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THE WILSON JOURNAL OF ORNITHOLOGY • Vo/. 123. No. 2. June 2011 
TABLE 4. Hatching intervals (days) of two and three-egg clutches in 2005. and three-egg clutches in 2006 and 2007 at 
Isla del Puerto. Argentina breeding colony. Values are mean ± SD with range in parentheses. 
Hatching interval (days) 
Clutch size 
A/B 
B/C 
A/C 
2005 
Two eggs 
Three eggs 
0.91 ± 0.83 (0-2) (n = 11) 
0.83 ± 0.75 (0-2) ( n = 6) 
NA 
1.33 ± 1.15 (0-2) ( n = 3) 
NA 
2.67 ± 0.58 (2-3) (n = 3) 
2006 
Three eggs 
2007 
Three eggs 
1.42 ± 0.77 (0-3) (n = 19) 
1.12 ± 1.05 (0-4) (n = 19) 
2.54 ± 1.39(1-6) (n= 13) 
2.29 ± 0.47 (2-3) (n = 13) 
3.69 ± 1.03 (2-6) (n = 13) 
3.15 ± 0.80 (2-4) (n = 13) 
NA = Not applicable. 
possible that pairs whose clutches were laid over 
several days but hatched synchronously, began 
incubation upon laying of the second egg and not 
immediately after the first egg was laid, delaying 
embryo development and eliciting some A- and 
B-eggs to hatch synchronously. 
The level of egg mortality (16.7%) and 
complete clutch loss (43.8%) reported during the 
incubation period in the study colony could have 
conservation implications through depressed 
breeding success for a vulnerable species. Spring 
tidal floodings are relatively common in the 
estuary of Bahia Blanca (Capelli and Campo 
2004) and can severely affect hatching and 
reproductive success of Olrog’s Gull by soaking 
or washing away eggs and chicks (L. F. La Sala, 
unpubl. data). However, no tidal flooding or 
extreme weather conditions were reported during 
incubation in 2005 suggesting predation of 
unattended eggs may have been substantial in 
the study colony. This assumption is supported by 
a recent report of depredation of Olrog’s Gull 
chicks by adult Kelp Gulls (La Sala and Martorelli 
2010) accompanied by a sustained growth of the 
Kelp Gull colony surrounding the studied Olrog’s 
Gull colony (P. F. Petracci, unpubl. data). The 
colony was left undisturbed in 2005 after nest 
selection and throughout the incubation period, 
but we cannot rule out investigator impact as a 
possible cause of some nest abandonment (Fet- 
terlof 1983) or interspecific aggression and 
depredation. 
The proportion of eggs hatched and young 
fledged often differ markedly in relation to nest 
location in colonies of many bird species. Some 
studies report gulls nesting in the central part of a 
colony hatch proportionately more eggs and raise 
more young than birds with nests on the periphery 
(e.g.. Dexheimer and Southern 1974), but other 
authors failed to find such differences (Ryder and 
Ryder 1981). The spatial distribution of selected 
nests in our study (peripheral and central in 200?. 
and only peripheral in 2006 and 2007) represent'' 
a potential source of bias; thus, our results of 
hatching success among years should be inter¬ 
preted with caution. 
Within and Between-clutch Egg Size Variu- 
rion .—Repeatability of egg size may be used to set 
an upper limit to the value of heritability. in 
addition to its use in assessing the reliability of 
multiple measurements of the same individual 
(Falconer 1981). Egg size varies greatly within 
many avian species with the largest eggs m J 
given population generally being at least 50‘v 
TABLE 5. Percent 
breeding colony. 
egg mortality and complete clutch loss in Olrog's Gull 
in 2005 at Isla del Puerto. Argentina 
Clutch size 
Variable 
One-egg 
Two-eggs Three-eggs 
Totals 
X 2 
Egg loss 
Complete clutch loss 
3.3 (4/120) 
26.0 (19/73) 
7.5 (9/120) 5.8 (7/120) 
13.7 (10/73) 4.1(3/73) 
16.7 (20/120) 
43.8 (32/73) 
P > 0.05 
P > 0.05 
--‘ 
