Baby, it’s cold outside

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Last week the winds howled and temperatures fell below freezing. Trees and branches cracked and sprawled across the ground. Leaving my house, I pulled my coat tight, drew my hat over my face, and in my fuzzy boots scuttled the 15 feet to my car. I blew a gust of frozen breath in the safety of the front seat. Then I looked out and saw a dark eyed junco, a tiny songbird pictured above, sitting on a branch, calm as can be.

How can a wee bird weighing no more than three pennies spend 24/7 in such cold temperatures without freezing to death?

Down, down, down

That jacket or blanket that you may have for extra cold days is stuffed with down for a reason. Unlike the long flight feathers on a bird’s wings and tail, the down feathers are short and can be fluffed up. This fluff creates air pockets that trap the warmth escaping from the bird’s body. The specialized down feathers can keep a bird’s body temperature at 104 degrees even in freezing weather!

Birds also undergo a fall molt in which they grow up to fifty percent the number of feathers on their body for greater insulation. That fluff is what makes these tiny birds look round as tennis balls in winter.

Have you ever seen birds poking their beak into the base of their tail, then rubbing their beak all over their bodies? They are collecting oil from the uropygial gland and spreading the oil over their feathers. Not only does the oil keep feathers preened and clean, but it also adds a weatherproofing layer that keeps them from getting wet. Can you even imagine the misery of having wet feathers in winter?

Wait, what about my feet?

No matter how warmly I dress, my feet and hands are usually the first thing to freeze in winter. So how do those featherless, little reptilian bird feet stay warm in winter?

For one thing, those sinewy feet have very few nerves, which reduces the cold that the birds feel through their feet. The scales are specialized to minimize heat loss. Birds also take turns lifting one foot and tucking it into their feathers to warm it up, or crouching down over their feet to warm them. 

But they have another brilliant adaptation as well: rete mirabile, which is Latin for “wonderful net.” In the rete mirabile, arteries that carry warm blood away from the heart intersect with veins that carry cold blood from the feet. A heat exchange occurs, and the blood from the feet warms before reaching the bird’s heart.

rete mirabile
This drawing illustrates the process of rete mirabile, whereby vessels carrying cold blood from the feet intersect with vessels carrying warm blood from the heart.

And then there is shivering

Birds are warm blooded creatures and, through a high metabolism, maintain a body temperature at about 105 degrees Fahrenheit. How challenging it must be to keep such a high body temperature in extremely cold weather! When all else fails, birds will start shivering their powerful chest muscles. The pumping of their strong flight muscles sends blood and heat surging through the body. But this relief comes at a cost. Half the calories that a bird needs to survive a wintry day could be depleted in the act of shivering. A chickadee, for example, requires the equivalent of 65 sunflower seeds to survive a cold winter day. Food to replace those lost calories is scarce in winter. Most insects and trees are dormant.

Birds are highly resourceful creatures, and they have adapted to find sustenance in seemingly barren environments. They have exceptional eyesight, and can tease out tiny cracks in tree bark serving as insect hideaways. As gardeners, we can help them by not removing our dead plantings in the fall. Those browned flower heads can contain seeds and insects that sustain birds through the winter (see Hidden Provisions blog entry). And if you are feeling extra generous, you can fill a feeder with sunflower seeds or suet, and enjoy the feeding frenzy that ensues.

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Cardinal waiting for his turn at bird feeder

 

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“What are you waiting for?,” this female cardinal must be thinking.

 

 

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Joining the junco and sparrow for a sunflower seed snack.
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Red-bellied woodpecker snags a seed

 

 

 

 

Hanging On

Most winter trees fall into one of two categories we readily recognize. There are the deciduous trees, whose leaves fall off in winter, and there are the evergreen trees, who keep their leaves throughout the winter. But there are trees that don’t squarely fit into these categories.

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In the photo above, you are looking at an American Beech, which is a deciduous tree. And although its leaves have wrinkled and faded, they hang on tenaciously to the tree. Take a walk in the Sheldrake woods and you’ll notice them right away, like a ballroom of debutantes frozen in time, not ready to hang up their petticoats for the season.

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There is a name for what these trees are doing: marcescence, or the retention of dead plant organs. In deciduous trees autumn triggers a chemical change that makes the tree grow a layer where the leaf stems connect to the branch. This layer creates a separation between the leaf and the branch, and prevents water and nutrients from reaching the leaf. That separation, also known as abscission, is the reason leaves change color, shrivel, and fall off. But in marcescent trees such as oaks, beeches, and witch hazel, even though the abscission layer forms, and the leaves shrivel and fade, they do not fall off.

Why? Why would a tree want to hold on to dead leaves that can no longer produce food?

This is one of those delicious questions that we actually do not know the answer to. There are theories, but in the absence of sufficient testing, we cannot say which is correct.

Hypothesis #1: Protection from browsers

It is possible that marcescent leaves may provide cover for tree buds from large browsing herbivores such as deer or moose. Crumbly zero-nutrition leaves may provide a layer of deterrence, since animals would have to eat through the leaves to get to the nutritious buds and twigs they like to eat, if they can even see them through the leaves!

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This beech tree bud is protected by its tough wrapping and pointy tip, and is also tucked away between the tree’s dead leaves.

In support of this theory is that marcescent leaves are primarily found on lower branches and young trees, whose buds are within reach of browsers.

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In the above follow the trunk of the beech tree with the marcescent leaves. You will see that all the leaves appear in the lower branches. The upper and middle branches are bare.

Hypothesis #2: Strategic decomposition

A second theory posits that there is an advantage to dropping the dead leaves in the spring, as new growth finally pushes off the old leaves. Marcescent trees such as beech and oak traditionally grow on infertile soil. It’s possible that by dropping the leaves in the spring, they will decompose at the time when the tree most needs the organic compost to spur new growth.

Personally I find this explanation less convincing, for it does not explain why only the younger trees or lower branches sport the dead leaves. Also, decomposition in winter is very slow due to the shorter hours of sunlight and the dormancy of many decomposers, so I am not certain there is much to be gained by waiting until spring to drop to the ground.

Hypothesis #3: Evolutionary vestiges

The first trees were all evergreens, keeping their waxy leaves through winter. As they succeeded and spread, they evolved to suit the different climates in which they found themselves. Some found it advantageous to shed their leaves over winter, to avoid freezing or depleting their nutrient store, and deciduous trees were born. It is possible that marcescent trees such as beeches and oaks are an evolutionary halfway point–deciduous trees that retain their (dead) leaves through the winter. Perhaps these clinging ghosts are an evolutionary vestige, like the human appendix or tailbone, which serves no function but has persisted over time. Lending support to this explanation is the fact that some beech and oak species are evergreens. The Southern Beeches are 43 species of evergreen trees and shrubs that until recently were classified in the same genus as beeches (They now have their own genus, Nothofagaceae, which translates to “bastard beeches.”). Their fossil record dates back to the Cretaceous period. Among oaks, the Quercus ilex is an evergreen.

What do you think? Which do you find most convincing? Whatever the explanation, take this time to walk in the woods and enjoy the rustle of the wind rattling these stubborn stragglers.

Jelly Fungus Among Us

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Photo credit: Svetlana Wasserman

They looked like a group of butterflies resting on a branch. But as I approached, the purplish wings morphed into something gelatinous, fleshy, and etched with veins. I touched them. There was something human and macabre about them, like severed decaying ears. I had mistakenly thought that winter was not a great time for mushrooms, but the rain of the previous day had launched an abundance of them.

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Photo credit: Svetlana Wasserman

What I was seeing was a member of an entire subgroup of mushroom–the jelly fungus, or for you Latin speakers, Basidiomycota. These particular ones are an example of Auricularia auricula, otherwise known as the Jelly Ear fungus. Perhaps not so pretty to look at, they perform a vital public service by decomposing dead wood. When weather is dry, they shrivel and harden, like an old cork. But once the rain returns, they quickly reabsorb water and recover their shape, size and color, and can begin to produce spores. This ability to dry, rehydrate, and revive with a small amount of moisture is a remarkable adaptation, allowing these fungi to endure.

Not ten paces from the first jelly fungus, I came across another oddity. Creamy yellow layers folded in and out of themselves, like buttercream roses. This was also a jelly fungus, growing right amid the turkey tail mushrooms that had already colonized this decomposing log (Some speculate that this fungus actually feeds on the turkey tails).

 

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Photo credit: Svetlana Wasserman

This particular fungus is called witch’s butter. Legend originating in Eastern Europe holds that this mushroom was hung on doorposts by people to rid themselves of a witch’s curse. Piercing the fungus was said to banish the curse.

Like other jelly fungi, witch’s butter is very absorbent, and after a rain it can soak up so much water that it looks more like a slime. Once the water evaporates, the fungus shrinks back and the fruiting body remains dormant until the next rain.

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Photo credit: Svetlana Wasserman

Two sunny side up eggs? No, this is also witch’s butter fungus, but with more water absorbed than the one above. In case it is making you hungry, witch’s butter is edible, although allegedly tasteless. Its gelatinous consistency makes it a good soup thickener and if you are an adventurous eater, you can find these for sale at Asian groceries.

The next time it rains don’t miss the opportunity to seek out the jelly fungus among us, at Sheldrake or another woods near you.