|The Question: We saw this large nest while hiking in Wisconsin. Who made it?
Submitted by: Claudia, Wisconsin, USA
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The Question: I’m trying to identify the burr/seed in this photo. I think it’s from a weed I’ve been pulling from my flower garden. I pulled off about 100 from my shirt. They grip like iron!
Submitted by: Jane, southwestern Connecticut, USA
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The Short Answer: These holes are classic foraging holes of the pileated woodpecker (Dryocopus pileatus), the largest woodpecker in North America (assuming Campephilus principalis, the ivory billed woodpecker, is extinct). The ones you found do look remarkably regular, as if made by a saw or a wood chisel. You must have an exceptionally neat woodpecker in your neighborhood. Carol Hartwig, a retired wildlife consultant who has studied pileated woodpeckers, says pileated woodpeckers often make square or rectangular holes. She speculates that the rectangular shape may be energy efficient. She also notes that the grain of western red cedar (Thuja plicata), which is what your tree trunk looks like, is particularly straight.
“The grain of western red cedar is so straight that when the woodpecker hammers with its bill into the trunk, large, straight pieces are excavated. This results in very square corners and a very clean rectangular shape. Their nest cavities are beautifully oval, however, so they are capable of making different shapes depending on the purpose and the type of wood they are excavating.”
Your holes are far too close to the ground to be attempts to build a nest cavity. More likely, the bird was looking for insects, its main source of food. Ants and termites are favorites, and beetle larvae and other tasty insect treats would be on the menu as well. Given the size of the hole, I’m guessing the bird was having good luck.
Tree Holes are in Demand: One group of researchers observing the nests of pileated woodpeckers saw wood ducks (Aix sponsa) repeatedly attempt to steal a choice pileated woodpecker cavity. The nest already contained three hatched chicks and the male woodpecker remained on guard a short distance away. When a wood duck hen would attempt to fly to the nest tree, the male woodpecker would intercept her and prevent her from going inside. However, the researchers noted one time when the wood duck hen entered the nest as the woodpeckers were off foraging and another time when the duck was so quick to the tree, the male woodpecker was caught off guard. In both instances, the male woodpecker entered the nest, a fight ensued, and the woodpecker eventually expelled the wood duck hen and retook his family’s nest. For animals that nest in tree cavities but can’t excavate their own, like the wood duck, the relatively large nest cavities of pileated woodpeckers are prime real estate.
Sources: Conner, RN, Shackelford, CE, Saenz, D, et al. (2001). Interactions between nesting pileated woodpeckers and wood ducks. The Wilson bulletin, 113(2), 250-253.
CONNER, RN, JONES, SD, & JONES, GD. (1994). Snag condition and woodpecker foraging ecology in a bottomland hardwood forest. The Wilson bulletin, 106(2), 242-257.
Hartwig, CL, Eastman, DS, & Harestad, AS. (2006). Characteristics of foraging sites and the use of structural elements by the pileated woodpecker (dryocopus pileatus) on southeastern vancouver island, british columbia, canada. Annales zoologici Fennici, 43(2), 186-197.
The Short Answer: I’m answering this question a tad late, in mid-April, so most of the snow in the U.S. is gone, but as deep snow melts, it often melts first around the base of trees, as Raymond mentions. Notice that a snow ring shows in this photo of a flower pot. This is a clue that it’s probably not that the trees are generating heat biologically, although some plants are capable of doing that. What happens with trees is that the tree branches keep some snow from reaching the ground. Suspended in the branches, that snow is subjected to more sun than snow distributed on the ground so it melts and never falls all the way to the ground under the tree. Also, when the sun is shining, the tree absorbs more heat than the surrounding snow. This happens because the tree is darkly colored — especially after the snow has melted so that the dark branches or evergreen needles are exposed. The tree trunk gets warm, and then radiates some of that heat back out, which melts the snow around the base. The same thing happens with the flower pot and other objects like telephone poles and fence posts.
More Information: This phenomenon occurs under deciduous trees that have dropped their leaves, and even more under evergreens because they catch more snow, and the dark color of their needles helps them absorb more heat, compared to deciduous trees, which are generally lighter colored. In fact, in northern evergreen forests that get large amounts of snow, the “snow wells” around trees can be two meters (6 feet) deep or more. This represents a serious danger to hikers and skiers who sometimes fall into the wells and can’t get back out.
This website gives information on the dangers of snow wells and tips for escape: http://www.deepsnowsafety.org/index.php/
Snow well paradox: Scientists study snow wells because the phenomenon that creates them has a significant effect on how heat and water are absorbed in northern forests. In their studies, they’ve found something surprising. Though the warmth radiating from the tree often melts the snow around the trunk, that same area also experiences the deepest frost. How does that happen? The bare ground at the base of the tree is exposed to the frigid winter air, especially at night or in cloudy weather when the sun is not warming the tree. The surrounding areas, covered by snow that is often quite deep, are insulated from the worst of the cold. Though the snow melts around the tree, paradoxically, that’s also where the frost goes deepest.