Wildlife Hoarders: The Case of the Acorn Woodpecker

^{Figure 1. Acorn Woodpeckers at a nesting hole. Parque Naciones Unidas, Honduras. Photo: Mario Ardón.}

^{The Acorn Woodpecker (Melanerpes formicivorus) is a striking and noisy bird that can often go unnoticed precisely because it’s so common and easy to spot. But here we’ll mention two compelling reasons why you might want to pay closer attention next time you see one (or likely several) of these birds.}

^{The Acorn Woodpecker is found from the western and southern United States, through Mexico and Central America, and into northern Colombia. It’s widely distributed in pine-oak forest associations but shows a particular preference for oak forests (Quercus species). Two standout features of this bird are its diet and social life. The Acorn Woodpecker diverges from most woodpecker species because it stores food and lives in families.}

• Feeding habits

^{True to its name, the Acorn Woodpecker feeds primarily on oak acorns and insects, though its diet is highly diverse and opportunistic. They may sip tree sap, either by drilling the bark themselves or by exploiting holes made by their cousins, the sap-sucker woodpeckers. They feed on oak catkins (flower clusters), and in urban parks where they live, they may even eat leftover human food. And if there are dogs nearby, you’d better hide the kibble—these birds will gladly snatch it. But the most interesting part isn’t that this bird might steal your pet’s food—it’s what they do with it afterward: they store it.}

^{Figure 2. (Left) Acorn Woodpecker feeding on flowers of an oak tree (Quercus sapotifolia). (Right) Individual carrying dog kibble.}

^{Acorn Woodpeckers are famous for storing acorns, but what makes them especially fascinating is their peculiar behavior of drilling thousands of holes (especially in North America) in bark and dead branches to store individual acorns. Within their territories, you can often find one or more trees designated exclusively for storage, called "granaries". These granaries, which hold an important food source, are defended from other animals. In North America, a granary can contain thousands of acorns, while in other parts of their range, they may contain hundreds. Such a majestic pantry is the result of the labor of multiple generations of Acorn Woodpeckers who inherit the territory.}

^{Figure 3. Acorn Woodpecker on a granary tree. Parque Naciones Unidas, Honduras.}

^{These birds are extremely busy. Gathering and storing acorns is no simple task. After collecting an acorn, they must find the right-sized hole to store it in. Over time, as the acorns dry and shrink, the birds must move them to smaller holes to keep them from falling out of the granary—a very labor-intensive job. And what happens if the acorn crop fails and the birds haven’t stored enough to survive the cold northern winter? They abandon their territory and migrate to other areas. This is why Acorn Woodpecker families tend to live in habitats with more than one oak species—if one species produces a poor crop, the others can compensate.}

^{When it comes to insect feeding, Acorn Woodpeckers are aerial hunters, excellent at catching flying insects—unlike most woodpeckers (family Picidae), which prefer to extract insects by drilling into trees. Their relatively long wing area and low wing loading give them high maneuverability at fast speeds (Bock 1970). But sticking to their hoarding behavior—if they catch large insects, whether in flight or by gleaning—they may store them in “anvils”, or crevices in tree bark, to eat later. Anvils are grooves on flat surfaces that allow them to wedge food in place and prevent the acorns from rolling while being eaten (MacRoberts & MacRoberts 1976).}

• Organización social

^{The Acorn Woodpecker’s social life is perhaps even more fascinating than its hoarding behavior. If you spot one, more are likely nearby, and you’ll be entertained by their loud vocalizations.}

^{These birds live in complex social groups made up of males and females of various ages who share and defend a multi-use territory (MacRoberts & MacRoberts 1976). Sometimes, you may see them in pairs, showing their social flexibility. Family groups can include up to 15 individuals, though groups of 6 are more common.}

^{Their mating system is especially unusual: polygynandry—two or more males mating with two or more females (Koenig et al. 1995). As if that weren’t unusual enough, they also have nest helpers—non-reproductive individuals who delay dispersal and stay in the family territory to help raise new offspring, much like older siblings. This behavior is known as cooperative breeding.}

^{Figure 4. Three Acorn Woodpeckers defying gravity.}

^{Group members participate in a range of shared activities, including: excavating nesting cavities, incubating eggs, protecting and feeding young, storing acorns, drilling storage holes, and defending the granaries. During all activities, the birds constantly vocalize, likely to stay in contact (Kattan 1988). Their signature “waka-waka” call (which to my ears sounds like “grruaka grruaka”) is often accompanied by a wing display considered a greeting.}

^{But not everything is peaceful within these woodpecker families. Reproductive males and females compete—males for mating, and females to lay the most eggs. Female competition can escalate to egg destruction—they may remove eggs from the nest cavity, and once outside, those eggs may be eaten by other group members, or even by the same female who removed them. In such a complex mating system, mechanisms must exist to avoid inbreeding. Non-breeding birds may migrate to other groups where a breeding position is available or inherit their family's territory. Competition to fill these vacancies can become aggressive, with direct confrontations between migrant birds and established breeders.}

^{Next time you get a chance to spot an Acorn Woodpecker, don’t miss the opportunity to observe this noisy, flashy, and remarkably interesting bird more closely.}

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By: Stefany Flores, Biologist

References

Bock CE. (1970). The ecology and behavior of the Lewis’ Woodpecker (Asyndesmus lewis). University of California Publications in Zoology 92: 1-100.

Kattan G. (1988). Food habits and social organization of Acorn Woodpecker in Colombia. Condor, 90(1): 100-106.

Koenig WD, PB Stacey, MT Stanback and RL Mumme. (1995). Acorn Woodpecker (Melanerpes formicivorus). The birdw of North America, No. 194 (ed. by Poole and F. Gill). Academy of Natural Sciences, Philadelphia. American Ornithologists’ Union, Washington DC.

MacRoberts MH and BR MacRoberts. 1976. Social organization and behavior of the Acorn Woodpecker in Central Coastal California. Ornithological Monographs No. 21.