The Wilson Journal of Ornithology 123(2):243—250, 2011 
BREEDING BIOLOGY OF OLROG’S GULL IN 
BAHIA BLANCA ESTUARY, ARGENTINA 
LUCIANO F. LA SALA, 1,4 ANDRES PEREZ, 2 SERGIO MARTORELLI, 1 AND JUDIT SMITS 3 
ABSTRACT.—Aspects of the breeding biology of the world largest Olrog's Gull (Lams atlanticus) colony, in the 
estuary of Bahia Blanca, Argentina, were assessed for 101, 66, and 47 nests in 2005. 2006, and 2007, respectively. Mean 
(± SD) clutch size in 2005 was 1.86 ± 0.73 eggs per nest and modal clutch size was two eggs (range = 1-3). The 
incubation period was 1.67 days longer for A-eggs than for B-eggs (27.44 ± 1.22 days vs. 25.77 ± 1.36 days, respectively; 
P < 0.001). Incubation length for C-eggs was 25.75 ± 0.96 days. The largest eggs were 31.5% (length), 21.3% (breadth), 
and 66.5% (volume) larger than the smallest eggs. Mean egg volume in 2006 and 2007 decreased with hatching order, but 
the magnitude of this change was more pronounced in 2007 than in 2006. Variation in all egg measurements was larger 
among than within clutches. Hatching success within three-egg clutches was 76.9% in 2005, 81.7% in 2006, and 91.3% in 
2007 IP = 0.20). Total egg loss in 2005 reached 16.7% and complete clutch loss was 43.8% during the incubation period. 
Parameters quantified in this study provide a comparative benchmark for future research on factors affecting breeding 
parameters in Olrog's Gull from this and other colonies, and lay the foundation for developing effective conservation 
strategies for the species. Received 20 September 2010. Accepted JO January 2011. 
Olrog's Gull {Larus atlanticus; Olrog 1958) is 
endemic to coastal wetlands along the Atlantic 
Coast of Argentina, Uruguay, and southern Brazil 
(Yorio et al. 2005). The limited geographical 
reproduction range of the species, small popula¬ 
tion size of —4,000-5,000 pairs, dietary special¬ 
ization, and high susceptibility to anthropogenic 
changes of the environment have led to its listing 
as a vulnerable species (BirdLife International 
2008). Few Olrog’s Gull breeding sites have been 
identified; all are in Argentina (Yorio el al. 2005). 
About 80% of the total breeding population is 
concentrated in the estuary of Bahia Blanca with 
the largest colony of —3,600 breeding pairs 
distributed among several subcolonies on Isla 
del Puerto (Delhey et al. 2001). 
Knowledge of the basic reproductive biology of 
seabirds of conservation concern is critical to 
recognize and identify factors affecting reproduc¬ 
tive success, such as anthropogenic environmental 
perturbations, and for developing effective con- 
Centro de Estudios Parasitol6gicos y de Vectorcs 
iCONICET-lJNLP). Calle 2 Nro. 584, 1900 La Plata, 
Buenos Aires, Argentina. 
Center for Animal Disease Modeling and Surveillance 
(CADMS), Department of Medicine and Epidemiology, 
School of Veterinary Medicine, University of California, 
Davis. CA 95616. USA; CONICET-Facultad dc Ciencias 
Vetcrinarias UNR. Boulevard, Ovidio Lagos y Ruta 33, 
2171) Castlda, Santa Fe. Argentina. 
Ecosystem and Public Health. Faculty of Veterinary 
Medicine, University of Calgary, TRW 2D20. 3280 
Hospital Drive NW, Calgary. AB T2N IN4, Canada. 
4 Corresponding author; 
e-mail: lucianolasala@yahoo.com.ar or 
lucianolasala@conicet.gov.ar 
servation strategics. Previous studies on the 
breeding biology of Olrog’s Gull have been 
limited to temporal breeding patterns (Devillers 
1977), breeding habitat requirements and selec¬ 
tion (Garcia Borboroglu and Yorio 2007a, b), and 
nest site characteristics (Yorio and Harris 1992, 
Delhey et al. 2001, Yorio ct al. 2001). A notable 
gap exists regarding other basic aspects of the 
reproductive biology of this vulnerable species. 
The objectives of our study were to provide data on 
previously undocumented life history traits of 
Olrog’s Gull including clutch size, egg measure¬ 
ments, within and among-clutch variation of egg 
size, incubation period, laying and hatching asyn¬ 
chrony, hatching success, and breeding success. 
METHODS 
Study Area .—Field work was conducted at Isla 
del Puerto, Argentina (38° 49' S, 62" 16' W) in 
2005, 2006. and 2007 during the Olrog’s Gull 
breeding season, which typically ranges from early 
September through late December. The study 
colony is on the northeastern margin of the island 
and is surrounded by a large colony of Kelp Gulls 
(Larus dominicanus I (Delhey et al. 2001). 
Data Collection .—Study nests were selected by 
convenience sampling in all three seasons, during 
the laying phase in 2005 and during late 
incubation in 2006 and 2007. Nests (« = 101) 
containing one fresh egg were selected in 2005 
(29 Sep-7 Oct) during the early egg-laying phase. 
These nests were selected from the periphery and 
inner areas of the colony and were visited eveiy 
second day until clutch completion to record 
approximate laying dates, laying order, and final 
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