Breeding biology of Olrog’s Gull in the Bahía Blanca Estuary, Argentina

LA SALA L.; PEREZ A.; MARTORELLI S.R.; SMITS J.

WILSON JOURNAL OF ORNITHOLOGY

WILSON ORNITHOLOGICAL SOC

Año: 2011 vol. 123 p. 243 - 250

1559-4491

Aspects of the breeding biology of the world largest Olrog’s Gull (Larus atlanticus) colony, which is located in the estuary of Bahía Blanca, Argentina, were assessed for 101, 66, and 47 nests in 2005, 2006, and 2007, respectively. Mean clutch size in 2005 was 1.86 ± 0.73 eggs per nest and modal clutch size was 2 eggs (range: 1-3). The incubation period was longer in A-eggs (27.26 ± 1.10 days) than in B-eggs (25.70 ± 1.03 days) in two-egg clutches but no difference between A-eggs and B-eggs was observed in three-egg clutches. Incubation for C-eggs was 25.75 ± 0.96 days. Laying and hatching orders were strongly correlated (Spearman r = 0.855, P < 0.001). Largest eggs were 31.5% (length), 21.3% (breadth) and 66.5% (volume) larger than smallest eggs. Egg volume was better predicted by egg maximum breadth than by egg length (r2 = 0.732, P < 0.001). In 2006 and 2007, mean egg volume decreased along the hatching sequence, 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. Variation among clutches was 53.8% for egg length, 70.0% for egg breadth and 61.6% for egg volume. Hatching success within three-egg clutches was 76.9% in 2005, 81.7% in 2006 and 91.3% in 2007. This trend was not significant (P = 0.20). In 2005, total egg loss reached 16.7% and complete clutch loss was 43.8% throughout the incubation period. Parameters quantified in this study provide a first comparative benchmark for future research on factors affecting breeding parameters in Olrog’s Gull from this and other colonies, and lay the foundations for developing effective management strategies for the species.