Woodpeckers are known for their characteristic pecking behavior as they search for insects hiding in trees. However, some species also store food like acorns for later consumption. This unusual behavior for a bird has long fascinated ornithologists and bird enthusiasts.
Do any woodpeckers store acorns?
Yes, there are a few species of woodpeckers that are known to cache or store acorns. The most common examples are:
- Acorn woodpecker – Found in western North America, they store acorns in specially drilled holes in trees.
- Lewis’s woodpecker – Another western species that eats and stores acorns.
- Red-headed woodpecker – Found in eastern North America, they sometimes store nuts and acorns in tree crevices.
Of these, the acorn woodpecker is the woodpecker most strongly associated with acorn storage behavior. Their entire social structure and breeding system is based on the collection and storage of acorn crops.
Why do acorn woodpeckers store acorns?
Acorn woodpeckers store acorns for consumption during times when their insect prey may be harder to find, such as winter or during migration. Acorns provide a vital backup food source when insects are scarce.
Acorns also serve as a long-term food investment. Acorns can be stored for up to several years before being eaten. Having a reliable stockpile of stored acorns allows acorn woodpeckers to survive seasonal changes and fluctuations in the insect populations they depend on.
How do acorn woodpeckers store acorns?
Acorn woodpeckers drill small holes into the bark of trees to create tightly fitting holes or depressions to store individual acorns. These holes are known as “granaries.”
The granary holes can be drilled into almost any part of a tree, including the trunk, branches, and even the undersides of large branches. A single tree can have hundreds or even thousands of acorn holes drilled into its bark.
Some key facts about acorn woodpecker granaries:
- Each hole is custom drilled to fit a single acorn snugly.
- The holes only go through the bark, not deep into the wood.
- The tight fit protects the acorn from moisture loss and insect damage.
- Birds may check on caches periodically and consume acorns as needed.
- Acorns can be stored in the same tree for up to 4 years before being eaten.
Granary trees
Acorn woodpeckers often select certain trees to heavily invest in and drill thousands of holes into to create a “granary tree.” These trees can stay in use for many years and successive generations of woodpeckers may continue to maintain them.
Preferred granary trees tend to be large, older oaks with thick, soft bark that is easy to drill into. The woodpeckers may also choose trees that regularly produce large acorn crops.
One study in California found that on average granary trees contained about 1,500 holes, with the maximum found being almost 6,000 holes in a single tree!
Do acorn woodpeckers live in granary trees?
In addition to acorn storage, mature granary trees are also the preferred sites for acorn woodpeckers to excavate nesting and roosting cavities. So in a sense, granary trees become the entire center of life for a woodpecker family group.
Acorn woodpeckers are unusual in that they live in social groups that cooperatively work together to maintain the granary tree. A family group may consist of up to 15 birds who breed communally and share responsibilities.
By living together in a granary tree, the group can efficiently defend their storehouse of acorns from theft by other animals.
What other unusual behaviors do acorn woodpeckers display?
In addition to their acorn caching behavior, acorn woodpeckers have many other unusual adaptations and behaviors compared to other North American woodpeckers:
- Highly social, cooperative breeding in family groups
- Polygamous breeding with multiple breeding males and females
- Long-term pair bonds between breeding males and females
- Helpers at the nest – young from previous years help feed nestlings
- Males and females share incubation duties
- Communal nesting in cavities excavated in granary trees
Their tight social structure seems to be an adaptation to allow successful maintenance of the granaries. The more birds available to watch for thieves, drill holes, and fill acorns, the more secure the food supply.
Do any other animals steal acorns from woodpecker granaries?
Many animals would happily help themselves to the acorn stores if given the chance. To defend against theft, acorn woodpeckers are highly vigilant and aggressive around their granary trees.
Some animals known to sometimes successfully raid woodpecker caches include:
- Other woodpecker species
- Nuthatches
- Chipmunks
- Squirrels
- Rat species
- Bear species
Stored acorns in higher, harder to access locations in the tree likely have a much higher chance of avoiding being stolen.
Squirrels
Squirrels in particular are major acorn thieves. This has led to an evolutionary arms race between woodpeckers and squirrels in some areas. Woodpeckers will try to peck deeper holes while squirrels evolve better techniques for prying acorns out.
How do woodpeckers find their stored acorns?
Considering a single granary tree may contain thousands of holes with acorns, how do acorn woodpeckers keep track of all their caches and recover them? This remains a bit of a mystery.
Researchers speculate they may use memory, sight, and smell clues to relocate acorn stores. Some possibilities include:
- Photographic memory for the location of cache sites
- Visual cues like recognizing patterns of drilled holes
- Olfactory clues from smelling nuts and fungi growing on caches
- Instinctive checking of sites during certain seasons
There is still much to learn about the exact methods they may use to keep track of extensive caches distributed across multiple trees.
How are acorn woodpecker populations doing?
Currently, acorn woodpeckers are considered a “Least Concern” species by the IUCN. Their population trend seems relatively stable, and they remain widespread across their historic range.
The availability of suitable oak trees for granaries likely shapes their population levels and distribution. As long as older oaks with soft bark persist, acorn woodpeckers should have a secure future.
There is concern that sudden oak death disease could potentially threaten western oak species and acorn woodpecker habitat, but impacts remain uncertain.
Compared to other more specialized woodpeckers, the acorn woodpecker’s unique flexibility with using both tree sap and cached acorns as a food source may make them more resilient.
Are woodpeckers and acorns related in any other interesting ways?
Beyond just storing them for food, woodpeckers and oaks have some other notable connections:
- Oaks produce hard nuts that require specialized bill tools to open, which woodpeckers possess.
- Acorns attract woodpecker prey insects like larvae and beetles.
- Oak trees provide good habitats with extensive bark surface area to forage on.
- Cavities created by woodpeckers are used by other wildlife in oaks like birds, squirrels, and raccoons.
There is likely a long, shared evolutionary history between certain woodpecker species and the oak forests they inhabit.
The granary behavior of acorn woodpeckers is a fascinating example of that relationship.
Conclusion
The habit of storing acorns in tree holes for later use is an intriguing adaptation found in some woodpecker species. The acorn woodpecker is most strongly associated with this behavior and depends on stored acorns to survive winters and periodic drops in insect populations.
Their impressive granary trees represent a major investment of resources and they have evolved complex social behaviors to maintain them. While woodpecker granaries attract theft attempts by other wildlife, the birds remain highly vigilant in defending their caches.
Understanding more about techniques woodpeckers employ to successfully manage their hidden acorn stores remains an area of continued research interest. The granary behavior of acorn woodpeckers highlights the remarkable abilities of some birds to plan for future needs.
