Laying date and its relationship with female body condition and breeding performance in the Imperial Shag (Phalacrocorax atriceps)

SVAGELJ, W. S.; QUINTANA, F.

Barcelona, España

Congreso; 31st Annual Meeting of the Waterbird Society; 2007

Waterbird Society, Universidad de Barcelona

It has been generally accepted that the timing of egg laying in birds results from an interaction between the evolutionary advantages of early breeding and physiological constraints operating on individual females during oogenesis. In this study we evaluated relationships between laying date and both female body condition and breeding success of Imperial Shags (Phalacrocorax atriceps) based in data collected from 625 nests monitored along three-consecutive breeding seasons (2004-2006) at Punta León colony (43º05´S, 64º30´W), Chubut, Argentina. Additionally, we estimated inter-annual repeatability in laying date and body condition for 105 individual females monitored on successive breeding attempts. Imperial Shags laid eggs between early October and early December. Although median laying date differed between breeding seasons, mean female body condition did not. Relationship between laying date and female body condition was uneven between seasons. As predicted, body condition decreased as laying date increases for 2004 breeding season, although no relationship was detected for 2005 and 2006. For all years, breeding success steadily decreased as breeding season progressed. Values of repeatability for laying date and female body condition were 0.35 and 0.28, respectively, indicating a higher degree of variation between rather than within individuals. In consequence, individual females characteristics (potentially genetic based) appears to be an important factor affecting both onset of egg-laying but also their body condition. Lack of a consistent relationship between laying date and female body condition in our data suggest that Imperial Shag´s females start to lay eggs after reaching a physiological threshold.