In long-lived seabirds, intermittent nonbreeding years in an adult life, the so called ‘sabbatical years’, are common. However, proximal causes that make animals decide whether to breed or not to breed are poorly known. In this study we assessed different components of the physiological state of a long-lived pelagic seabird, the Cory's shearwaters Calonectris diomedea, during the pre-laying period to understand which components are influencing breeding decisions. We analyzed plasma biochemistry, haematocrit, leukocyte counts and stable isotope (δ15N and δ13C) values in blood and claws. We also recorded body mass, ectoparasite loads and fault bars in primary and tail feathers. All parameters were compared between breeders and sabbaticals and between males and females. Among all biochemical variables analyzed we found triglyceride levels in plasma to be significantly greater in breeders than in sabbaticals and in females than males. H/L ratio was lower in breeders than in sabbaticals and greater in males than females. These results suggest the ability to store fat and the immunological condition are important components compromising breeding decisions. Our study also suggests that H/L ratio can be used as a good predictor of the breeding decisions

J. Navarro was supported by a postgraduate grant of the Ministerio de Educación y Ciencia (MEyC) of Spain and J. González-Solís was supported by a contract of the Program Ramon y Cajal funded by the MEyC and Fondos FEDER. Financial support was provided by the projects REN2002-01164/GLO, 2005GR00744 and the Fundación Banco Bilbao Vizcaya-Argentaria (FBBVA 2005)

5 pages, 3 figures, 2 tables

Peer reviewed