Cooperative Breeding-Why Do Acorn Woodpeckers Live As A Family?

Cooperative breeding is when non-parent members of a group help in caring for the young in the group. 9% of bird species demonstrate cooperative breeding.

Families are messy. Parents impose their will on you, siblings steal your clothes and there’s always that one cousin who outshines everyone with their unattainable achievements. Most of us tolerate our families because, deep down, at our core, we love them. However, families, this group of whom you’ve most likely grown up with and seen throughout your life, help one deal with the chaos of the world.

But if you think human families are messy, you’ve never heard of Acorn Woodpeckers. Their family will stick together through thick and thin!

The family that helps together, stays together

Acorn woodpeckers, a small species of woodpecker with a red feather patch on its head that looks like a bald patch of an old man, practice cooperative breeding. Several adults will cooperate to raise the young in the family, even if they aren’t directly related to the young. This means that there are one to three sole reproducers—parents, if you will—who mate and lay their eggs, while the rest of the members, the extended family, protects and cares for the rest of the family.

Acorn woodpeckers live in large joint families, often with more than a dozen members, usually about 15 or so, and they are all more or less related to each other. There are usually several breeding males—brothers, sons, fathers or uncles to each other—and usually one to three breeding females who might be mothers, daughters, sisters or aunts to one another.

The rest of the family are ‘helpers-at-the-nest’ and are typically the children of the breeders from a past mating season. Females lay their eggs together in a single nest.

They inhabit ttall trees in North America, like the oak, in which they create hundreds of pockmarks. Within their tree caves lie their granaries of acorns for the hard seasons, as well as the resultant holes that are used for their nest.

Acorn Woodpecker on a tree in California(Jean-Edouard Rozey)s

The Acorn Woodpecker’s granary tree storing their acorns (Photo Credit : Jean-Edouard Rozey/Shutterstock)

For such a large family, a similarly large territory is required. Acorn woodpeckers can occupy territories stretching as far as 15 acres (almost 15 football fields), which family members fiercely defend from their competition, other acorn woodpecker families in the vicinity, or from scrub jays, birds that looks like a clear summer morning with light blue, white, and grey feathers.

In these large families with immense properties, the helpers do most of the defending by calling out alarms, making sure the little ones are safe and sound, and foraging, but in doing so, they give up their reproductive rights.

Larger families, larger issue

Don’t be fooled by the word cooperative, thinking that these birds are one massive, happy family, coexisting in a utopia of love and companionship. As with any family, these birds have their feuds and conflicts.

A huge concern for these birds is incest. In this large group, where every bird is related to some degree to another bird, avoiding incest is paramount. Male helpers remain so because they cannot breed with their mother and all the available positions for a male breeder are occupied.

Helpers at the nest are always on the lookout for an opportunity to rise up the hierarchy and become breeders, whether that is in their own group, or in another group, which would require abandoning their family.

“But how do they know the difference between their cousin and their sister?” you might rightfully wonder.

She went to another group meme

Family disputes

Acorn woodpeckers have triadic awareness, the awareness of knowing who’s who in their own and other groups, and what relationship each on has with the other. That’s like if you go to a family wedding and you know not only who your second cousin’s son is, but also what kind of relationship they share with each other and who the wedding crasher is at the party.

If a breeder dies in a family, the competition to ascend to breeder status is tough. Birds from within the group will compete with each other and members from outside groups.

If a female dies in the group, male helpers who couldn’t mate with her because she was their mother would now be able to breed with another female. Thus, a father and son can both be breeders with one female.

Among the female birds, there is competition for the survival of their own children. In families where there are several female breeders, one will often sabotage the other’s eggs by tossing them out of their common nest. Egg tossing is so prevalent that almost one-third of eggs are lost in this behavior.

There appears to be a size limit to these families. If the family is too small, there aren’t enough helpers to nurture and protect the young. If the family is too large, success also goes down, often because competition amongst mates increases. Like most things in life, there is an optimal number of members that give the family the highest reproductive success. For the acorn woodpeckers, having a family any larger than 7 or 8 members is detrimental for success. Pairs not in a group might produce more children individually, but overall, groups with multiple breeders have more children in total.

Why cooperate?

This question is at the heart of research on cooperative breeding. In most cases, life is predisposed to protect its own interests, namely its own genetic interests. Our selfish genes are looking out for their own domination.

Why, then, would 9% of bird species, the Acorn woodpecker only being one of the most extensively studied, choose to live in families where some members never get a chance to breed?

Florida scrub jay (Aphelocoma coerulescens) singing in a tree(Agami Photo Agency)s

The Florida Scrub-Jay, another bird species that show cooperative breeding (Photo Credit : Agami Photo Agency/Shutterstock)

One hypothesis is kin selection and inclusive fitness. The theory, first proposed by W. D. Hamilton, states that some members of a group relinquish (or delay) their reproductive potential in order for the greater benefit of the group. Honeybees are a classic example of perfectly social animals, although cooperative breeding societies are less perfectly social than those of busy bees.

Supporters of kin selection say that helpers, who tend to be family members in many avian cooperative breeding families, derive indirect fitness (a word used to refer to reproductive success). As important as this hypothesis is, it doesn’t adequately explain the development of cooperation, especially considering that many members of a family are non-relatives who have moved out of their original family.

The role of ecological factors has gained interest in recent years. Evidence shows that cooperative breeding develops in two opposite types of environments—stable environments and unstable/turbulent environments. A stable environment would have “filled up”, where individuals experience high competition for resources, making it difficult to breed. This has been called “ecological constraints” or “habitat saturation”. An unstable environment would not provide food or shelter with certainty.

The ideas are contradictory and have perplexed many studying the field. Many species of animals face “ecological constraints” and severe competition for resources in some of the toughest terrains, yet cooperative breeding has only evolved in a few species.

It appears that both of these conditions are necessary. It appears that initial conditions require solitary species to come together to form family groups that don’t yet cooperate and actively help care for the young. However, disturbed and uncertain variation between seasons is required for the non-cooperative family structure to evolve into one that cooperatively breeds. Much of this was brought to light by a research group studying cooperative breeding in Siberian jays.

The above isn’t a truth set in stone. Conditions in nature rarely conform to our made-up hypotheses and simply observing current ecological behaviors doesn’t give us the entire picture. For the complicated acorn woodpeckers, the above seems to hold true, yet there are other species where there remains a great deal of debate.

It has been more than 50 years since scientists began studying the messy families of Acorn woodpeckers. From their ability to hold an entire familial yellow pages in their head to a family system that rivals the most complex system humans have attempted, these are incredible creatures. We often like to think of human beings as somewhat superior or separate from the animal kingdom, yet we are very much a part of it, exemplified in our shared quality of having such messy families!

References:

  1. Journal: Proceedings of the Royal Society B 
  2. Journal: The American Naturalist
  3. Journal: PLOS Biology
  4. The International Encyclopedia of Primatology
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Salama has a degree in Life Science and Biochemistry from St. Xavier’s College, Mumbai. She enjoys being in the water much more than being on land. She is passionate about science and wants to declutter science from its jargon to make people understand its beauty.

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