Get to Know the Acorn Woodpecker
Treat yourself to a fun bird watching activity that can only be done along the US west coast, the southwest, and down into the highlands of Mexico and Central America: viewing an Acorn Woodpecker family group! These groups of the conspicuous, clown-faced, acorn-storing woodpeckers usually consist of more than a dozen birds cooperatively raising young in pine-oak woodlands of North America. The species is best known for storing acorns in individually drilled holes in storage trees called granaries, which is unique to this species. Storing the food source for the winter, the holes do not compromise living trees—a good thing since granaries of up to 50,000 holes have been found! Even more amazing is the time it took to drill that many holes: studies estimate a family group might take over 100 years to achieve that! Group members periodically check the fit of stored acorns as they dry out and move them to smaller holes to keep a tight fit. Acorn Woodpeckers are highly territorial in defending this precious food resource and each primary and secondary granary has guards that alert the group with their comical waka waka waka calls when birds and other animals invade.
Although acorns are a major portion of the Acorn Woodpecker winter diet, throughout the year the species also forages on seeds, sap, and insects by flycatching and bark-gleaning. Overall, the birds are non-migratory, but when groups eat through their stores, they often abandon their territories and wander in search of other food sources. Interestingly, a unique local population near the Huachuca Mountains is a regular migrant and undertakes a completely different breeding strategy. These birds do not create granaries and in most years they run out of food and leave until spring when adults return to reoccupy their previous territories where cooperative breeding is absent.
Among most Acorn Woodpeckers, cooperative breeding is the norm with family groups containing 1–8 male breeders, 1–4 egg-laying females, and up to 10 male and female non-breeding helpers. In groups with more than one breeding female, all eggs are laid into a single nest cavity and any existing eggs are usually destroyed until all the females are laying simultaneously. Unbelievably, more than one third of all eggs laid in joint nests are destroyed.
In Southeast Arizona, look for Acorn Woodpeckers in the oak woodlands of all the Sky Island ranges. Near Tucson, Bear Canyon in the Catalinas and Madera Canyon in the Santa Ritas are excellent spots, and even Reid Park has had one bird wintering the last few years!
For more woodpecker fun, watch the excellent new Nature episode.
Sources: Birdsoftheworld.org, Allaboutbirds.org, Whatbird.com, eBird.org, and personal observations.