpretation and generality of our findings. Taxonomy Together with the Galliformes, the order Anseriformes (waterfowl) belongs to one of the oldest lineages of modern (neognathous) birds. Recent evidence suggests that they originated during the Cretaceous period (Clarke et al. 2005). Although waterfowl phylogeny is still partly unresolved, one extinct (Cnemiornithidae, New Zealand geese) and three extant families are recognised: the Anhimidae (screamers), Anseranatidae (with a single representative, the magpie goose, Anseranas semipalmata) and the Anatidae, which includes over 140 species of ducks, geese and swans.
Chapter
The evolution and regulation of clutch size has long been a central issue in ornithology. Early ornithologists realized that females of each species of bird lay a characteristic number of eggs, and we have been trying to determine ever since why this is so. In pursuit of the answer to this seemingly simple question, ornithologists have not only accumulated a wealth of egg data, but also have made important contributions to such diverse topics as life-history strategies, population regulation and group selection. Yet how clutch size is determined remains a controversial issue. The consensus that was once sought in the form of a central theory (Lack, 1968; Cody, 1966; Klomp, 1970; von Haartman, 1971) has disappeared in a sea of specific hypotheses. In this review we attempt to organize and summarize clutch size theories as they emerge in modified form from recent research and evaluate their ability to explain observed patterns in clutch size variation. We concentrate on the literature and concepts published since the review of Klomp (1970), but we incorporate earlier work when necessary.