La Sala el al • OLROG'S GULL BREEDING BIOLOGY 
249 
larger and, at times, twice as large as the smallest 
eggs. Approximately 70% of the variation in egg 
size is generally due to variation among, rather 
than within, clutches (repeatability) (reviewed by 
Williams 1994, Christians 2002). 
.Ml egg measurements in the studied colony 
exhibited large variation and, as expected, most of 
the phenotypic variation in egg size was attribut¬ 
able to differences among, rather than within, 
females. Repeatability of egg volume was 6.5% 
lower than the mean (68%. range = 35-92) 
reported by Christians (2002) in a review of egg 
size variation spanning 22 avian species. 
Egg size has been shown to vary systematically 
with position in the laying sequence of a clutch, 
which may relate to reproductive strategies of the 
female (Slagsvold et al. 1984). The decrease in 
egg size that occurs with laying order in many 
avian species creates a size hierarchy among 
nestlings, which is exacerbated by hatching 
asynchrony, resulting in last hatched chicks with 
lower body mass (Pierotti and Bellrose 1986. 
Magrath 1992. Williams 1994, Aparicio 1999) 
and reduced competitive abilities/higher mortality 
rates relative to their older siblings (Pierotti and 
Bellrose 1986, Bollinger et al. 1990, Sydeman and 
Emslie 1992, Stoleson and Beissinger 1995). 
Thus, studying variation of egg size both within 
and between clutches becomes an elemental, first 
step towards better interpretation of other life 
history traits such as sibling rivalry, nest-level 
mortality, and breeding success in vulnerable 
populations. 
The decrease in egg size that occurred with 
hatching order in this study created a clear size 
hierarchy among nestlings. Egg volume decreased 
along the hatching sequence both in 2006 and 
-007. The intensity of this trend depended 
strongly on the year of sampling, being much 
more pronounced in 2007. This difference, 
however, would be attributable to considerably 
larger A-eggs in 2007 compared with 2006. while 
'he volume of B- and C-eggs does not seem to 
have contributed to the inter-annual variation 
observed. 
Egg size appears to he a characteristic of 
Individual females, and increases slightly with age 
•n many species (Christians 2002). Previous work 
w »h lands has shown that older birds lay larger 
dutches, initiate breeding earlier each season, and 
hatch/fledge a greater proportion of their eggs and 
c hicks (Pyle et al. 1991, Sydeman et al. 1991). 
Mills (1979) reported egg size variation between 
seasons and presented evidence that the most 
efficient foragers and older females produce the 
largest eggs. 
There are no studies of the breeding phenology 
of Olrog’s Gull in the estuary of Bahia Blanca. 
Long-term monitoring would be necessary to 
understand inter-annual variations in egg size, 
but it is possible that in 2006 we sampled a greater 
proportion of nests of late and younger breeders 
which laid smaller eggs and, most notably, 
smaller A-eggs compared with 2007. Following 
this hypothesis, the smaller volume of three-egg 
clutches from 2006 would have been produced by 
younger, less experienced females. 
ACKNOWLEDGMENTS 
The authors especially thank Club Nautico Bahia Blanca, 
Nicolas Acosta. Joaquin Cereguetli. Martin Sotelo, Lucre- 
cia Dia/. and the Fernandez family for their unwavering 
support. We thank the anonymous reviewers whose 
suggestions resulted in substantial improvements to this 
article. This study was partly funded by Agencia Nacional 
de Promocidn CienliFtca y Tecnologica (PICT 34412/05). 
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