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Biology for the Global Citizen

Spreading the Wealth – Barn Owl Chicks

by George Shiflet
Copyright © 2023 George Shiflet. All rights reserved.

If you have siblings, you may have experienced some competition and conflict as you grew up. Sibling rivalry occurs in human families, but it also occurs in other animals. In many bird species, eggs do not all hatch at the same time, and environmental factors, like food supply, may determine which of the chicks survive. For instance, Blue-Footed Boobies (Sula nebouxi) breed on many islands of the eastern Pacific Ocean (Figure 1). On average, the female lays two eggs, four to five days apart. Under these circumstances, the oldest hatchling has a head start on growth over the chick developing from the second egg. Being larger, healthier, and stronger, the first chick can demand a bit more attention from its parents. If food is plentiful, both chicks will survive to adulthood. On the other hand, if food is scarce, the older chick may monopolize the food, attack, or even kill the younger bird. In some cases, the elder chick drags the younger sibling out of the nest to die. Although this behavior seems cruel, it does limit the number of new animals to the number that can survive, given available food. Moreover, there are many other examples of such behavior among bird species.


Figure 1a. Blue-Footed Booby (left). Figure 1b. Blue-Footed Booby on the nest, eggs warmed with her feet (right).

1a. Copyright © George Shiflet

1b. Copyright © kuhnmi at https://commons.wikimedia.org/wiki/File:Blue_Footed_Booby_Breeding_(49304360538).jpg


Recently, scientists have made extensive observations of barn owls and their young and observed some surprising behavior (Figure 2). Barn owl mothers may lay up to nine eggs, and the competition for sufficient food can be intense. Surprisingly, they found that when there was sufficient quantities of food, the older and larger chicks would share their food with younger, smaller siblings.


baby barn owl

Figure 2. Young barn owl

Copyright © Peter Trimming at https://commons.wikimedia.org/wiki/File:Baby_Barn_Owl_(18034959998).jpg


However, this behavior is not entirely altruistic, because the older birds usually provide food only to younger chicks that have groomed them. Grooming behavior is found in many animals, establishing and reinforcing kinship and social structure, but crucially ridding the feathers, fur, and so on of fleas and ticks. Thus, both siblings benefit, increasing the number of siblings that survive to adulthood, which improves the chances of keeping shared genes in the gene pool of the population.



Ducouret, Pauline, Andrea Romano, Amélie N. Dreiss, Patrick Marmaroli, Xavier Falourd, Manon Bincteux, and Alexandre Roulin. 2020. "Elder barn owl nestlings flexibly redistribute parental food according to siblings’ need or in return for allopreening." Am. Nat. 2020 Aug;196(2):257-269. doi: 10.1086/709106.