Hatching asynchrony as a reproductive strategy in birds may explain the hatching failure of the last eggs of the clutch

Hatching failure, due to infertility or embryo mortality, is an important factor contributing to reduced reproductive success in birds. Although hatching failure and its possible causes have been widely investigated, the stage of development at which embryo death occurs and its association with laying order have rarely been studied. The relative size of eggs laid in different positions within the laying order is a key factor that can lead to different parental reproductive strategies, such as hatching asynchrony. Here we investigate hatching failure in relation to laying order by establishing the developmental stage of dead embryos found in unhatched European Pied Flycatcher Ficedula hypoleuca eggs and considering possible causes of failure. We found that variation in egg dimensions showed a quadratic relationship with laying order, with relatively large volumes and sizes in the first and last positions of the clutch. Egg position in the laying sequence was also related to hatching failure, with the first and last positions being more susceptible to failure. The death of embryos late in development was more likely for eggs laid later in the sequence. To our knowledge, this is the first study showing that females may be adaptively allocating more resources to last-laid eggs to avoid competitive disadvantages between siblings, a strategy that seems to fail becasue these eggs suffer greater embryo mortality.