Treebeard's Homepage : Stumpers

Treebeard's Stumper Answer
3 December 1999

Oh, Nuts!

Thanksgiving is our harvest festival. It's also harvest time in nature, especially for fruit and nuts, so here are two "nutty" stumpers. Lately, I've been noticing walnut shells on the tennis courts at school and on nearby roads. But there are no walnut trees growing in the immediate area. How do the shells get here? I've also been noticing the noisy Acorn Woodpeckers stuffing new acorns into trees and telephone poles. Woodpeckers are insectivores that mostly eat ants and grubs. So why do these woodpeckers hoard so many acorns?

Walnut shells on the tennis courts. Acorn Woodpecker granary in telephone pole.
Walnut shells that I collected on the school tennis courts. The large nuts
are cultivated English Walnuts from a nearby orchard. The smaller thick nuts
are from the native Southern California Black Walnut (Juglans californica)
that grows along Alamo Pintado Creek about a mile away.
Acorn Woodpecker (Melanerpes formicivorus)
granary in utility pole. Note that the scientific
name literally means "ant-eating woodpecker".
This pole will soon have to be replaced!


Smart crows drop the walnuts on the tennis courts to crack them open. Crows in Japan are known to place the nuts under the wheels of cars at stop lights, and then gather the pieces after the lights change. This is discovery, not instinct! I first thought that Acorn Woodpeckers hoard acorns to harvest the insect larvae (mostly weevils?) that often live within, a kind of simple ranching. I was wrong. They do eat the nutritious nuts. Maybe there's not much difference to a woodpecker between the acorn and the insect grub it becomes!

Notes:

I put these two stumpers together because I thought they both involved bird intelligence and foresight. But I think I have to eat crow on the second part of this stumper.

I've always assumed that the Acorn Woodpeckers harvest the grubs from the acorns. That would be even more remarkable since Acorn Woodpeckers live in complex social groups of 5 - 20 birds, so it's a kind of cooperative agriculture or ranching. But all the sources I've checked state that they eat the acorns and depend on them through the winter months when insects are scarce.

I'm not entirely convinced. It's not enough to see woodpeckers eat their stored acorns from a distance, since we can't see whether they're eating the acorns or the grubs within. It would be interesting to sample the acorns stored in a woodpecker granary and count how many contained a grub. I'm sure it depends on the oak species that is hoarded. I've found that Coast Live Oak (Quercus agrifolia) is often infested, but Valley Oak (Quercus lobata) is often intact. This is not a good acorn year at school, and all the woodpecker granary trees (and utility poles) are empty in the lower parts that I can reach. Maybe next year.

Even without grubs, Acorns are highly nutritious. Here is a nutritional breakdown (showing percent by weight) that I found on the Web:

  Water Protein Fats Fiber Carbohydrate
Coast Live Oak29.104.8813.059.0442.52
Valley Oak40.803.193.606.1544.91
wheat11.511.41.00.2 75.40

Acorns are especially high in fat content compared with cereal grains like wheat. This makes them more meat-like, and maybe more acceptible to carnivores. This is an alternate answer to my stumper.

The annual acorn crop is fickle. This is a pretty good acorn year in the Santa Ynez Mountains where I live, despite our great snow storm last spring just when the oaks were starting to flower. (Our Live Oaks make acorns in one year, unlike some other oaks.) We never see Acorn Woodpeckers up here in the mountains for some reason, but we're seeing flocks of Bandtailed Pigeons this year in our canyon that like to eat the acorns whole.

There's a real acorn stumper here. Acorns are the staple wildlife food here in Central California, but years of plenty are often followed by years with no acorns at all. What environmental factors does acorn production depend on? How does this variability effect local wildlife? How did this effect the native American Chumash people who also depended on acorns?

Graybear apologized and then got it right:

Since this is a naturalist stumper, I doubt I can add anything to your knowledge of the situation. My guess is that the nuts get to the tennis courts probably by squirrels but perhaps by birds. I think they bring the nuts to the courts to smash them and eat them. My guess about the woodpeckers is either 1) they are like bears and eat whatever is available when their preferred foods aren't, or 2) they are actually 'ranching' their future food by supply by providing a place for the grubs to grow, which would be pretty interesting.
I was thinking of your (2), but your (1) is probably right. We don't have tree squirrels here at school because the Santa Ynez Valley is mostly an oak savanah with widely spaced trees. It's definately the crows who drop the walnuts on the school tennis courts. I've watched them.

Crows aren't the only ones. Gulls sometimes drop clams on rocks (or roads) to break them open. Eagles in Greece carry tortoises high into the air and drop them on the rocks below. An ancient story is told that Aeschylus, founder of Greek tragedy, heard from the Oracle that he would die when a house fell on him. (Or was it a bolt from heaven?) An eagle mistook his bald head for a rock and dropped a tortoise (in his "house") on him, killing him.

Who gave commission to that fatal weight
The tortoise, cold and hard, to drop on thee
And crush thee, -- better cover thy bald head;

Elizabeth Barrett Browning, "Aurora Leigh"

I like crows, but I don't want them moving in next door...

We occasionally get a large, noisy colony of crows roosting in the trees by our house. They drive out hawks and owls and other birds. They're a nuisance, but they're smart and organized! They move on after a while.

We have two kinds of walnuts here in Central California. There are familiar cultivated English Walnuts (Juglans regia) that have large nuts with thin shells. We also have native Southern California Black Walnuts (Juglans californica) that have smaller nuts with thick shells (and tastier meat). The natives mostly grow along Alamo Pintado Creek about a mile away from school.

I noticed that the shells on the tennis courts are mostly Black Walnuts. I collected all the walnut shells from the tennis courts last Friday, and then again on Monday to see what was new over the weekend. By volume, you can see there are many more black walnuts (on the left):

Walnut shells on the tennis courts.

I did an informal survey with the help of DMS students Nick, Andrew, Erik, and Kevin. We only counted pieces thumbnail-size or larger. Here are our results:

 Last weekMondayTotal
English Walnuts46 pieces128 g10 pieces71 g56 pieces184 g
Black Walnuts123 pieces480 g38 pieces159 g161 pieces641 g

Black Walnuts outnumber English Walnuts by 3 to 1. I think this is a testament to the intelligence of crows. English Walnuts are thin-shelled and pretty easy to crack. But Black Walnuts have thicker shells that are much harder to crack. Spencer and Erik tossed a few on the tennis courts and found that they take two or three drops before they crack. English walnuts crack every time.

I figure the crows are smart enough to realize they can drop English Walnuts on the closest road, but the Black Walnuts are worth the trip to our tennis courts for the harder conrete surface. Asphalt won't do.

The real stumper is whether this behavior is instinct or learned. Crows around the world drop nuts on rocks and roads, suggesting this is instinct. But placing nuts under cars and selectively dropping harder nuts on concrete suggests otherwise. I found no walnut shells on the new outdoor concrete basketball court built last year across the road from the Middle School. Maybe the crows haven't found it yet and instructed their young. I think they will!

Here are a few Web links for further research:

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Copyright © 1999 by Marc Kummel / mkummel@rain.org