La Sala et al. • OLROG’S GULL BREEDING BIOLOGY 
247 
I 
I 
B-eggs 
100 
90 
80 
70 
60 
2006 2007 
C-eggs 
Clutch volume 
FIG. 1. Distribution of egg volume for different egg types in three-egg clutches, and total clutch volume for Olrog’s 
Gull, lsla del Puerto, Argentina breeding colony. The middle 50% of the data (box). 25th and 75th percentiles (lower and 
upper hinges), median (line), mean (cross) and minimum and maximum (whiskers) are displayed by year. P-v alues show 
statistical difference between years. 
FIG. 2. Volume of Olrog’s Gull eggs at lsla del Puerto. 
Argentina breeding colony in 2006 and 2007. The effect of 
hatching order on egg volume was significant in both years 
and depended on the year of sampling, being more 
pronounced in 2007 Numbers (%) represent effect size 
statistic (CL) comparing egg volume between egg types 
sampled in 2006 (below the line) and 2007 (above the line), 
and indicate the probability that an egg randomly selected 
from the category on the left of CL will be larger than a 
randomly sampled egg from the category on the right in the 
same year. 
by other authors for some gull species (Beer 1966, 
Yorio et al. 1996), but does not correspond with 
the modal clutch size of three eggs most 
commonly reported in gulls (Lack 1968, Winkler 
and Walters 1983, Reid 1987). Selection of study 
nests in 2005 was based on the presence of one 
egg (A-egg) and were visited every second day, 
which precluded accurately estimating clutch 
initiation dates and incubation periods in that 
year. Thus, the length of incubation for A-eggs 
was likely underestimated while that for B- and C- 
eggs may have been overestimated by a maximum 
of 48 brs. 
Hatching asynchrony in all seasons led to 
broods with hatching-order-dependent age hierar¬ 
chy among siblings, as has been reported for other 
species of gulls (Graves et al. 1984, Pierotti and 
Bellrose 1986, Sydeman and Emslie 1992. Royle 
and Hamer 1998). Synchronous hatching of some 
eggs in all seasons may be more a reflection of our 
daily nest visit schedule, which affected our 
ability to detect exact hatching times, ft is also 
