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Page last updated at 04:20 GMT, Wednesday, 9 February 2011
Ravens stressed by 'gang life'
By Ella Davies
Earth News reporter

Raven flock
Life is tough in gangs

Ravens living in juvenile gangs are more stressed than those in adult pairs, a new study reveals.

Scientists analysing droppings found higher levels of stress hormones among birds living in groups.

The findings contradict theories that living in territorial pairs is more energetically demanding than co-operative group life.

Researchers now suggest that stress could be a driving factor in ravens' maturation from groups to pairs.

Ravens are members of the intelligent corvid family that includes crows and magpies.

Young birds live together in social groups and co-operatively share food.

Adult birds, meanwhile, form pairs, often for life, and aggressively defend their breeding territory.

Dr Selva takes samples of the raven droppings (c) Adam Wajrak
Our study shows that life in groups is not so heavenly as traditionally thought
Dr Nuria Selva

Scientists studying the birds in Bialowieza Forest, Poland, investigated how stressful these different social systems were.

Their findings, published in the journal Biology Letters, caused researcher Dr Nuria Selva some surprise.

"In the case of ravens, it is clear that food finding and sharing is easier when a group of 30 ravens is searching for a carcass, than when only two ravens do it," she said

"But our study shows that life in groups is not so heavenly as traditionally thought."

Previous theories have identified the benefits of group living because young ravens do not have to defend territory or forage alone.

However, the new study's analysis of raven droppings on the forest floor told a different story.

The juvenile gangs' droppings contained much higher levels of the stress hormone corticosterone than the adult pairs'.

The new evidence suggests it may be more energetically demanding to live in groups than to maintain a territory.

Competition for dominance could cause the increased stress, the researchers say.

A group of ravens is sometimes called a congress
Common ravens mostly feed on carrion and will store surplus food
They are also known to store shiny and unusual objects

Dr Selva also says stress could be a significant factor in ravens' maturation.

"We think that having high stress levels can be an important reason to leave the group," she said.

The pressures of gang life could drive young ravens to set up home in a more stable adult "relationship".

"Somehow, we feel it has many similarities with human life - stressful life in teenage gangs versus a more peaceful live in a pair," said Dr Selva.

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