Acorn woodpeckers turned out to be lovers of watching the battles of their relatives

Acorn woodpeckers spend a lot of time to observe the conflicts of their relatives: some of the spectators even travel several kilometres for this purpose. As noted in an article for the journal Current Biology, the purpose of such behaviour is not entertainment, but the collection of social information about neighbouring groups.

Acorn woodpeckers (Melanerpes formicivorus), which live in the West of North America and in some regions of South America, are famous for their "storerooms". In order to have a stable food source during the winter, these birds hollow out many holes in the bark of trees and store acorns in them: there can be up to fifty thousand holes in a single tree. However, this is not the only unusual feature of acorn woodpeckers. Scientists, for example, have long been interested in the social relationships of this species. In some parts of their range, woodpeckers live in colonies of 15-20 individuals, and birds of the same sex are related to each other. In each group, there are several breeding males and females, as well as non — breeding assistants who help to feed the Chicks, as well as protect the territory and "storerooms" from competing groups.

The best chance for helpers to start breeding themselves is to take up a vacant position in a neighbouring territory after the death of one of the breeding individuals. Such opportunities are rare, so birds from several groups can apply for one job at once. Same-sex assistants from the same group join in coalitions that are fiercely fighting for the opportunity to take the vacant seat. Participants in fights shout threateningly, open their wings and fight, and some individuals even die in the process. A single conflict can involve up to twelve different coalitions.

A team of ornithologists led By Sahas Barve) from old dominion University, I decided to learn more about acorn woodpecker battles. To do this, researchers captured birds and equipped them with light radio sensors that allow them to monitor their movements. All the work was carried out in Central California, where a population of woodpeckers lives, which has been closely studied since 1973. In total, there are 50 groups of acorn woodpeckers, and vacant breeding sites appear in a quarter of them every year.

During the study, the authors managed to track three conflicts. They occurred in May and August 2018, as well as in April 2019. In all cases, the battle was fought for the vacancy of a breeding female, so female assistants from several territories participated in the battles.

Researchers have found that woodpeckers are willing to spend a lot of time and effort for the sake of the opportunity to reproduce, especially if we are talking about a vacancy in a valuable territory with rich storerooms. In one case, a confrontation between two coalitions lasted for four days, with the birds spending more than ten hours on the battlefield each day (the victory was won by a third coalition whose members were not marked by radio sensors). However, conflicts attracted not only female helpers but also birds that could not apply for vacant positions in neighbouring groups — males and reproducing individuals of either sex. These birds did not take part in the battles, but only watched them. Researchers have long known about the existence of such viewers, but thanks to radio tags, they have for the first time been able to understand how much time such individuals spend on observation.

On average, the audience spent about an hour a day on the battlefield. This is 113 minutes less than the direct participants in the conflict, but still quite a lot. In addition, the spectators made more effort to get to the place of the fight. Researchers found that they covered an average of 1,432 meters, while for warriors this figure was 644 meters. Some of the spectators came from territories located more than three kilometres from the site of the battle.

Thus, birds that were not involved in the conflict still spent time and energy to observe it. Some of them even left their own territories unattended. According to the authors, the audience's goal was to collect information about neighbouring groups. For such highly social birds as acorn woodpeckers, it is extremely valuable.

Woodpeckers are quite pugnacious birds. For example, they drive most birds of their own size from their feeders. This conclusion was reached by ornithologists from Cornell University in New York. With the help of 20 thousand Amateur ornithologists, they managed to create a hierarchy of all North American birds visiting feeders.

Photo: Ingrid Taylar / Flickr

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