Mothers under stress? Hatching sex ratio in relation to maternal baseline corticosterone in the common tern (Sterna hirundo)

Sex ratio of progeny should be balanced if costs and benefits of rearing sons and daughters are equal. However, shifts in sex ratio have been demonstrated across bird species and it was suggested that females are able to adjust the primary sex ratio. One possible mechanism is the glucocorticoid corticosterone which rises under stressful conditions and can be deposited into egg yolk by mothers. We analysed primary sex ratio of common terns Sterna hirundo from 2006 to 2008 and related it to maternal baseline corticosterone level, laying date and year. Therefore, we took 101 blood samples of 71 breeding females via blood sucking bugs, a method with negligible stress for the birds. Sex ratio did not differ from parity in any of the analysed years, which were characterized by poor food availability and breeding success. Only within 1 year there was a tendency for more females in the last hatched chick. Neither corticosterone level nor laying date or year showed an influence on hatching sex ratio. The negative result concerning primary sex ratio and maternal baseline corticosterone level might suggest conditions to be good enough for mothers to prevent them from depositing high levels of corticosterone into eggs.