Acute stress affects the corticosterone level in bird eggs: a case study with great tits (Parus major)

In vertebrates, stress experienced by mothers during the early stages of reproduction is an important source of epigenetic modifications in their offspring. Birds represent excellent models to test such effects as their maternal investment can be quantified in terms of egg quality. Recently, it has been demonstrated that corticosterone (CORT) can be transmitted from a female bird into its eggs. However, there is little published evidence about maternal effects that are mediated by acute stress. In this study, we demonstrated that female great tits Parus major facing an aerial predator during egg formation increased CORT concentration in eggs that were laid the morning after the treatment. By presenting a predator model to each experimental nest twice a day, we found that maternal stress influences corticosterone content in eggs during a time period from albumen production in the magnum until the initial phase of shell secretion, when additional water is added to the egg in the shell gland. We also found a positive correlation between the duration of parental alarm calls and CORT concentration in eggs. In conclusion, the response of female passerines to predatory cues brings about a maternal effect that may have negative consequences for offspring performance. We also suggest that the total duration of the behavioral response to a stressor is an important determinant of CORT levels in the albumen of bird eggs.