Vertebrate embryos are exposed to maternal hormones, possibly mediating maternal effects (hormone mediated maternal effects), which can have consequences for the offspring phenotype. Egg laying species have been used extensively to study such effects as the embryo develops inside the egg, where the mother deposits her hormones, facilitating experimental manipulations of this exposure without interfering with the mother. Bird species are a particularly suitable model since bird ecology is often well-known and can easily be studied in the field, facilitating studies of the adaptive significance of maternal effects.Several studies have injected hormones into bird eggs, mimicking hormonal variation in egg yolk, and found a wide array of effects...