Bird song

corvids are songbirds… so why don’t they sing?

One day in early November, after a morning of plonking about on snowshoes for the first time this year, I sat myself on a snowbank and leaned against an old alder tree—time out for a snack. My companion perched at the base of another alder a few feet away. We’d spent the morning looking at animal tracks in a meadow. As soon as we opened our packs to dig out our lunches, we were visited by a raven, who called and attracted another one. But they ignored our (admittedly) small offerings and departed.

Thinking about the raven calls reminded me of some recent reading about the singing behavior of birds and a lingering question. First, some background: ravens, crows, and jays are classified as songbirds on the basis of both morphology and genetics. But their singing behavior differs from that of other songbirds in an interesting way.

Most songbirds sing during the nesting season, sometimes all day long, sometimes mostly in the morning. Males may use one song before dawn and another after sunrise, and there is usually a dawn chorus of many species all vocalizing at once. Morning bird songs are something that lots of folks, not just birders, look forward to in temperate-zone springtime and even sometimes in the tropics.

Bird song has many functions. Males advertise their territory ownership, telling others of that species to stay away. Sometimes, two males face each other aggressively at a territory border and have song duels, each one trying to out-shout the other. Males’ songs also attract females to their territories, the vigor of their songs indicating their state of health and motivation. Female songbirds may sing too—telling other females to stay away from that territory. Or, in some cases, a female might sing to tell her mate to bring her a snack.

The ‘voice box’ of birds is totally different from that of humans, other mammals, and reptiles. We have a cartilaginous larynx at the upper end of the trachea (windpipe). The larynx is thought to have evolved from a valve involved in swallowing (and when we swallow, the larynx moves). We sing or talk by moving air out of the trachea and through the larynx, causing vocal membranes to vibrate; muscles linked to the cartilages control the sound. The closest living relatives of birds are crocodiles and they too use a larynx for vocalization. However, although birds have a larynx, it’s not used for making sounds; they use another structure, called a syrinx. The avian syrinx is located at the lower end of the trachea where it branches into two passage-ways (bronchii) going to the lungs. The syrinx apparently evolved, not from a valve, but from structural supports for the branching airway, and very different muscles are involved in control of its vibrating membranes.

Because the syrinx is in two parts, at the beginnings of the two bronchii, songbirds can sing two distinct notes at the same time, something a larynx cannot do. This is not the same as Mongolian throat singing, in which singers learn to exploit the harmonics (overtones) of the fundamental note. The two distinct notes sung by a songbird are not harmonically related. Air is moved from the lungs into the trachea, as most song is produced by exhaling; but birds can also produce some song when they inhale. Loudness and pitch are changed by altering air pressure and by changing the tension of the muscles that control the elastic, vibrating membranes of the syrinx.

Jays, crows, and ravens have the avian syrinx, but they don’t use it in the same way as other songbirds. They can make a huge variety of sounds; the loud, raucous ones are most familiar to us. But they are perfectly capable of making musical sounds and do so, quietly, upon occasion. If you listen carefully, you might hear them! But these songbirds have no dawn choruses, no song-battles for territory, no lovely singing to attract mates. That begs the question: Why not?

Sheep Creek Valley

nest-building, a song chorus, and a wildflower show

In early June, Parks & Rec hikers went up the Sheep Creek trail on a day of fitful rain showers and intermittent sunshine. This is a favorite trail, but it was clear that the trail could use some work! The uphill portion of the trail is seriously eroded by water coursing down the trail. The long traverse below the road is cut by deep erosional gullies and the edge of the trail is collapsing in spots. Along this stretch, cow parsnip overhangs and obscures the trail. Once in the valley proper, the going is easier, although several wind-shattered cottonwoods and sagging willows lie across the trail and there are more erosion cuts. Some of these things are easily fixed, while others are significantly more challenging.

This was a good time to go up into the valley, because it is rich in nesting, singing songbirds. Even though the P&R summer hikes begin well after the early-morning chorus of bird song (and my hearing is not as good as it once was), I identified the songs of twelve songbird species, plus hooters on the hillsides. One species, in particular, was a treat: Swainson’s thrushes commonly nest up there but they arrive later than the others; I don’t usually hear them until June. By that time, robins and fox sparrows are feeding chicks and juncos have fledglings.

swainson's-thrush-by-bob-armstrong
Swainson’s thrush with nest material. Photo by Bob Armstrong

Swainson’s thrushes nest all across northern North America and down along the Rockies. They spend the winter mostly in southern Central America and northern South America, although some go as far as northern Argentina. When they at last arrive here in spring, the female builds a nest, usually in the understory, lays her eggs and incubates them, while the male sings. But both parents tend the chicks. Hearing the song of that species is certainly a treat for me, but my favorite remains the ruby-crowned kinglet’s cheering carols from the canopy.

Right next to the trail we found a very large scat of a carnivore, full of fur and bones, artistically arranged. A wolf (or possibly a bear) had dined well, probably on marmot.

On this hike, some of the wild flowers were appearing—lots of buttercups, some chocolate lilies and miner’s lettuce, three kinds of violets, a few enchanter’s nightshade. Occasional salmonberry canes bore flowers, but there were wide stands of dead canes, some of which showed no evidence (?yet) of new canes coming up at the bases of the old ones. Does that bode ill for salmonberry production up here this year?

A special floral sighting was a clump of some kind of saxifrage, growing on boulder. We’d seen this on previous hikes too and noted the leaves with three sharp terminal teeth. That made identification simple—the three-toothed saxifrage. The leaf margins have scattered hairs, a feature that led us astray for a while, but consultation with real botanists eliminated the confusion and confirmed the name. This species is not common in our area, but it seems to be the only saxifrage here with three-toothed leaves. The white petals have reddish spots on them (so does another species, but that one has different leaves). It’s fun to try to figure out such things and learn new species; now if I can just remember all the distinguishing features…

Of course, having the right name is just a small part of any story! Many questions lie in wait for curious naturalists. What insects pollinate this plant? What is the function of the spots on the petals? Do the marginal hairs on the leaf deter some herbivores? Does this plant typically grow on rocks? And so on. That’s where the real interest and fun lie!

Just for fun of a different sort, on a completely different topic: I put up a peanut butter feeder on my deck this spring. A simple thing, it consists of a small block of wood with pits (for peanut butter) drilled into both sides of it. This dangles on a hook where I can see it easily, while lazing in my big comfortable chair. The chickadees found it almost immediately and visit it regularly. Did they know that this funny-looking thing might have food or are they just curious? Once there, one experimental peck would tell them there were goodies to be had, worth coming back for. For several weeks, I saw only chickadees there. Then the juncos began to come. Maybe they saw that the chickadees were making repeat visits and decided to check it out. They are considerably larger and much less acrobatic than chickadees, but they somewhat clumsily began to perch on top and reach down to the peanut-butter-laden holes. As time went on, they became more adept and more skillful at extracting several nice bites before losing their balance and fluttering down. Clearly they were learning how to exploit a new resource!

Occasionally other birds came too; a hairy woodpecker clung to the side of the feeder and reached quite easily over to the food source. A Steller’s jay sat on the deck railing, scoped out the situation, and flew straight at one of the gobs of peanut butter, snatching out a good mouthful on its way back to the railing. That worked, so it repeated the maneuver a couple of times. But it has not been seen again.

 

Vocalization and predation

begging chicks, whispering whales, and clicking moths

Four baby juncos, in a nest tucked under a dropping clump of grass, lie low and are very quiet. Only when their parents come with food do they raise their heads and beg. When the parents leave, the chicks again are still. There’s a good reason for this: a nestful of lively, loud chicks would probably attract predators, who are always on the watch for succulent little morsels. Even the repeated back-and-forth trips of attentive parents are often enough to alert watchful predators to the location of a nest.

The nestlings of many other songbirds (robins, sparrows, warblers, etc.) behave in the same way, for the same reasons. They nest in open-cup nests, which are vulnerable to all comers. Cavity-nesters, such as woodpeckers, can be a little more brash in their protective holes. Predators may come, but only some of them can enter or reach into a deep cavity, and if they do, they may face a barrage of sharp beaks. Predations rates on cavity-nesters are much lower than on open-cup nesters. As the chicks get bigger, they sometimes perch right next to their front door, poking their heads out and yelling for food (that’s how I easily found the nest of a black-backed woodpecker, some years ago).

Ducks and shorebirds do it differently. Wherever they nest, the chicks leave the nest soon after hatching, typically following a parent around but feeding themselves. When incubation is done, there are no back-and-forth parental feeding trips to lure a predator to a particular place. The family is now a moving target, not a stationary one.

Among mammals that are subject to predation, we find a similar dichotomy. The young of some species follow their mothers, but in other species the babies cuddle in a nest. Humpback whale calves stay close to their mothers, and recent research shows that they talk to them in ‘whispers’—soft vocalizations that cannot be heard at any distance. This may reduce the risk of killer whale attacks. The young of deer, moose, zebras, elephants and other large herbivores are also able to accompany their mothers soon after birth; I wonder if they whisper too!

Young marmots stay near the den, which offers a quick retreat when danger threatens, and young beavers gain some protection from the pond outside the lodge, a haven, when the alarm sounds. Smaller mammals have a variety of arrangements, mostly depending on being cryptic and hard to find.

But some small rodents add another feature that improves safety: They can produce ultrasound: too high-pitched for human ears (hence, “ultra”), these sounds have a very short wavelength along with the very high frequency. Such sounds attenuate rapidly with distance, so they do not carry very far; they are more directional than low-frequency sounds, but they get scattered by reflecting off twigs and leaves. Young lemmings, mice, and rats emit ultrasounds to call their mothers, if they have become separated too long; the calls prompt the mother to retrieve the wandering pups. Although many predators of small rodents cannot hear ultrasound, some can (e.g., dogs and cats and their relatives), and a short-range call of distress might reduce the risk of predation from such carnivores.

Adult small rodents also use ultrasound as a form of social communication within a group, quiet talk among companions. Certain ground squirrels emit ultrasounds that alert others to the presence of a distant threat, the rapid attenuation ensuring that the sound does not carry as far as the potential threat.

On the other hand, some predators have evolved the ability to use ultrasound in hunting—as an aid to predation (rather than a way to avoid it). The toothed whales use echolocation (sonar), much of it in the ultrasonic range, to navigate in turbid waters and to detect their prey. Our resident killer whales, for instance, use ultrasound to locate and capture their fish; the transient killer whales, however, seldom use it while hunting their prey of marine mammals. Not only are the prey mammals much larger and easier to see than the prey fish, typically, they are also more likely to be able to hear the sonar calls of the hunting killer whales. So the transients usually hunt silently.

Bats are perhaps the best-studied predators that hunt using ultrasound. The short wavelengths permit the sounds to bounce off small prey, such as insects, and bats emit very high intensity (‘loud’) ultrasounds as they close in on a hapless bug. Not all insects are hapless, however! Some toxic tiger moths make ultrasonic clicks to warn off approaching bats, which then often abort their attack. Other, nontoxic, tiger moths use their ultrasound to jam the sonar of an attacking bat, making the attack less likely to be successful.

Echolocating calls sometimes also allow the bats to communicate with each other, as they are looking for roosts or food. Because the calls can be individually recognizable, young bats can communicate with their mothers, and friends can talk to each other (although others may eavesdrop). There remains much to be learned about the social uses of sound in bats.