The Acorn Woodpecker is a medium-sized woodpecker that is native to western North America. Their diet consists mainly of acorns, insects, sap, and fruit.
Acorn Woodpecker Diet
Acorn Woodpeckers get their name from their habit of storing acorns in storage holes they drill in trees. Acorns make up 50-60% of their diet. They forage for acorns on the ground and in trees. The acorns provide them with carbohydrates and some protein.
Insects make up to 40% of an Acorn Woodpecker’s diet. They consume ants, beetle larvae, caterpillars, spiders, and other insects. Insects provide crucial protein for the woodpeckers. Acorn Woodpeckers often forage for insects on the trunks and branches of trees by probing into crevices in the bark.
Acorn Woodpeckers will also drink sap from holes they drill in trees. The sap provides sugar for energy. They supplement their diet with fruits such as berries, apples, oranges, peaches and pears when available. The fruits provide important nutrients like vitamin C.
Foraging Behavior
Acorn Woodpeckers forage for food in a variety of ways. One of their most distinctive foraging behaviors is drilling holes in trees to store acorns. They will drill small holes in the bark of trees in their territory. Once the hole is drilled, they will force acorns into the holes, called granaries, for storage. A single tree can have thousands of acorn granary holes drilled in the trunk and branches.
Acorn Woodpeckers will often forage on the ground for fallen acorns. They hop along the ground probing the leaf litter and grass with their beak searching for acorns. They may dig up shallow holes in the dirt looking for insects like beetle larvae to eat as well.
On trees, Acorn Woodpeckers forage by probing into crevices and under bark for insects. They use their straight, chisel-tipped bill to hammer away at the tree surface to get access to hidden insects. They have a long, sticky tongue that helps pull insects out once a hole is made.
Acorn Woodpeckers will sometimes sally out from a perch to catch insects in flight as well. They return to the same perch to eat the insect prey. This flycatching behavior is more common during nesting season when the demand for protein is high for the growing nestlings.
Adaptations for Diet
Acorn Woodpeckers have several key adaptations that allow them to specialize in eating acorns:
- Chisel-tipped bill – The straight, sharp bill helps them drill into trees to create acorn storage granaries.
- Sticky saliva – Their saliva helps them glue acorns into the storage holes.
- Long tongue – Their tongue has a barb at the tip that helps extract insects from trees and other crevices.
- Strong beak muscles – Powerful muscles in their beak allow them to hammer on tree trunks.
- Zygodactyl feet – Two toes point forward and two point backward to better grip surfaces while climbing.
In addition, Acorn Woodpeckers live in small family groups that cooperate in collecting and storing acorns. Their social behavior helps them create centralized acorn stores that the entire group shares access to when other food is scarce.
Importance of Acorn Storage
The acorn storage behavior of Acorn Woodpeckers is key to their survival, especially in winter when insect prey is scarce. Acorns are an important source of carbohydrates and protein for the woodpeckers. Their habit of drilling small holes into trees to store acorns is unique among birds.
A single Acorn Woodpecker group may store up to 50,000 acorns in a season. Their storage holes, or granaries, allow them to keep a stable food source available even when acorns are out of season. The abundance of acorns also allows them to feed nestlings without having to forage extensively.
Acorn Woodpeckers will aggressively defend their storage trees from other woodpeckers trying to steal acorns. Having centralized food storage helps Acorn Woodpeckers survive cold winters when insects are not available.
Seasonal Changes in Diet
The diet of Acorn Woodpeckers changes depending on the season and availability of different food sources:
- Spring – More insects like beetle larvae, caterpillars, ants. Sap intake increases.
- Summer – Consume insects, fruits, acorns. Increase acorn storage.
- Fall – Forage for acorn mast crops. Maximize acorn storage.
- Winter – Rely heavily on stored acorns and some insects.
In spring, the woodpeckers consume more sap and insects as they become more abundant. In summer and fall, they focus on acorn storage but still eat some insects and fruit. In winter, stored acorns become the primary part of the diet along with some persisting insects.
Comparison to Other Woodpeckers
The Acorn Woodpecker is unique among North American woodpeckers in its strong dependence on acorns. Other woodpeckers may eat some acorns, but do not store them to the extent of Acorn Woodpeckers.
Other differences include:
- Northern Flicker – More emphasis on ants. Eats beetles, fruits, seeds.
- Downy Woodpecker – Mainly eats insects like beetle larvae. Some seeds, sap.
- Pileated Woodpecker – Feeds heavily on carpenter ants and beetle larvae.
- Red-headed Woodpecker – Eats mostly insects, beetles, grasshoppers.
The Acorn Woodpecker is the only North American woodpecker that constructs such extensive external acorn granaries. This adaptation allows it to thrive in oak woodlands that produce intermittent heavy acorn crops.
Granary Trees
Acorn Woodpeckers selectively choose certain trees for their acorn storage granaries. They prefer large, older trees that produce lots of acorns. Some key granary tree species include:
- Oaks – Coast live oak, canyon live oak, California black oak
- Pine – Coulter pine, ponderosa pine, Jeffrey pine
- Sycamore
- California bay laurel
- Bigcone Douglas fir
The woodpeckers choose trees that regularly produce large acorn crops. The live oaks are a favorite because they hold their acorns for two seasons, providing a steady supply. The woodpeckers also prefer trees with smooth bark that makes the holes easier to drill.
Dead granary trees continue to be used for as long as they remain standing and solid enough to hold acorns. Some granary trees have been found still in use over 80 years after they died.
Ideal Granary Tree Characteristics
- Produces abundant acorn crops
- Holds acorns for multiple seasons
- Thick, solid trunk
- Smooth bark surface
- Large mature size
- Located in territory of woodpecker clan
Benefits to Trees
While Acorn Woodpeckers damage trees by drilling holes for acorn storage, researchers think the storage behavior may actually benefit the trees in some ways:
- Seed dispersal – Acorns are distributed as woodpeckers drop them.
- Pollination – Woodpeckers carry pollen on their bodies as they forage.
- Pest removal – Woodpeckers eat insects that can damage trees.
- Pruning – Their drilling may accelerate limb pruning.
- Defense – Birds harass and drive off squirrels and other acorn thieves.
Having Acorn Woodpeckers present may help protect granary trees from major damage by other pests. The trees appear to thrive despite targeted drilling damage from the birds.
Acorn Woodpecker Population
Acorn Woodpeckers are found in western North America in parts of California, Oregon, Washington, Arizona, New Mexico, Colorado, and Mexico. They thrive in oak woodlands habitats where acorns are abundant.
Their population was estimated at 1.5 million individuals in the 1960s when acorn woodlands were more extensive. However, habitat loss has caused an estimated 60% decline in their numbers since then. Current population estimates stand around 610,000 birds.
Conservation efforts are aimed at preserving intact oak woodlands to ensure healthy Acorn Woodpecker populations into the future. The unique ecological services they provide further add to the importance of maintaining their numbers through habitat protection.
Conclusion
In conclusion, Acorn Woodpeckers are fascinating birds uniquely adapted to specialize on a diet of acorns and insects. Their acorn drilling and storage behavior allows them to thrive in western oak woodlands. Acorns provide a crucial carbohydrate and protein source, especially in winter when insect prey is scarce.
The Acorn Woodpecker’s specialized acorn habits, social behavior, and adaptations like chisel bills and granary trees allow it to exploit an ecological niche based on mast acorn crops. Maintaining healthy oak woodland habitats will be key to conserving populations of these unique woodpeckers into the future.
