Many Wren Species Found To Be Pack Animals

They Don’t Carry Heavy Loads, But They Do Hang Out in Packs

Dec 30, 2008

While little is mentioned in standard references, the social system of wrens is more complex than that of most bird species and has only recently been an object of study.

Many of the nearly 80 species of wrens worldwide are seen in pairs, but others exhibit highly social behavior, foraging in flocks. Studies are just emerging to indicate which species are actually cooperative breeders, where multiple adults join efforts in nest building, hunting for food, incubating eggs, and feeding nestlings.

Behavioral Ecology

It wasn’t until the mid-20th century that isolated naturalists (behavioral ecologists) became aware of this phenomenon in wild mammals, and still later that it was documented in birds. Of the approximately 9000 categorized bird species, only about 220 are known to be cooperative breeders according to a recently published book Living in Groups, 2005, by Jans Krause and Graeme D. Ruxton, pg.114.

Cooperative Breeding in Coyotes

Wildlife biologists have marveled at cooperative breeding in wild canids. Packs normally consist of a breeding pair and several “helpers” or “betas”, young from previous years which assist in gathering food for, educating, and raising the puppies of the alpha pair. The entire pack functions like a human family. Over years of observation, they stay together, taking part in obviously happy play unrestricted to the calendar or breeding seasons (since the breeding pair normally mates for life), with each other and each year’s new offspring in a closed and balanced family social system.

Dr. Jonathan G. Way, who heads up Eastern Coyote Research on Cape Cod, has studied this social system among coyotes for over ten years. He maintains in his book Suburban Howls (2007) that cooperative breeding in mammals indicates highly evolved social systems akin to humans and that participating individuals not only note the death or disappearance of members, but literally mourn their loss.

Wrens in Family Groups

While there are no such known studies regarding birds, cooperative breeding has been observed in many bird species, including some woodpeckers and wrens. Although the term pack generally refers to canines, it’s interesting that similar terms exist for groups of wrens, including flock and herd. Ornithologists suggest that the common frequency of wrens being observed in family groups is evidence of members intentionally congregating and foraging together, rather than simply an uncharacteristic exhibition of territorial negligence.

Carolina wrens are documented cooperative breeders, where pairs observed colonizing a new territory appear to be alone the first year, but after the initial brood has fledged, groups are then seen. Parents may raise several broods per season, but numbers do not always correlate to the maximum size of the brood(s), so while it is possible some die, it is assumed some young may disperse, even as wild canids do. Long term studies may confirm if the helpers remain only one or more years.

According to the doctoral studies (1998) of behavioral ecologist Dr. J. Jordan Price at the University of North Carolina at Chapel Hill, the stripe-backed wrens of Caracas, Venezuela are also cooperative breeders “because they live in family social groups on communally defended territories,” Price explained. “Within each group, the dominant male and female act as the principal breeding pair while up to 12 other birds--usually previous offspring--do not breed, but help construct the nest and feed the young.” Price’s results came from investigating about 30 groups of wrens.

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