- 
246 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 2, June 2011 
TABLE 3. Olrog’s Gull egg measurements. Length (mm), maximum breadth (mm), and volume (cm 3 ) are reported for 
each egg-type in three-egg clutches from 2006 and 2007 at Isla del Puerto, Argentina breeding colony. Measurements are 
mean ± SD. Sample size 
in parentheses. 
Hatching order 
A-egg 
B-egg 
C-egg 
2006 
Length 
69.96 ± 3.36 (26) 
69.28 
± 2.96 (25) 
69.07 
± 2.60 (24) 
Breadth 
48.80 ± 1.68 (26) 
49.03 
± 1.58 (25) 
48.01 
± 1.63 (24) 
Volume 
79.44 ± 7.04 (26) 
79.44 
±7.11 (25) 
75.90 
±6.15 (24) 
2007 
Length 
73.32 ± 2.53 (24) 
70.17 
± 3.01 (23) 
69.23 
± 2.69 (23) 
Breadth 
49.02 ± 0.91 (24) 
49.12 
± 1.17 (23) 
47.94 
± 1.28 (29) 
Volume 
83.90 ± 4.28 (24) 
80.63 
± 5.33 (23) 
75.80 
± 5.38 (29) 
varied 4.2%, egg breadth 3.2%, and egg volume 
8.1% (all clutch sizes considered). Mean volume 
of each egg type and total clutch volume within 
three-egg clutches varied between 2006 and 2007 
(Table 3: egg measurements; Fig. 1: egg and 
clutch volume), The volume of A-eggs was 
4.46 cm 3 larger (P = 0.010) in 2007, but the 
volume of B- (P = 0.52) and C-eggs (P = 0.95) 
was not different between years (Figs. 1, 2). Total 
clutch volume in three-egg clutches was 6.37 cm’ 
larger (P = 0.022) in 2007 (Fig. 1). 
The results of the final model assessing the 
variables that best explained variation in egg 
volume varied (Table 2). Mean egg volume 
decreased with hatching order in three-egg 
clutches in 2006 and 2007. The magnitude of this 
change was significant in both years, and was 
more pronounced in 2007 (Fig. 2). B- and C-eggs in 
2006 were 0.11 and 3.50 cm’ smaller, respectively, 
than A-eggs, and C-eggs were 3.39 cm' smaller 
than B-eggs. B- and C-eggs in 2007 were 3.33 and 
7.73 cm 3 smaller, respectively, than A-eggs. and 
C-eggs were 4.40 cm 1 smaller than B-eggs (Fig. 2). 
The variance component models for egg size 
revealed variation in all egg measurements was 
larger among than within clutches. Egg length 
varied 56.8% among clutches and 43.2% within 
clutches, egg breadth varied 65.9% among clutches 
and 34.1% within clutches, and egg volume varied 
61.5% among clutches and 38.5% within clutches. 
Hatching. Hatching began on 5 October in 
2005 and during the last week of September in 
2007. The date of hatching onset could not be 
ascertained in 2006, because we arrived at the 
colony after hatching began. Eggs hatched 
asynchronous^ in the order they were laid in 
2005 (Spearman r = 0.855, 95% Cl: 0 . 77 - 0.91 
P > 0.001) with the exception of a few eggs (5 
pairs of A-B eggs and 1 pair of B-C eggs) which 
hatched synchronously despite having been laid 
days apart. Similarly, hatching in 2006 and 2007 
was asynchronous in most clutches with a lew 
eggs (2006: 5 pairs of A-B eggs; 2007: 5 pairs of 
A-B eggs) hatching synchronously. Hatching 
intervals between eggs varied (Table 4). 
Hatching success in three-egg clutches was 
76.9% (30/39) in 2005, 81.7% "(58/71) in 2006, 
and 91.3% (63/69) in 2007 for trend = 6,00, 
df = 4, P = 0.20). Breeding success was 2.42 
chicks per nest in 2006, and 2.74 chicks per nesi 
in 2007. Hatching and breeding success were 
likely overestimated in 2006 and 2007 due to our 
method of nest selection. Breeding success could 
not be ascertained in 2005 due to the low sample 
size of three-egg clutches in which all eggs could 
be monitored. Hatching success in 2005 was not 
different among different clutch-sizes (1 
68.4%; 2 eggs: 75.4%; 3 eggs: 76.9%; x 2 for trend 
= 6.00, df = 4, P = 0.20). Total egg loss and 
complete clutch loss in 2005 in the colony was 
similar among different clutch sizes (Table 5). 
DISCUSSION 
Quantifying the reproductive biology of species 
of conservation concern is vital for developing 
effective conservation strategies. Despite the 
existence of additional, albeit smaller reproducing 
colonies ol Olrog's Gull, comparative information 
from those colonies is not available. Therefore 
our findings cannot be completely correlated with 
existing research on this species. 
Laying, Incubation, and Hatching.— The modal 
clutch size of two eggs in 2005 agrees with report 
