Woodpeckers are known for drilling holes in trees to find insects, building nests in tree cavities, and making loud drumming noises to communicate and attract mates. However, there is one particular species of woodpecker that exhibits the rather unusual behavior of collecting and storing acorns: the acorn woodpecker.
The acorn woodpecker (Melanerpes formicivorus) is a medium-sized woodpecker found in oak woodlands across the western United States and Mexico. As their name suggests, acorn woodpeckers have a strong ecological association with oak trees and acorns.
Acorn woodpeckers collect acorns and store them in unique structures called “granaries” or “acorn woodpecker granary trees.” These granaries are holes drilled into dead trees, living tree limbs, telephone poles, or the side of buildings. The holes are drilled in such a tight matrix that it gives the appearance of honeycomb. Each hole contains one acorn that is so tightly wedged inside that other animals can’t easily remove it.
Reasons for Acorn Caching
There are several reasons why acorn woodpeckers cache acorns in granary trees:
- Food storage – Acorn caching provides the woodpeckers with a food source during harsh winters and periods when insect prey is scarce.
- Territory defense – Trees with granaries are an important resource that acorn woodpeckers will vigorously defend against other birds.
- Breeding sites – Cavities in granary trees are used as nesting sites to raise young.
Acorn Woodpecker Behavior and Adaptations
Acorn woodpeckers exhibit a number of behavioral adaptations and characteristics that allow them to collect and store acorns:
- Loud, distracting vocalizations – Acorn woodpeckers make repeated loud vocalizations as they collect and store acorns, which may help distract other animals.
- Rapid drilling – Their strong beaks and swift drilling abilities allow them to quickly create acorn-sized holes in trees before other animals can steal the acorns.
- Tight storage – Wedging acorns tightly into custom-drilled holes helps prevent theft from other animals.
- Social living – Acorn woodpeckers live in family groups that cooperate to defend granary trees.
- Short-term memory – Studies suggest acorn woodpeckers may have spatial memory adaptations that help them remember where they’ve cached each acorn in a granary tree.
Unique Anatomy
Acorn woodpeckers also have some distinctive anatomical features that aide their acorn-caching lifestyle:
- Strong beak – Their beaks are robust and tapered, ideal for drilling into wood.
- Sticky saliva – Their saliva is incredibly viscous, which helps them wedge acorns tightly into holes.
- Long tongue – Their tongue is extra long and can be protruded up to 4 inches, helping collect and maneuver acorns.
Granary Trees
The granary trees created by acorn woodpeckers are an impressive sight. They strategically select dead trees in open areas that allow easy access and clear sight lines for defense. A single granary tree can contain tens of thousands of acorns wedged into thousands of holes.
Here are some key facts about acorn woodpecker granary trees:
- Holes are approximately 1-2 inches deep and spaced 2-3 inches apart.
- Preferred tree species are those with soft wood, such as pine, alder, sycamore or cedar.
- A granary tree is often used for multiple generations.
- The record is a tree in California with over 50,000 acorn holes.
- Acorns can be so tightly wedged that it takes 600-700 pounds of force to remove them.
Granary Construction and Maintenance
Acorn woodpeckers follow a set process to construct and maintain their elaborate granaries:
- Select a dead or dying tree in an open setting.
- Excavate tight, acorn-sized holes in close proximity across the trunk and limbs.
- Collect acorns from nearby oak trees and tightly wedge one into each hole.
- Continue to defend the tree and patrol for any stolen acorns.
- Periodically replace rotting wood around holes and replace stolen or insect-infested acorns.
This continuous construction and maintenance can go on for decades. It’s common for a woodpecker family group to reuse a granary tree for many generations.
Diet
The acorn woodpecker’s unique diet consists of:
- Acorns (50-90% of diet)
- Insects
- Sap
- Fruit
- Nuts
- Seeds
- Occasionally: eggs, nestlings, lizards, mice
However, acorns clearly dominate their diet, especially in winter. Acorns are high in fats and carbohydrates which provide the birds the energy they need.
Acorns as a Key Food Source
Acorns play a critical role in the survival and reproduction of acorn woodpeckers:
- Essential winter food when insects are scarce.
- Allow the birds to remain in their territories year-round.
- Provide nutrition needed for breeding.
- Enable larger group sizes and cooperative breeding.
Some researchers believe the availability of acorn crops may actually dictate the acorn woodpecker’s intricate social structure and breeding system.
Breeding and Social Behavior
Acorn woodpeckers are highly social and live in territorial group of up to 15 birds. These family groups consist of:
- Dominant breeding pair
- Related and unrelated helpers of both sexes
- Current year’s offspring
Cooperative breeding is essential to defend resources. Even adolescent female helpers participate in incubating eggs and feeding nestlings.
Complex Social Dynamics
Acorn woodpeckers exhibit a fascinating and complex set of social dynamics within each family group:
- Polygynandrous breeding system (multiple males mate with multiple females).
- Rank hierarchy determines access to resources and mates.
- Dominant breeders try to suppress reproduction in subordinates.
- Fierce competition to become a breeder if a vacancy opens up.
- Helpers may get a chance to breed within their group or disperse to find a breeding vacancy elsewhere.
The opportunity to breed, attract a mate and parent offspring is a key driver governing the structure and stability of acorn woodpecker clans.
Distribution and Habitat
Acorn woodpeckers are found in the following regions:
- Southwest United States
- California
- Oregon
- New Mexico
- Arizona
- Western Mexico
They inhabit oak and pine-oak woodlands up to 9,500 feet in elevation. Critical habitat features include:
- Open canopy
- Scattered large trees for granaries (dead or dying)
- Nearby oak trees producing acorn crops
In winter they may descend to lower elevations and urban areas where acorn crops are reliable.
Range Map
Here is a map showing the acorn woodpecker’s range across the western U.S. and Mexico:
Image source: Audubon
Threats and Conservation
While still relatively common, acorn woodpecker populations face some key threats:
- Habitat loss from development and fire suppression.
- Limited oak regeneration in some areas due to habitat loss.
- Competition for acorns from non-native animals like rats and pigs.
- Predation from hawks, snakes, squirrels, rats, and other animals.
Conservation measures include:
- Protecting remaining oak woodland habitats.
- Letting natural fires cycle through ecosystems.
- Monitoring acorn crop yields.
- Restricting rodenticides and insecticides which limit prey.
Dependence on Acorns
The acorn woodpecker’s specialized dependency on acorn crops also makes it vulnerable. Oak trees have variable mast years, so a series of bad acorn production years could have significant impacts on acorn woodpecker populations.
Researchers recommend land managers promote healthy oak woodlands with recurrent natural fires to encourage acorn production. Ensuring an abundance of acorn crops will allow acorn woodpecker communities to thrive into the future.
Conclusion
The acorn woodpecker is a remarkable bird with its distinctive acorn collecting and storing behaviors. This species demonstrates incredible natural adaptations that allow it to drill into trees, wedge acorns tightly into holes, defend granary trees, and remember storage locations.
Acorns are crucial to the acorn woodpecker’s diet and breeding success. The availability of acorn crops dictates group living dynamics, social structures, reproduction, and overall population health. These unique birds serve as an important example of how specialized food resources can shape the natural history of a species.
While acorn woodpeckers still thrive in areas with healthy oak woodlands, continued habitat conservation efforts are needed to protect granary trees and maintain abundant acorn production. Ensuring stable acorn resources will enable these charismatic birds to continue their fascinating acorn hoarding behaviors into the future.