Why parent birds play favourites

Most birds hatch their broods asynchronously, which leads to reduced growth and higher mortality of last-hatched nestlings1,2,3. Why parent birds confer handicaps on some of their progeny, and advantages on others, remains controversial, although a consensus is emerging that there is no single adaptive explanation2,3. Our studies of red-winged blackbirds provide confirmation of an insurance hypothesis4,5,6,7,8,9 where the marginal offspring created by hatching asynchrony serve as replacements for failed earlier-hatched or ‘core’ offspring.