Hammerhead, harpoon-tongue

We’ve had a Hairy Woodpecker (Picoides villosus) hanging around the house for the last week or two. For the most part it’s remained some distance away, up high or hidden by leaves, hard to observe but easy to detect through its tap-tap-tapping as it chips away bark looking for insects. Hairy Woodpeckers have always struck me as being less common than the smaller Downy Woodpeckers, as I find I don’t encounter them nearly as often when birding, and I’ve always found it a treat to have one hanging about.

The second edition of the Atlas of the Breeding Birds of Ontario includes maps of abundance levels throughout the province for most species (anything that can be regularly detected on point counts), and it confirms this assessment. It indicates that the Downy is at its peak abundance, up to 7 birds per 25 point counts (the standard unit of measurement for the abundance maps) through most of the Carolinian forest region extending southwest from Toronto, and patchily throughout the rest of the province. The Hairy, meanwhile, is encountered at a rate of less than 1 bird per 25 point counts through the same region. Even in the areas where it reaches peak abundance, through the more rugged Canadian Shield, its abundance is still only a maximum of 3 birds per 25 point counts – less than half of what the Downy reaches in its peak zones.

Here in the Frontenac Axis we’re right in one of those peak abundance zones for Hairys (it’s incidentally also a good spot for Downy and Pileated, likely simply due to all the deciduous and mixed forest around here). We hear the other guys around, but the only one to spend much time on our property has been the Hairy. Although the Hairy is noticeably larger than the Downy, without aid of a size reference, the best way to tell them apart is by the bill: a Hairy’s bill is about the same length as his head, while a Downy’s is only half the size of his head and has a rather petite look to it. Incidentally, although the two species look virtually identical, they’re not all that closely related. Their similar patterning is a wonderful example of convergent evolution – two species acheiving the same result by two separate, independent evolutionary paths.

Yesterday morning she was not far off from the deck, and she was unusually low. The sexes are told apart by red at the nape of the crown in males, while the back of the head on females is all black. This is true also for Downys, and virtually all other woodpecker species have some form of dimorphism between the sexes, although the particular feather region in question varies.

I watched her hopping up the tree trunk. There are a few groups of trunk-foragers, which have all developed their own special niches. Woodpeckers are borers, in that they’ll drill holes or chip off bark to get at the bugs sequestered underneath. They hang upright off the side of the tree to do this, using their sharp little claws to grip the bark and their stiff tail feathers like a prop. Hopefully you’ll never have occassion to handle a wild bird, since that probably means it’s run into a window or some other human structure, but should you have the unfortunate (for the bird) opportunity to find and handle one, or happen to visit a banding operation where they catch a (live, healthy) woodpecker, take a moment to check out these specialized features. Of all the non-predatory birds I’ve handled, woodpeckers have by far the sharpest nails. In this photo she’s caught mid-hop, pushing off with her powerful feet and leaping vertically, then grabbing the trunk again with her feet.

The other two groups are nuthatches, which have the peculiar habit of descending down a tree head-first as they forage, and creepers, which go the other direction, up and upright. Both of these groups prod in the cracks of bark and under loose pieces, looking for insects that are hiding there. Their different foraging directions means they can exploit cracks that go unnoticed by the other group.

You can just see her tongue between her two mandibles as she probes here for an insect. Woodpeckers have amazing adaptations to their foraging strategy. The first is the length of their tongue. The grubs they target make tunnels into the bark, which the birds chip or drill into the bark to expose. Once they’ve found a tunnel, the grub can sometimes be down the tunnel at the other end. The bird inserts its tongue, which it uses like a spear to snag the grub from the bottom of its burrow. The woodpecker’s tongue has a stiff cartilaginous tip, covered in sharp little barbs, perfect for pulling things back that are out of reach.

And the bird’s tongue is looong. The tongue is a cross between the muscular structure we humans have, and a rigid bony structure. It’s got a long, branching Y-shaped bone, called the hyoid, that splits at the back of the throat and wraps up behind the head, sometimes ending as far forward as the eye socket or nostril, where it’s secured by elastic tissue. There are muscles that are paired with the horns of the Y, affixed to the back of the jaw, and running to the tip of the hyoid bone. Contraction of these muscles pulls the tips of the hyoid bone around the back of the head and back down toward the mouth, and in doing so, pushes the tongue out the mouth. When the muscles relax, the elastic tissue returns the bone to its original position, and the tongue is pulled back in.

If you part a woodpecker’s feathers on the back of its head you can usually see the hyoid bones and muscles under the skin there. Humans have a hyoid apparatus, too, but ours is very much reduced and not used in the same way (ours is mostly support, and a base for some muscles). Because of the length of these hyoid bones wrapping around the back of its skull, a woodpecker is capable of extending its tongue twice the length of its head, useful when the prey is buried deep. Check out some of the photos that Hilton Pond Center has posted at this link – pretty amazing. The incredible structure and mechanism that operates the tongue (also described on Hilton Pond’s site) are sometimes used as an example against evolution, on the basis that such a wonderfully complex system couldn’t have evolved on its own. Really, though, it’s just a bigger, fancier version of the tongue structure found in chickens. A baby woodpecker starts out with hyoid horns that resemble a chicken’s, and they grow into woodpecker-length as the baby grows.

The other adaptation a woodpecker has evolved has to do with the hammering. Can you just imagine the sort of headache you would have if you had to do this for your dinner every day? A woodpecker’s beak, when it’s hammering against a tree, is moving at about 7 meters per second – the equivalent of about 25 km/h or 15.6 mph – which would be sufficient to cause brain damage in humans when met with an abrupt stop (the force of which would be about 1000 times that of gravity). Most animals, ourselves included, have brains that sort of “float” in the brain cavity, surrounded by a thin layer of fluid and membranes, which allows the brain to keep moving and “crash” into the bone when the skull suddenly stops, causing bruising and brain damage. A woodpecker’s is tightly packed inside the skull by spongy bone that protects it, so it stops when the skull stops. This page notes a couple additional secondary adaptations that also help in reducing the impact of the impacts.

But what about it’s beak? Here the bird is using it to chip away some loosened wood. The beak is used as a multitude of tools, including crowbar and chisel, but it’s mostly used as a drill (or hammer, depending on how you look at it). Surely the force of such constant and repeated impacts (up to 20 times per second) would weaken or shatter it. With a normal bird’s beak it probably would, but woodpeckers have special grooves that run at an angle to the direction of force, which strengthen the structure. It’s like trying to stand a piece of paper up on its thin side. Put a crease in it and it’s suddenly got much more strength and rigidity.

I watched the Hairy work the trees for a bit. She was remarkably docile, and seemed unperturbed by my being there. I ran off well over 100 photos of her poking about the trees, but won’t share them all here. Here’s a few more from the collection, though.

She stops for a quick preen. Here she’s reaching back to a little gland that’s at the base of the tail, on their top surface (at the bottom of their back), which has the fancy name of uropygial gland, but the more casual name of oil gland or preen gland. If you look at it up close (again on that unfortunate window-strike, or the luckier banding capture), it looks a bit like a wart, yellow and slightly greasy looking. It produces an oil that’s incredibly important to birds for waterproofing. When you see a bird running its beak through its feathers it’s doing one of two things. If it’s simply working the feathers then it’s reorganizing all the barbs and smoothing them out so they create a flat surface (ruffled barbs disrupt aerodynamics and mean the bird burns more energy when flying; also they can allow more heat/temperature loss). However, if it’s reaching around to its tail regularly, it’s squeezing the gland to produce oil that it then works through the feathers for waterproofing.

She showed interest in this cavity. I thought it was a little late for her to still be nesting, but not impossible, and it would explain why she’d been hanging around the area so closely.

She paused and looked around, then looked inside again, before going in.

The hole was fairly deep, and she slipped down, first just her tail feathers protruding, then disappearing entirely.

She was only gone a moment before her head popped back up, though. She did this twice during the span that I watched. Neither time did she seem to go in with food. The second time she came out with what appeared to be a wood chip. I didn’t hear any cheeping, and baby woodpeckers are notoriously noisy, so if there were any in there I would’ve heard. But she wasn’t staying in, so she must not have eggs. I had to conclude that she didn’t have a nest after all, but maybe was scoping out potential sites for next year?

Then she checked out this little knot. And pulled out a grub. Maybe she was just looking for food after all.

I’d been watching her a good twenty minutes or so, running off constant photos, curiously following where she went and what she was doing, when she headed up this slanted branch. She swung around to its underside, and then as she twisted her head to probe into a crack in the bark…

…I could see red in his crown! Yes, all along it had been a boy. Young males, males that were hatched this summer, won’t have that red nape yet, but they will have red speckled through the crown as this one does. It’s funny that I didn’t notice it at all in the 20 minutes prior. Especially since, once I got all the photos onto my computer, it was incredibly obvious in so many of the photos. I had to pick through them carefully to make sure I chose ones that didn’t show any red for the images above! Females can actually show a bit of red in the crown as juveniles, but it’s rarely extensive (this is on the intermediate side of extensive, and it could be hard to call this either way, but I’d lean toward boy). I’ll blame my lack of attention on the lighting. Or being so caught up in watching his behaviour. Or something.