It takes exactly 3 weeks to hatch chickens. This timing is incredibly consistent: once you place chicken eggs in a warm, plastic incubator, precisely 21 days later you have cute baby chicks. In the wild, no such plastic box exists to heat eggs, so instead mother hens ‘brood’ – they sit atop their eggs to warm them with body heat, and the eggs hatch exactly 21 days later. In other bird species, too, incubation periods are precise enough for mothers to do some family planning. Birds lay one egg at a time, usually over the course of a couple days. Sometimes, mothers don't heat the eggs until they're all laid; they incubate them in a batch, so all hatchlings emerge on the same day. But other birds, such as blackbirds (Turdus merula), sometimes incubate each egg as soon as it comes out, so incubation times are staggered and some siblings are older than others. However, older siblings steal all the food, which means the youngest hatchlings often starve and die. Still, many birds plan their families this way, incubating eggs at different times and leaving the youngest to suffer. Why would a mother willingly be so cruel?
Biologists once believed this was a strategy to save food in hard times: starve the youngest so the oldest can eat. But Juan José Soler and colleagues (Universidad de Grenada, Spain) weren't convinced – after all, mothers still feed the dying hatchling for over a week, wasting precious food. Instead, Soler's team questioned whether parents act differently with a hungrier mouth to feed – maybe a begging baby bird would motivate the father to gather more food?
To investigate, Soler's team watched blackbird nests outside throughout the spring breeding season. When eggs started to hatch, the researchers gingerly redistributed baby birds among the nests so that each bird family was structured the same: a father, a mother and four hatchlings. But in some families, Soler and colleagues substituted a hatchling that was a couple days younger than the rest. The lab tracked the food each parent brought back, tallying every insect fed to the chicks. It turns out all the families collected the same amount of food, but parents worked differently when there was a younger nestling. Mothers worked hardest when all the siblings were the same age, but they provided less food when there was a younger nestling constantly begging to be fed. Fathers took up the slack, diligently collecting food for the family.
Soler and team also tracked where the food went by labelling each nestling with fluorescent markers and tallying how much they ate and which parent fed them. Parents played favourites. Fathers fed the largest nestling but neglected the small adoptee. Mothers spread food more evenly, but even so, they underfed the runt of the litter. As Soler and colleagues underline, mothers fed the youngest hatchling only enough to prolong its life, but not enough to save it. And as long as there was a hungry and vocal hatchling crying out for food, fathers worked much harder to collect food for the family.
It is unlikely such a cruel egg-hatching strategy is without benefit. Soler's team points out blackbirds breed often, and females must avoid burnout if they're planning on a new family. So, while blackbirds condemn their youngest to death, it isn't in vain. These nestlings are sacrificed so their mothers might foster a new nest.