Avian egg size: variation within species and inflexibility within individuals

TLDR: Egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear, and the available evidence suggests that egg size may be more flexible within individuals.

Abstract: Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly understood. Egg size varies greatly within many avian species, with the largest egg in a population generally being at least 50% bigger, and sometimes twice as large, as the smallest. Generally, approximately 70% of the variation in egg mass is due to variation between rather than within clutches, although there are some cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between females, this trait is highly consistent within individuals between breeding attempts; the repeatability of egg size is generally above 0.6 and tends to be higher than that of clutch size or laying date. Heritability estimates also tend to be much higher for egg size (> 0.5) than for clutch size or laying date (< 0.5). As expected, given the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on this trait in 18 out of 28 (64%) studies. Where dietary supplements do increase egg size, the effect is never more than 13% of the control values and is generally much less. Similarly, ambient temperature during egg formation generally explains less than 15% of the variation in egg size. In short, egg size appears to be a characteristic of individual females, and yet the traits of a female that determine egg size are not clear. Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally less than 10%. Female mass and size rarely explain more than 20% of the variation in egg size within species. A female's egg size is not consistently related to other aspects of reproductive performance such as clutch size, laying date, or the pair's ability to rear young. Physiological characteristics of the female (e.g. endogenous protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size is often correlated with offspring mass and size within the first week after hatching, but the evidence for more long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude of egg-size variation within populations is often as great or greater than that observed within avian populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian species (particularly fish and turtles), it is more common for female mass or size to explain a substantial proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.

Figures

Table 6. (cont.)

Table 5. (cont.)

Fig. 1. The ratio of the size of the largest egg in the population to that of the smallest in 39 studies ; mean egg size of the clutch was used where provided. Data are from Montevecchi et al. (1983); Bancroft (1984); Birkhead (1984); Nol et al. (1984); Murphy (1986a) ; Duncan (1987); Muma & Ankney (1987); Arcese & Smith (1988); Eldridge & Krapu (1988); Ja$ rvinen & Pryl (1989); Leblanc (1989); Amundsen & Stokland (1990); Coleman & Whittall (1990); Wiggins (1990); Hendricks (1991); Martin & Arnold (1991); Pehrsson (1991); Croxall et al. (1992); Flint & Sedinger (1992); Swennen & Meer (1992); Meathrel et al. (1993a) ; Nilsson & Svensson (1993a) ; Potti (1993); Robertson & Cooke (1993); Smith et al. (1993); Arnold (1994); Simmons (1994); Amundsen (1995); Wiebe & Bortolotti (1995); Amundsen et al. (1996); Dufva (1996); Weidinger (1996); Williams (1996a) ; Williams et al. (1996); Ashkenazi & Yom-Tov (1997); Blomqvist et al. (1997); Erikstad et al. (1998); Smith & Bruun (1998); Reed et al. (1999).

Fig. 2. Proportion of variance in egg size due to variation between rather than within clutches in 26 studies. Where more than one estimate was provided per species per study, I selected the lowest. Data are from Grant (1982); Zach (1982); Ja$ rvinen & Va$ isa$ nen (1983); Bancroft (1984); Ricklefs (1984); Lank et al. (1985); Poole (1985); Redmond (1986); Rohwer (1986); Hepp et al. (1987); Muma & Ankney (1987); Galbraith (1988); Rohwer & Eisenhauer (1989); Amundsen & Stokland (1990); Coleman & Whittall (1990); Wiggins (1990); Grant (1991); Thompson & Hale (1991); Arnold (1992); Magrath (1992a) ; Swennen & Meer (1992); Nilsson & Svensson (1993a) ; Smith et al. (1993); Wiebe & Bortolotti (1995); Nol et al. (1997); Erikstad et al. (1998).

Table 4. Changes in egg size, clutch size and laying date with age (or experience, where noted) (C¯ increase with age with a decline in the oldest age classes. Other symbols are as in Table 3.)

Table 1. Repeatabilities of egg size, clutch size and laying date between breeding attempts

Full text

Biol. Rev. (2002), 77, pp. 1–26 " Cambridge Philosophical Society
DOI: 10.1017\S1464793101005784 Printed in the United Kingdom
1
Avian egg size: variation within species and
inflexibility within individuals
JULIAN K. CHRISTIANS*
Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada
(Received 3 November 2000; revised 24 July 2001 ; accepted 24 July 2001)
ABSTRACT
Egg size is a widely-studied trait and yet the causes and consequences of variation in this trait remain poorly
understood. Egg size varies greatly within many avian species, with the largest egg in a population generally
being at least 50% bigger, and sometimes twice as large, as the smallest. Generally, approximately 70% of
the variation in egg mass is due to variation between rather than within clutches, although there are some
cases of extreme intra-clutch egg-size variation. Despite the large amount of variation in egg size between
females, this trait is highly consistent within individuals between breeding attempts ; the repeatability of egg
size is generally above 0n6 and tends to be higher than that of clutch size or laying date. Heritability estimates
also tend to be much higher for egg size ( 0n5) than for clutch size or laying date ( 0n5). As expected, given
the high repeatability and heritability of egg size, supplemental food had no statistically significant effect on
this trait in 18 out of 28 (64 %) studies. Where dietary supplements do increase egg size, the effect is never
more than 13% of the control values and is generally much less. Similarly, ambient temperature during egg
formation generally explains less than 15 % of the variation in egg size. In short, egg size appears to be a
characteristic of individual females, and yet the traits of a female that determine egg size are not clear.
Although egg size often increases with female age (17 out of 37 studies), the change in egg size is generally
less than 10%. Female mass and size rarely explain more than 20% of the variation in egg size within species.
A female’s egg size is not consistently related to other aspects of reproductive performance such as clutch size,
laying date, or the pair’s ability to rear young. Physiological characteristics of the female (e.g. endogenous
protein stores, oviduct mass, rate of protein uptake by ovarian follicles) show more promise as potential
determinants of egg size. With regards to the consequences of egg-size variation for offspring fitness, egg size
is often correlated with offspring mass and size within the first week after hatching, but the evidence for more
long-lasting effects on chick growth and survival is equivocal. In other oviparous vertebrates, the magnitude
of egg-size variation within populations is often as great or greater than that observed within avian
populations. Although there are much fewer estimates of the repeatability of egg size in other taxa, the
available evidence suggests that egg size may be more flexible within individuals. Furthermore, in non-avian
species (particularly fish and turtles), it is more common for female mass or size to explain a substantial
proportion of the variation in egg size. Further research into the physiological basis of egg-size variation is
needed to shed light on both the proximate and ultimate causes of intraspecific variation in this trait in birds.
Key words: age, egg mass, food, intraspecific variation, optimal egg size theory, phenotypic plasticity,
physiology, reproductive performance, temperature.
CONTENTS
I. Introduction ............................................................................................................................ 2
II. Intraspecific variation in egg size ............................................................................................ 3
* Address for correspondence : Julian K. Christians, Institute of Cell, Animal and Population Biology, University of
Edinburgh, Ashworth Laboratories, King’s Buildings, West Mains Road, Edinburgh, EH9 3JT, UK. Tel.: j44 (0)131
650 7334; fax : j44 (0)131 650 6564; e-mail: julian.christians!ed.ac.uk

2 Julian K. Christians
III. Environmental effects on egg size............................................................................................ 6
(1) Food supplementation....................................................................................................... 6
(2) Temperature ..................................................................................................................... 6
IV. Female characters .................................................................................................................... 6
(1) Age and experience........................................................................................................... 6
(2) Female mass and size ........................................................................................................ 9
(3) Reproductive performance................................................................................................ 9
(4) Other aspects of female phenotype ................................................................................... 11
V. Proximate considerations ......................................................................................................... 14
(1) What determines egg size? ................................................................................................ 14
(2) Lack of plasticity within females ....................................................................................... 15
VI. Ultimate considerations ........................................................................................................... 15
(1) Consequences for offspring fitness ..................................................................................... 15
(2) Consequences for maternal fitness..................................................................................... 15
VII. Patterns of egg-size variation in other taxonomic groups ........................................................ 16
(1) Other oviparous vertebrates.............................................................................................. 16
(2) Arthropods ........................................................................................................................ 18
VIII. Conclusions.............................................................................................................................. 18
IX. Acknowledgements .................................................................................................................. 19
X. References................................................................................................................................ 19
I. INTRODUCTION
How much should parents invest in each of their
progeny? Some life-history models predict that there
is an optimum amount of effort that organisms
should invest in each offspring (e.g. Smith &
Fretwell, 1974 ; McGinley, Temme & Geber, 1987).
However, propagule size, one component of the
effort expended per offspring, often shows tremen-
dous variation within species (Bernardo, 1996).
Intraspecific variation in propagule size could be
due to differences in optima between individ-
uals and environments, or to temporal changes in
the optimum effort-per-offspring. Alternatively,
nutritional or physical limitations might constrain
the amount of resource invested in each offspring
(e.g. Congdon & Gibbons, 1987). However, to date
the ubiquitous variation in propagule size within
populations remains unexplained (Bernardo, 1996).
The development of better models of parental
investment in offspring will require knowledge of the
patterns of variation within species (e.g. whether
variation occurs within or between individuals;
whether variation is related to resource availability),
and of the aspects of maternal phenotype that
influence propagule size (Bernardo, 1996). In these
respects, the ornithological literature provides a rich
resource. Perhaps more than in any other taxonomic
group, most aspects of egg-size variation have been
extensively documented : variation within and be-
tween clutches, repeatability between clutches
produced by an individual female, heritability,
response to food supplementation, correlations with
female phenotypes, and correlations with offspring
growth and survival. In this review, I examine the
patterns of intraspecific egg-size variation within
avian populations and draw attention to (a) the
large intraspecific variation in egg size within avian
populations, (b) the lack of plasticity in egg size
within individual females, and (c) our lack of
understanding of the proximate or ultimate causes of
intraspecific egg-size variation.
I begin by summarizing the variation in egg size
within avian populations, as well as repeatability
and heritability estimates of this trait. Repeatability
of egg size could be due to the repeatability of
environmental conditions within individuals (e.g.
food supply), and so I review the effects of
environmental factors (food and temperature) on
egg size. Next, I summarize relationships between
female characteristics (e.g. age, size) and egg size,
giving special attention to physiological traits of the
female that, although less intensively studied, are
crucial to understanding variation in reproductive
performance (Bernardo, 1996). The proximate
causes and ultimate consequences of intraspecific
egg-size variation are then considered. Finally, to
assess the extent to which the observed patterns are
unique to birds, I briefly review intraspecific egg-size
variation in other taxonomic groups.
I have not included studies of domesticated species
in this review because the causes of intraspecific
variation present in natural populations may have
been obscured or eliminated by the long history of

3Intraspecific egg-size variation in birds
artificial selection on reproductive traits. However,
studies of captive, non-domesticated birds have been
included since such populations commonly exhibit
patterns of egg-size variation similar to those
observed in nature (e.g. Williams, 1996a). Due to
the enormous number of studies that have reported
data on avian egg size, I have focused on variation in
egg size between individuals, and generally do not
consider the pattern of variation within clutches
since this variation is small compared to the variation
between clutches (see below) and has been reviewed
elsewhere (Slagsvold et al., 1984). Throughout, I use
the term egg size to refer to egg mass or volume,
which are highly correlated (generally r
#
 0n8 ; e.g.
Duncan, 1987 ; Reid & Boersma, 1990 ; Arnold,
1992; Magrath, 1992 a; Meathrel et al., 1993 a;
Smith, Ottosson & Ohlsson, 1993; Nol, Blanken &
Flynn, 1997 ; Vin
4
uela, 1997 ; but see Flint & Grand,
1999); where authors have provided data on both
mass and volume, I use the former. Although I do
not explicitly consider variation in egg quality, fresh
egg mass is generally correlated with the dry
20
18
16
14
12
10
8
6
4
2
0
<1.40 1.40–1.59 1.60 –1.79 1.80 –1.99
>1.99
Ratio of largest egg to smallest egg
Number of studies
Fig. 1. The ratio of the size of the largest egg in the
population to that of the smallest in 39 studies; mean egg
size of the clutch was used where provided. Data are from
Montevecchi et al. (1983) ; Bancroft (1984); Birkhead
(1984); Nol et al. (1984); Murphy (1986a) ; Duncan
(1987); Muma & Ankney (1987); Arcese & Smith
(1988); Eldridge & Krapu (1988); Ja
$
rvinen & Pryl
(1989); Leblanc (1989) ; Amundsen & Stokland (1990) ;
Coleman & Whittall (1990); Wiggins (1990) ; Hendricks
(1991); Martin & Arnold (1991); Pehrsson (1991) ;
Croxall et al. (1992); Flint & Sedinger (1992); Swennen
& Meer (1992); Meathrel et al. (1993 a); Nilsson &
Svensson (1993 a); Potti (1993); Robertson & Cooke
(1993); Smith et al. (1993); Arnold (1994); Simmons
(1994); Amundsen (1995) ; Wiebe & Bortolotti (1995);
Amundsen et al. (1996) ; Dufva (1996); Weidinger (1996) ;
Williams (1996a) ; Williams et al. (1996); Ashkenazi &
Yom-Tov (1997); Blomqvist et al. (1997) ; Erikstad et al.
(1998); Smith & Bruun (1998) ; Reed et al. (1999).
components of the yolk and albumen (e.g. St. Clair,
1996; Kennamer, Alsum & Colwell, 1997 ; Flint &
Grand, 1999) and thus is a good measure of quality
in terms of macronutrient composition (see Williams,
1994 and Hill, 1995 for reviews).
II. INTRASPECIFIC VARIATION IN EGG SIZE
Within many species of birds, there is a large range
in egg size. The largest egg in a population is
generally at least 50% bigger, and sometimes twice
as large, as the smallest (Fig. 1). This variation is also
seen in species with one-egg clutches, where egg size
is the only means by which females can adjust the
amount of nutrients and energy invested in eggs (e.g.
Montevecchi et al., 1983 ; Croxall, Rothery & Crisp,
1992; Meathrel et al., 1993 a; Amundsen, 1995 ;
Amundsen, Lorentsen & Tveraa, 1996; Weidinger,
1996). Generally, approximately 70% of the vari-
ation in egg size is due to variation between rather
than within clutches (Fig. 2). Notable exceptions to
this pattern are found in the crested penguins
(Eudyptes spp.) that exhibit extreme egg-size di-
morphism, with differences of 30–60 % between eggs
within a clutch (Williams, 1990; St. Clair, 1996).
10
8
6
4
2
0
<50 50–59 60–69 70–79
>79
Proportion of variance in egg size due to
variation between clutches
Number of studies
Fig. 2. Proportion of variance in egg size due to variation
between rather than within clutches in 26 studies. Where
more than one estimate was provided per species per
study, I selected the lowest. Data are from Grant (1982);
Zach (1982); Ja
$
rvinen & Va
$
isa
$
nen (1983); Bancroft
(1984); Ricklefs (1984) ; Lank et al. (1985) ; Poole (1985);
Redmond (1986) ; Rohwer (1986); Hepp et al. (1987);
Muma & Ankney (1987) ; Galbraith (1988) ; Rohwer &
Eisenhauer (1989); Amundsen & Stokland (1990) ;
Coleman & Whittall (1990); Wiggins (1990); Grant
(1991); Thompson & Hale (1991); Arnold (1992) ;
Magrath (1992 a); Swennen & Meer (1992) ; Nilsson &
Svensson (1993 a); Smith et al. (1993); Wiebe & Bortolotti
(1995); Nol et al. (1997) ; Erikstad et al. (1998).

4 Julian K. Christians
Table 1. Repeatabilities of egg size, clutch size and laying date between breeding attempts
Latin name Common name Repeatability Reference
Egg size
Aegolius funereus Tengmalm’s Owl 0n51 Hakkarainen & Korpima
$
ki (1994)
Anas acuta Northern Pintail 0n61 Duncan (1987)
Anas acuta Northern Pintail 0n89 Flint & Grand (1996)
Anas platyrhynchos Mallard 0n62 Batt & Prince (1979)
Aythya marila Greater Scaup 0n36 Flint & Grand (1999)
Branta bernicla nigricans Black Brant 0n78 Flint & Sedinger (1992)
Branta canadensis Canada Goose 0n92 Leblanc (1989)
Charadrius semipalmatus Semipalmated Plover 0n68 Nol et al. (1997)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n80 Lessells et al. (1989)
Falco sparverius American Kestrel 0n71 Wiebe & Bortolotti (1995)
Ficedula hypoleuca Pied Flycatcher 0n61 Potti (1993)
Haematopus palliates American Oystercatcher 0n35 Nol et al. (1984)
Lagopus lagopus scoticus Red Grouse 0n73–0n77 Moss & Watson (1982)
Parus major Great Tit 0n58–0n72 Noordwijk (1987)
Parus major Great Tit 0n64 Ho
4
rak et al. (1995)
Parus major Great Tit 0n68 Dufva (1996)
Stercorarius parasiticus Arctic Skua 0n63, 0n73 Phillips & Furness (1998)
Sturnus vulgaris European Starling 0n76 Smith et al. (1993)
Sturnus vulgaris European Starling 0n76 Christians & Williams (2001b)
Tachycineta bicolor Tree Swallow 0n77 Wiggins (1990)
Taeniopygia guttata Zebra Finch 0n74 Williams (1996a)
Tringa tetanus Redshank 0n73, 0n87 Thompson & Hale (1991)
Clutch size
Anas platyrhynchos Mallard 0n54 Batt & Prince (1979)
Branta bernicla nigricans Black Brant 0n14 Flint & Sedinger (1992)
Bucephala albeola Bufflehead 0n55 Gauthier (1989)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n26 Findlay & Cooke (1987)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n15 Lessells et al. (1989)
Falco tinnunculus Kestrel 0n19 Meijer et al. (1988)
Geospiza fortis Darwin’s Medium
Ground Finch
0n08 Gibbs (1988)
Parus major Great Tit 0n51 Perrins & Jones (1974)
Parus major Great Tit 0n30–0n54 Noordwijk (1987)
Parus major Great Tit 0n86 Dufva (1996)
Taeniopygia guttata Zebra Finch 0n59 Williams (1996a)
Laying date
Accipiter nisus Eurasian Sparrowhawk 0n18, 0n26
a
Newton & Marquiss (1984)
Anas platyrhynchos Mallard 0n57 Batt & Prince (1979)
Bucephala albeola Bufflehead 0n57 Gauthier (1989)
Charadrius semipalmatus Semipalmated Plover 0n00
b
Nol et al. (1997)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n22 Hamann & Cooke (1989)
Falco tinnunculus Kestrel 0n00 Meijer et al. (1988)
Fulica atra European Coot 0n32 Perdeck & Cave
!
(1992)
Haematopus palliates American Oystercatcher 0n73 Nol et al. (1984)
Parus major Great Tit 0n19–0n44 Noordwijk (1987)
Stercorarius parasiticus Arctic Skua k0n16, 0n50 Phillips & Furness (1998)
Uria aalge Common Murre 0n20 Sydeman & Eddy (1995)
a
Corrected values from Lessells & Boag (1987).
b
Repeatability of clutch completion date, not laying date.

5Intraspecific egg-size variation in birds
Table 2. Heritability estimates for egg size, clutch size and laying date
(NS l heritability estimate was not significantly different from zero and was not provided.)
Latin name Common name Heritability Reference
Egg size
Anas acuta Northern Pintail NS Duncan (1987)
Anas platyrhynchos Mallard 0n55 Prince et al. (1970)
Branta leucopsis Barnacle Goose 0n67 Larsson & Forslund (1992)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n53 Lessells et al. (1989)
Ficedula hypoleuca Pied Flycatcher NS Potti (1993)
Ficedula hypoleuca Pied Flycatcher 0n55 Potti (1999)
Lagopus lagopus scoticus Red Grouse 0n66 Moss & Watson (1982)
Parus major Great Tit 0n86 Ojanen et al. (1979)
Parus major Great Tit 0n66–0n86 Noordwijk (1987)
Parus major Great Tit 0n81 Ho
4
rak et al. (1995)
Clutch size
Anas platyrhynchos Mallard 0n46 Prince et al. (1970)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n20 Findlay & Cooke (1987)
Chen caerulescens
caerulescens
Lesser Snow Goose 0n17 Lessells et al. (1989)
Ficedula albicollis Collared Flycatcher 0n33 Schluter & Gustafsson (1993)
Ficedula albicollis Collared Flycatcher 0n35 Merila & Sheldon (2000)
Geospiza fortis Darwin’s Medium
Ground Finches
NS Gibbs (1988)
Parus major Great Tit 0n48
a
Perrins & Jones (1974)
Parus major Great Tit 0n25–0n50 Noordwijk (1987)
Sturnus vulgaris European Starling 0n34 Flux & Flux (1982)
Laying date
Accipiter nisus Eurasian Sparrowhawk NS Newton & Marquiss (1984)
Ficedula albicollis Collared Flycatcher 0n41 Merila & Sheldon (2000)
Fulica atra Coot 0n02 Perdeck & Cave
!
(1992)
Parus major Great Tit k0n08–0n45 Noordwijk (1987)
a
Cited in Hailman (1986).
Despite such extreme dimorphism, the masses of the
first- and second-laid eggs are correlated within
clutches in the Macaroni Penguin (E. chrysocome;
Williams, 1990), i.e. females still differ from one
another in the size of egg they produce.
Egg size is highly consistent within individual
females between breeding attempts. Repeatability is
a measure of ‘the proportion of variance in a
character that occurs among rather than within
individuals’ (Lessells & Boag, 1987 : p. 116; see also
Falconer & Mackay, 1996) and is generally higher
for egg size ( 0n6) than for clutch size or timing of
egg laying ( 0n6; Table 1), as observed by Boag &
Noordwijk (1987) and Lessells, Cooke & Rockwell
(1989). Based on the data from Table 1, the mean
repeatabilities for egg size, clutch size and timing of
laying are 0n68, 0n40 and 0n35, respectively. Even in
the Macaroni Penguin, a species that shows extreme
egg-size variation within clutches, the masses of the
first- and second-laid eggs are positively correlated
with the respective egg masses of the same female in
the subsequent year (r
#
l 0n27–0n67 ; Williams &
Croxall, 1991).
Estimates of heritability, the proportion of pheno-
typic variation due to additive genetic variance
(Falconer & Mackay, 1996), also tend to be higher
for egg size ( 0n5) than for clutch size or timing of
egg laying ( 0n5 ; Table 2) (see also Hailman,
1986; Boag & Noordwijk, 1987; Lessells et al., 1989).
Based on the data from Table 2, the mean
heritabilities for egg size, clutch size and timing of
laying are 0n66, 0n35 and 0n24, respectively. It should
be noted that there are a number of potential
problems associated with measuring heritability in

Frequently asked questions

Q1. How did the anti-estrogen tamoxifen reduce egg size?

The anti-estrogen tamoxifen also reduced egg size (15%), perhaps byreducing the plasma concentration of yolk precursors below the level required to maintain normal yolk formation (Williams, 2000). 

Q2. What is the effect of egg size on offspring fitness?

The consequences of egg size for offspring fitness also tend to be more apparent inoviparous vertebrates ( fish : Heath & Blouw, 1998; Einum & Fleming, 1999, 2000; amphibians : Kaplan, 1998; lizards : Sinervo et al., 1992), probably because there is less parental care in these species. 

Q3. What is the effect of age on the size of the eggs?

In some species, egg size increases from young to intermediate ages and then decreases, the greatest changes being 12–13% (Sydeman & Emslie, 1992; Weimerskirch, 1992), whereas in others only a decline with age is evident (Reid, 1988; Potti, 1993). 

Q4. What was the effect of handicapping on the size of the female?

handicapping reduced clutch size but had little (2–6%) or no effect on egg size (Slagsvold & Lifjeld, 1988, 1990; Winkler & Allen, 1995), even though there was some evidence that feather clipping reduced female condition as measured by breastmuscle thickness (Winkler & Allen, 1995). 

Q5. What is the reason for the high heritability of egg size?

the high heritability of egg size could be due to relatively high additive genetic variance or relatively low environmental and non-additive genetic effects compared to other life-history traits. 

Q6. What is the role of female size and/or mass in determining egg size in fish?

Female size and}or mass is also believed to play an important role in determining egg size in fish, and the variation between females is often attributed to differences to growth history (Morita et al., 1999; Berg et al., 2001; see also references in Elliott & Hurley, 1998). 

Q7. Why do the authors have no clues as to what aspects of female phenotype would determine?

For instance, while variation could be due to differences in optimum egg size between indi-viduals, the authors have no clues as to what aspects of female phenotype would determine this optimum, or why optima would vary greatly between females but not between breeding attempts within a female. 

Q8. Why did Hipfner et al. find a negative relationship between egg size and?

Decreases in egg size with laying date may be due to younger birds laying smaller eggs later (e.g. Hipfner et al., 1997; Vin4 uela, 1997), rather than an effect of laying date per se. 

Q9. How many studies found a seasonal decline in egg size?

In those that did find a statistically significant effect, a seasonal decline in egg size was observed frequently (20 studies), but only six studies found an increase in egg size throughout the season. 

Q10. How much of the variation in egg size was explained by supplemental food?

supplemental food increased egg size in only one-third of the studies examined, and this effect was always small (! 15%) compared to the large variation between females. 

Q11. Why is the egg size variation in avian species unclear?

the available evidence suggests that in other oviparous vertebrates and arthropods egg size may be more flexible within individuals. 

Q12. What is the effect of age on egg size in fish?

Much of this variation is an effect of maternal age … but in many arthropods there is substantial variation in egg size within individual clutches of eggs ’’ (Fox & Czesak, 2000: p. 355). 

Q13. How many studies found that supplemental food increased egg size?

Considering only statistically significant effects, 36% of studies (10}28) found that supplemental food or enhanced food quality increased egg size (Table 3).