Nest Negotiations: Why Owls Groom Siblings for Food

Juvenile birds jammed together in nests compete furiously for food. Firstborn nestlings shove their weaker siblings aside as they crane their necks to beg from their parents. Sometimes, all the jostling leads to the younger ones accidentally falling out. However, the older and stronger offspring of some birds aren’t above getting rid of the competition. But the young of one species are a great deal more considerate.

At nesting time, barn owls lay as many as nine white oval eggs once every two to three days, which hatch at staggered intervals. The eldest nestling can be as much as a month older than the youngest. Until the firstborn is between two and three weeks old, the mother stays with them, enveloping the naked chicks with her warmth, while the father fetches food for the family. She rips his offerings into pieces and stuffs them into the hungry mouths.

As the growing nestlings sprout white downy feathers, they can control their body temperature and don’t need their mother to keep them warm anymore. She leaves them home alone to share her partner’s burden of keeping the growing chicks well-fed.

Many years ago, my husband and I reared a barn owlet which looked nothing like the beautiful adults with their jet-black round eyes set in a heart-shaped white face. Being the runt, it had been abandoned by its parents. The bare-faced chick was the ugliest baby I had ever seen. Every time we approached its cardboard box nest, it raised a screeching din that quietened only after one of us jammed a piece of meat into its gullet. Since it was an only nestling, we didn’t realise then the complex family life of the species.

Throughout the afternoon, the nestlings preen each other. The younger ones comb the feathers of their older siblings’ head, neck and back, removing ticks and mites from those hard-to-reach spots. Although the larger chicks don’t return the favour, they groom others closer to their age.

I tried to spend time with the barn owlet, but it had no interest in me. Once all the meat disappeared, it ignored me, staring somewhere else. When I made a sound or talked to it, it swivelled its head and blinked at me as if surprised I was still there. That haughtiness put paid to any illusion I belaboured of being a companion.

In the wild, young barn owlets rush towards the parent when it brings the first meal of the night. Neither of the adults cuts up the fare into bite-sized pieces as the mother did when she was nest-bound. Since they bring whole rodents, one at a time, whom should they feed? The older barn owl nestlings take the initial feeds, but this is where the similarity with other bird species ends. They then step aside to let their younger siblings get their share.

Between parental visits, the other owlets engage in noisy chatter. Scientists say they “negotiate” whose turn it is to receive the next meal. The hungriest one lets the others know how famished it is by consistently making long hissing calls. When a parent arrives with food, the hungry nestling positions itself at the front of the nest and screeches to be fed while its brothers and sisters hang back and allow it to get priority.

Not only do barn owlets negotiate with their siblings, but also engage in barter. Researchers from France, Italy, and Switzerland supplemented the diet of approximately 130 young birds in nearly 30 nests with two white mice each. Instead of feeding every birdling, they left the rodents inside the chamber. The parents also delivered prey to the youngsters as usual. After eating their fill, the larger ones decided which of their siblings to reward with the surplus food. They picked up a mouse with their beaks and laid it at the feet or fed the one that had preened them.

This barter works for both parties, say the researchers. The hungry younger ones get prey while the older nestlings, which are infested with parasites from having lived longer within the confines of the nest, receive an essential service at no extra cost to themselves.

This food-for-preening exchange appears to occur more often in nests of single parents. With many bellies to fill, the hard-working adults cannot waste time figuring out who to feed next. So they deliver the rodent prey to the older offspring and set off to hunt.

The younger birds, however, weren’t held to ransom. They didn’t starve even if they didn’t curry favour by scratching the older ones’ backs. The firstborns responded to their calls of hunger and fed them anyway. Every one of the over 100 birdlings tried either preening or calling, but not both at the same time.

“We do not know why,” says Andrea Romano of the University of Lausanne, Switzerland, and one of the researchers who studied the barter system. She hypothesises that grooming others and calling “might be two distinct, and maybe mutually exclusive, strategies to obtain food from a sibling”.

Tending to others is a common behaviour among non-human primates, bats and courting adult birds. But researchers know of no other species whose nestlings behave in this manner.

When there’s a food shortage, however, things turn tragic as older barn owlets can kill and eat weaker siblings. The terrible recourse at least improves the chances of the seniors’ survival. The younger ones can’t negotiate and barter their way out of that fate.