The Natural History of Dark-Phase Hawks

I wish I had photos to illustrate this post, but I don’t, it’s just something that’s been on my mind.

I’m not sure I’d ever laid eyes on a dark-phase Red-tailed Hawk before I moved to Eastern Oregon. Now I see them all the time. (At first, I’m embarrassed to admit, I think I mistook a few of them for Golden Eagles. Western raptors are still a new thing for me.) Click here for a photo of a typical Red-tailed Hawk, and here for a photo of the dark variety. Same species, two very different-looking birds.

A number of other Buteo raptors also have dark and light morphs – Swainson’s Hawks, Rough-legged Hawks, and Ferruginous Hawks are all found here and all include both dark and light birds. This variation is genetic (think of hair color in humans), and while dark birds are generally less common that light birds in every species, this varies by geography – you’re more likely to see a dark Red-tailed or Ferruginous Hawk out west, more likely to see a dark Rough-legged Hawk back east. What I want to know is, why? Is natural selection at work here, and if so, how are different color morphs adaptive for different regions?


A Walk in an Aspen Stand

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Back in Wisconsin, aspen suckers grew like weeds after an area was logged. It came as a surprise to learn, when I moved out West, that here aspens are in serious decline and the focus of conservation efforts. Above is a stand that we’re going to be building a fence around soon at work, to protect it from damage from cattle, deer, and elk.

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It’s doubtful that elk actually spend much time in this pasture anymore (though they’re definitely around in the hills), but the dark scars on the trunks of the trees come from elk scraping at the bark with their teeth to get at the nutritious, photosynthetic layer of bark under the white outer layer.

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While aspen trees do produce flowers, most of their reproduction is vegetative, in the form of new shoots or “suckers” growing from existing root systems. Young, tender suckers are super tasty food for deer – the ones in the photo above, growing in the shelter of a fallen adult, have been heavily browsed. Fencing the deer out of the stand will help young trees get established.

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The way suckering works is actually pretty interesting – the crown of the tree produces a hormone called auxin that inhibits the production of suckers. When the tree falls and auxin is no longer produced, the growth of new suckers increases in response to keep the stand going. An aspen stand is really one big organism, interconnected by the root system that keeps on living even as individual trees die and are replaced. One particular aspen clone in Utah is a candidate for the world’s largest, oldest organism.

There are multiple reasons for aspen decline in the West, including the removal of top predators from ecosystems (no wolves -> more elk and deer -> more browsing of aspen) and the suppression of natural wildfires, which allows other trees like junipers to become established and crowd out aspen. Climate change is almost certainly playing a role, as well. More information:


Fire and Fireweed

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This is what the “forest” I was in for work yesterday looked like. One of the teenagers I was with asked me why someone didn’t just cut down all the standing dead trees, and I explained that snags like this are actually important habitat for wildlife such as woodpeckers. It does look pretty bleak, but even on a hot, dry, dusty August day, there were splashes of color.

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That layer of purple is the aptly-named Fireweed (Chamerion angustifolium). It’s a “weed” because it’s a pioneer species, quickly colonizing disturbed habitat – especially (as in this case) burned-over forest, but I also saw it growing along roadsides back in Wisconsin. However, unlike a lot of weedy, fast-growing roadside plants, this is a native species. This is one beautiful, welcome weed.


In the natural order of things, the trees that re-grow after a fire (you can see plenty of baby trees in the first two photos if you look) will soon out-compete the sun-loving pioneer weeds. Everything has its place and time, and even a bleak-looking forest of charred tree trunks is full of life.



Red oak in September:

Red oak in October:

Red oak today:

See all the dead leaves still clinging to its branches? This is called marcescence. A lot of oaks and beeches, especially younger ones (and especially on lower branches, as seen here), hang onto the past season’s old dead leaves throughout the winter. The dead leaves finally drop off in the spring when new ones grow in their place. No one is quite sure why some trees do this, but there are a few theories – well, a lot of theories.

  • Dropping dead leaves at the beginning of spring could provide a burst of fertilizer to start off the growing season as the leaves decompose.
  • Keeping the leaves in the winter could help trap more snow around the bases of the trees, giving them extra moisture when the snow melts in the spring.
  • The dead leaves could even provide some protection from cold and frost for the tree’s buds or even deter browsing by herbivores.

For more information, check out this great article on marcescence by a professional forester in Vermont. Are any trees marcescing (did I just make up a word?) in your area?


The Strange Tale of the Deciduous Conifer

We have a long list of native conifers here: red, white, and jack pine, black and white spruce, balsam fir, eastern hemlock, eastern red cedar, and… tamarack. Tamarack is different. Tamarack is the only one that does this.

I really like tamaracks. I’ve written about them before. I keep wondering about them, though. Why are they deciduous, losing their needles every autumn, while every other species of conifer found here is evergreen? Things like this usually don’t just happen at random – there must be some of sort of adaptive advantage to shedding their needles, right?

Boreal forests are mostly dominated by evergreens. Keeping your needles year-round cuts down on the amount of nutrients you need (you don’t need to manufacture new leaves as often) and lets you photosynthesize during the winter, instead of relying on the short northern growing season. So why are tamaracks and other larches among the most successful boreal trees of all, growing far north of the Arctic Circle? Some Googling led me to a 1990 paper on the subject by S.T. Gower and J.H. Richards.

According to the paper, the longer a leaf lives, the lower its photosynthetic rate: individual tamarack needles don’t live as long as those of evergreen conifers but they can photosynthesize like crazy in the long days of the northern summer. What’s more, since tamarack needles don’t have to survive the cold, dry conditions of winter, tamaracks don’t have to invest nutrients in constructing thick protective cuticles them. Ever stroke the foliage of a tamarack? It’s very soft. Tamaracks still get the advantages of having a conifers’ needles-shaped leaves and cone-shaped canopy, though, so they can compete where spreading, broad-leaved deciduous trees like maples and birches can’t. (Among other things, being cone-shaped helps conifers get the maximize their access to sunlight.)

Everything in nature is a trade-off. I still have a zoology department t-shirt from my undergraduate days featuring a drawing of a giraffe bending down to drink captioned with the words “Life is a Compromise.” Tamaracks are a compromise between two different tree lifestyles, and it must work well for them, because they thrive in some of the harshest environments around.


What Does an Oak Tree Sound Like in September?

If you blindfolded me and dropped me somewhere on the property at random right now, I would still be able to tell you whether or not there was an oak tree nearby, based on the sounds alone.

The chattering and scolding of the red squirrels. The screams of the Blue Jays. The knock-knock-knock of the jays’ bills against the acorns’ tough outer husks. The loud smacks of discarded caps and husks falling to the forest floor. It’s harvest season, and the animals are taking full advantage of this year’s acorn mast, which I first wrote about a month ago.

Like squirrels, jays are voracious acorn eaters (you’ll have to excuse the fact that I couldn’t get a good photo of one with my point-and-shoot camera). Researchers have found that a single bird can harvest and cache about 110 nuts a day. What they can’t eat right away, they cache (store for later) by burying it in the soil and leaf litter, and since they don’t find and eat every acorn they cache they’re also planting the next generation of oak trees. For more information about the relationship between jays and oaks, check out the article Jays Plant Acorns from the University of California’s Oak Woodland Conservation Workgroup.

Last winter I wrote about squirrel caches and posted a photo of the scraps of pine cone left behind after a squirrel’s meal – I suspect that this winter those middens will contain acorn husks as well!

Next week I will be leading a backpacking trip to the Porcupine Mountains for my students, so (obviously) I won’t be on the internet at all. I have three fabulous guest posts scheduled for you, but I won’t be replying to comments etc. until I get back, at which point I’ll hopefully have lots of new photos and stories to share.


What’s With the Little Holes in Acorns?

I’m sure you’ve all seen acorns with small holes in them, often right under the cap, like on this one. What causes that? To the best of my knowledge it’s actually an insect called acorn weevil, an insect in the genus Curculio. The female lays her egg in an immature acorn. The larva develops inside the acorn, and then when the nut ripens and falls it bores its way out to live in the soil until it’s ready to mature into an adult. If you want to see what the weevil actually looks like, it’s actually the subject of possibly one of the greatest insect photos of all time. For more information, the Michigan Entomological Society has a great page on acorn insects.

And now, because I’m talking about a species of Curculio, I’m sorry but I cannot resist posting the weevil joke from the naval movie Master and Commander. Again.

Capt. Jack Aubrey: Do you see those two weevils, doctor?
Dr. Stephen Maturin: I do.
Capt. Jack Aubrey: Which would you choose?
Dr. Stephen Maturin: Neither; there is not a scrap a difference between them. They are the same species of Curculio.
Capt. Jack Aubrey: If you had to choose. If you were forced to make a choice. If there was no other response…
Dr. Stephen Maturin: Well then, if you are going to push me… I would choose the right hand weevil. It has significant advantage in both length and breadth.
[the captain thumps his fist in the table]
Capt. Jack Aubrey: There, I have you! You’re completely dished! Do you not know that in the service, one must always choose the lesser of two weevils!

You’re welcome.


A Forest After a Fire

Over the weekend a friend and I checked out a stand of trees that burned back in May. It wasn’t a natural forest, rather a plantation of red pines, but forest fires aren’t nearly as common here around the Great Lakes as they are out west and it was interesting to see up-close what woods look like in the aftermath of a fire. Because this particular fire only burned for a few hours, the trees are still standing, but they are mostly dead and dying with blackened trunks.

In some spots a lush carpet of grass and ferns had sprung up, probably in response to the burst of nutrients the fire released.

Wood-boring beetles are slowly working on demolishing the standing dead trees, and we could literally see and hear the process. Sawdust slowly drifted through the air around us and settled at the bases of the trunks.

Most amazing of all, we could literally hear the sound of the beetles chewing and gnawing all around us. Listen!

At the edge of the stand of pines a fire break was created to keep the blaze contained, and the contrast between burned on one side and not-burned on the other was sharp.

Not a bad way to spend a Saturday morning!


How Does a Sapsucker Select a Tree?

Last spring I was in the woods with a group of kids and stopped to point out rows of small, neat holes that had been drilled into the trunk of a paper birch, standing out dark against the white bark. “That’s from a sapsucker,” I told them. We’d already looked at the gaping cavities the Pileated Woodpeckers had excavated in the nearby cedars, but now I explained how Yellow-bellied Sapsuckers, another woodpecker species, make small, shallow holes like this and ate the sap that seeps out, rather than tearing the wood apart in search of insects.

“So is their favorite tree the sugar maple?” asked one of the kids.

What a good question – and not one I knew the answer to. We talked a bit about what trade-offs sapsuckers would face when selecting a tree, like the sugar content of the sap versus the hardness of the wood, and then we moved on to something else. The idea stuck with me, though. Do sapsuckers have a favorite tree species? What factors affect which trees they make their sap wells in?


Oak Mast

We currently seem to be experiencing a bumper crop of acorns – enough so that it’s noticeable even though oaks aren’t all that common here. The technical term for this is mast. We are having a mast year.

Oaks don’t produce large amounts of acorns every year (same goes for other nut trees). Instead, the trees in area will synchronize themselves so that, at irregular intervals, they all go acorn-crazy at the same time. The most popular theory as to why is that when all of the trees go all out at the same time, there are just so many nuts around that the squirrels and other nut-eating critters can’t eat them all even if they gorge themselves, so at least a few will get to germinate and grow. If one oak tree were to be a rebel and produce a lot of acorns in a year when the other trees were holding back, all the squirrels in the forest would descend on that one tree and it wouldn’t get to reproduce. (Squirrels aren’t the only animals that eat acorns, of course – earlier this summer one of the people I work with spotted a black bear way up in the highest branches of an oak tree, feasting on them.)

Of course, the real question here is how the heck are a bunch of trees spread across a forest communicating with each other and reaching a consensus on when to produce mast? And how are they balancing this cooperation with competition, natural selection favoring the trees that manage to reproduce more than their neighbors?

The simple answer seems to be that no one is really sure, but that doesn’t mean there aren’t a couple ideas. Plants can communicate with each other through chemical signals, messages like “look out everybody, there are lots of herbivores around.” A new study that’s gotten some media coverage even suggests that some communicate by producing clicking sounds in their roots that other plants can sense – plants don’t have ears, but maybe they can sense vibrations? So maybe our oaks are doing something like this, but they’re not in dense stands here like the maples and hemlocks are, they’re scattered in among the other more common trees. It’s questionable whether chemicals and clicks would really be effective at transmitting information across an entire forest.

There are other ideas too, like maybe trees are responding to some environmental cue and we just don’t know what it is, or maybe they need a certain number of years to store up energy before they flower and, since they need each other’s pollen to produce acorns, eventually they all either get on the same cycle or fail to pass on their genes.

In any case, tree mast is a really cool phenomenon, and if you know more about it than this feel free to jump in via the comments. Is it a mast year where you are?