LOVE, O.P.*; WILLIAMS, T.D.; Simon Fraser University, British Columbia, Canada; Simon Fraser University, British Columbia, Canada: Corticosterone provides a link between maternal condition and sex-specific adjustment of offspring quality

Sex allocation theory predicts that females in better condition are expected to invest in the more expensive sex (most often sons), while those in poor condition should invest in the less expensive sex (most often daughters). Accumulating evidence strongly indicates that female birds can sex-specifically adjust the quality of their offspring in relation to their own maternal condition, potentially allowing them to fine-tune the quantity and quality of offspring to prevailing environments. Although few physiological mechanisms have yet been proposed as to how mothers might achieve this, glucocorticoids may provide a link. Using a wild colony of sexually-size dimorphic European starlings (Sturnus vulgaris), we manipulated corticosterone (CORT) levels in pre-laying females during 2002 and 2003 to examine the potential role of this stress hormone in shaping offspring quality in relation to sex. Elevated levels of CORT may be considered a sign of poor maternal condition since food-restriction and low body condition are correlated with elevated CORT levels in wild birds. Preliminary analysis has revealed that male offspring of CORT-implanted mothers hatch at significantly lower masses and grow more slowly than males from sham-implanted mothers, while female siblings are unaffected. Males may be differentially affected by poor maternal condition because in our colony male starlings grow faster and fledge at a larger structural size and mass than their female siblings. We suggest that CORT may act as a physiological signal enabling mothers to fine-tune the quantity and quality of offspring to prevailing environments in order to maximize their fitness. The potential role of CORT as a mediator between maternal condition and adaptive sex-ratio adjustment will be discussed.