Greenery in avian nests

why do some birds add fresh sprigs?

Most birds build nests chiefly of plant parts—branches, twigs, grass blades, mosses; in some cases, mud is a major component. Feathers, lichen, spider webs, plant down, hair, and other materials may be included by various species.

Less well-known (to non-ornithologists) is that many birds, from songbirds to raptors and herons, also add fresh, green-leafy, nonstructural material to the nest. In general, the added greenery is from species that have aromatic leaves, rich in volatile compounds; these plants are a highly non-random, carefully selected portion of the plants available in the nesting habitat. The persistence of such a habit in so many species suggests that the use of greenery contributes in some way to reproductive success and reproductive fitness. The search for fitness consequences has led to numerous studies, but many questions still remain tantalizingly unanswered.

But before we go into all that, let’s first establish that—contrary to much conventional ‘wisdom’—birds have a decent-to-excellent sense of smell. Depending on the species, they use it to locate insects in leaf litter or krill in the sea or carrion, to identify individuals, to locate a nest burrow when returning to it at night, and I bet that’s how they found my peanut-butter feeders when I first hung them up.

Although many ideas about the function of nest greenery have been suggested, three ideas have been examined most extensively.

Two species of starling express their interest in nest greenery entirely during the time of courtship and pair formation. Males then carry green material into the nest cavity in the presence of a female before egg-laying; that activity is correlated with testosterone levels. Some studies have shown that an increase of greenery led to larger clutches and more male chicks but experimental removal of greenery reduced the likelihood that a female laid eggs in that nest. Perhaps females use the presentation of greenery to judge the quality of the males?? But more testosterone in the males was associated with less paternal care of chicks and more greenery also led to more aggression among females. So the results of the several studies indicate some positive and some negative effects. In addition, other experiments found that nesting (already paired) females with nests decorated by their mates often left the greenery in place during incubation (although they commonly removed greenery added by an experimenter), and one recent study showed that the presence of greenery somehow induced more steady incubation behavior of the female, a shorter incubation period, and bigger chicks.

 Perhaps the most popular idea about the function of nest greenery is that the volatile compounds from the leaves help deter parasites and pathogens. Many of the birds that use greenery add the greens after egg-laying (unlike starlings), during incubation and nestling periods. The volatile compounds are known to have negative effects on microbes and bugs in other situations. In line with that idea, birds that re-use old nests or nest in cavities (places where residual debris could house dormant pathogens and parasites) are more likely to put greenery into nests than birds that don’t re-use old nests and don’t nest in cavities. And one study of many species of songbirds in Argentina showed that botfly parasitism was much less in nests of species that added greenery.

However, studies of starlings found that the anti-parasite effects differed in different populations. And other results were conflicting too: if parasites were reduced, the nestlings were not measurably healthier than those in parasitized nests; contrarily, another study found that improved offspring survival. Then again, even if parasites were not reduced by greenery, nestlings in greened nests did better anyhow, possible because their immune systems were some bolstered (in some still-to-be-determined way) or because the females became better incubators.

Clearly, this common natural history phenomenon needs a lot more study—of different bird species, in different habitats, with different parasites and pathogens, with breeding birds of different ages and with different stress levels, and so on. The reproductive fitness consequences are there to be found, and they are likely to differ among species and situations.

Mark Twain once remarked that “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” And that’s true! But, contrary to Mr. Twain’s implication that conjecture is just hot air, it’s the starting point for science to proceed. T. H. Huxley, famous British biologist and staunch Darwinian, noted that, without conjecture, we rarely get as far as actual facts: Reformulate those conjectures based on observations and limited data into testable hypotheses. Those are necessary steps to discovering real facts—such as those that lie beneath disparate, conflicting results. That’s how science works.

Two species

an encounter with a wary ermine… and some thoughts about tree swallow nests

On a murky day toward the end of February, I went with a friend on the Boy Scout camp trail. Rain and warm temperatures had turned the snow to unpleasant deep slush and puddles in some places. As usual, we were just looking to see what we could see—and it wasn’t much. There were some crows picking through the wrack on the beach, a tiny group of bufflehead moving farther offshore, and a few geese on the far side of the big meadow. Not even any curious seals popping up to inspect us, no sea lions cruising by. A bit disappointing!

We cut through some of the groves on the big berm behind the beach, where the mosses were happily showing off their many shades of green. One spreading tree sheltered several duck decoys. Then, as I was stepping over a few roots, a movement near the toe of my boot made me stop. A small white head with bright black eyes was peering up out of squirrel-size hole in the ground. I signaled to my friend (who walks faster than I do) to come back. Meanwhile the white head disappeared, but briefly, only to re-emerge once more for a quick look-see. The owner of the head did not like two monsters looking at it, so even though we backed well away and waited, it did not reappear. With its wintry white coat, the ermine (a.k.a. short-tailed weasel; called a stoat in the U.K.) would have been very conspicuous on the snowless ground under the trees. We don’t see ermine very often, and this was the highlight of the walk that day.

The same day, in the afternoon, three female mallards arrived on my icy home pond. One of them had scouted the place two days earlier, and now brought along a couple of friends. They were out of luck, though; no open water and no seeds on the ice. The ducks weren’t the only critters that were anticipating spring, however. The previous week, a bear had crossed the ice into my yard, no doubt allured by the aroma of the peanut butter feeders, and left dirty footprints on my downstairs windows. That was not the only bear report for the Valley—ADFG tells me that there have been other early risers (or poor sleepers) this winter.

Some recent reading included a book called White Feathers, by famous naturalist Bernd Heinrich. It’s about tree swallows, those beautiful aerial acrobats that also sing sweetly—some birds seem to have it all! They are cavity nesters, using natural tree holes and readily using nest boxes.

Among many other observations, Heinrich noted that the tree swallows using his nest boxes had a strong interest in white or light-colored feathers, sometimes collecting them from some distance away. Male swallows were especially interested, although females sometimes showed interest too. Small feathers might make a cozy nest, but they had a special use for long, whitish feathers, chiefly during the later stages of egg-laying and the incubation period.

Of course, I wanted to know if our local tree swallows collected white and light-colored feathers too. And they do: inspection of nest boxes here and in Gustavus found white and whitish feathers around the clutches of eggs.

Those long, white feathers are arranged around the edge of the cup that holds the eggs, placed with the quills poked into the bottom of the nest around the eggs, so that the plumes stand up and arch over the eggs. The feathers clearly are not a cuddly cushion for the eggs, and not a snuggly blanket around them; smaller feathers might do that. They might conceal the eggs, but feathers of any color could do that. So why white ones? Are tree swallows the only species that adorns its nests in this particular way?

Photo by Jessica Millsap. This image was taken as part of the Audubon Tree Swallow Project, under permits from the Alaska Department of Fish and Game and the US Fish and Wildlife Service.

Tree swallows are fiercely territorial, aggressively defending an area and sky-space near the nest, sometimes engaging in knock-down-drag-out fights that end in injuries. They defend a chosen nest cavity against other tree swallows and other cavity-nesting species, including wrens, woodpeckers, bluebirds, starlings, chickadees, and others. The supply of suitable cavities is generally limited and competition for them can be ferocious. In some cases, tree swallows even oust chickadees that have already laid eggs and appropriate the cavity.

The long, whitish feathers, arranged to arch over the eggs, would show up well in dark cavities, easily visible from the nest opening. Heinrich suggests that they might possibly be a visible signal that tree swallows occupy that cavity. When the adult swallows are out foraging, such a signal could be useful in turning away other cavity-seekers and thus avoiding injurious battles. More observation and research needed!

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.

 

Bird stories

nest-builders, scat-shifters, and spring singers

One day in late April, two friends and I scrambled up a steep stream-side slope to a perch on a cliff below a waterfall. We hoped to locate a nest of American dippers, which have nested in this spot for many years. Although a dipper sat near the pool below the falls, it eventually just flew up over the falls, and we were no wiser about a possible nest location.

However, as we surveyed the pool and falls, another bird was busy, attending to a clump of moss on a spruce branch above us. A male Pacific wren (formerly known as the winter wren) zipped back and forth, carrying twiglets to that mossy clump, which was obviously intended to become a nest. Male wrens commonly build more than one nest, which are inspected by females during the courtship process. When a female selects one of these male-built nests, she adds a little threshold to the entrance, claiming that nest as her own. If a male builds several good nests, he may attract two or even three females who will raise his chicks.

arnie-dipper1
American dipper. Photo by Arnie Hanger

This nest- building male became disturbed at our presence, fidgeting about while peering at us and then finding an elevated perch from which he sang loudly, as if to make sure we knew we were not welcome (songs are how songbirds advertise ownership of their territories). Birds really do not like to be observed when nest-building—egg- and chick-predators such as Steller’s jays are always on the watch for tasty morsels, and the busy activity of a bird carrying nesting material gives away a prospective nest for the jay or other predator to raid.

We go the male’s message and backed away a little. Although he was still nervous, he resumed carrying small twigs and fibers to the growing nest in the ball of moss. Suddenly, the entire bottom of the nest ball fell out! Apparently, the scrawny twigs of the spruce branch weren’t sufficiently substantial to support the structure or all the in-and-out visits of the builder. The wren vanished into the forest.

A couple of weeks later, I returned to this site. Now the tattered remains of the wren’s nest were tipped catty-wompus, barely clinging to the frail spruce twigs. The wren had clearly abandoned this effort and decided to build elsewhere. I could hear him singing, a little deeper into the woods.

On this visit, however, I did see the dippers in action. They were not building in their traditional site in the cliff beside the falls, but under a mid-stream log, instead. These dippers were a bit late in getting started; dippers on some other streams were already incubating clutches of eggs, the incubating females sometimes fed by the male.

In the middle of May, Gold Ridge still had lots of snow, attracting brave or foolhardy skiers up the trail. Ravens were soaring and cavorting, as usual, over the end of the ridge, sometimes peeling away from the group to roll and tumble acrobatically or to chase a passing eagle.

Two ravens perched on a rock outcrop. Both birds picked up something lumpy and white and moved behind another outcrop just uphill. They came back to the first outcrop without the white lumps and picked up two more. They flew downhill a little way, and there they deposited these objects, carefully placing them in nooks and crevices of the rock. Then they flew away.

This time I could see where the white lumps were placed, so of course I went to look. The lumps were the old scats of a wolf and perhaps a bear, all dry and winter-whitened. What in the world did these ravens want with these old scats? Were they playing some kind of game?

Robins and fox sparrows were singing all over the shrubby slopes above the tram. Above the cross, snow still covered much of the ground, and ptarmigan had left the digested remains of their dinners in the places where they had burrowed under the snow in winter. A flock of pipits flew in and began to forage for insects and perhaps a few seeds in the snow-free patches. Pipits look more slender than sparrows and they typically walk and run instead of hop. They sometimes nest high in the alpine tundra on the ridges.

On my home pond, the mallard battles are over. As many as four males hang out amicably, eating seeds that drop from the hanging feeder and sleeping next to each other. No need to fight now; all the females are incubating clutches of fertilized eggs. This is a big contrast to early-season relationships, when each male fiercely defended his female from the attentions of other males. That doesn’t always work, by the way—the interlopers are sometimes successful. Meanwhile, up on Gold Creek, a pair of harlequin ducks was consorting and foraging. She will nest up there somewhere, and when the clutch of eggs is complete, she will incubate them and he will go back out to sea to lollygag with his chums on some rocky point. That’s the way of it, with ducks!

Birds in a snowy land

nest-building ravens, cocoon-tearing chickadees, and cockle-dropping crows

In the middle of March, I made a quick visit to Gustavus. It was snowing heavily, so the ferry ride was a ride in white-out most of the way. Good for taking naps (not to mention second breakfast and more than one cup of tea), making up for having to get up early and getting my gear on the luggage cart. A peaceful sort of trip.

Naturalists love to look for animal tracks in the snow and conjure up stories to go with them, but there was so much fresh snow falling that tracks were covered quickly. So animal-tracking was not very exciting, but bird-watching offered compensations.

A thick blanket of powdery snow lay on the ground, and snow continued to fall. But that didn’t deter a pair of ravens. They flew back and forth between a tree behind the house, where they had nested last year, and a flat area just across a small river. Coming back from over the river, they often carried big wads of moss; on other trips, bundles of long strips of plant fiber dangled from their bills.

My friends said that the long fibers came from dead cottonwood trees, so we went over to look. Beavers had felled cottonwoods and willows here, and moose had left the marks of their lower incisors on the fallen willows. On a cottonwood log, the loose outer bark had been pulled away and dropped in small pieces on the snow, and the fibrous inner bark had been peeled off, exposing the bare wood. This was where the ravens had been at work.

The ravens were clearly lining a twiggy nest basket with moss and bark, and lots of it—a cushy bed for the eggs still to come. An eagle cruised up the river and received a rough welcome from the ravens, which escorted it off into the distance. Maybe the ravens were just making sure that this eagle knew there was a no-fly zone here, ahead of the time when the nest would have occupants.

One day we saw a raven flopping about in the deep, fluffy snow—taking a snow-bath. It pushed its head forward into the snow, rubbing on both sides, then vigorously threw snow over its body with flapping wings. Moving to a new, still undisturbed, spot, it repeated the process. I wonder if snow works as well as water, for a bath.

raven-rolling-in-the-snow-1-by-bob-armstrong
Rolling in the snow. Photo by Bob Armstrong

The ravens aren’t the only ones who know that spring is coming. Oregon juncos are singing and the sapsuckers are back from winter quarters. Although magpies are still around and so are slate-colored juncos, these will soon head for the Interior, where they nest.

Other birds were out foraging in the snowy landscape. A little group of pine grosbeaks flitted through the shrubbery, chatting quietly with each other and nibbling willow buds. One of them dropped down to the snow and ate the seeds from a fuzzy seed-head that poked up from the snow at bird’s-eye level. Of course, we had to determine what kind of seeds they were, which led to some discussion and then back to the books. Ah, they were the seeds of big-leaf avens, a fairly common plant of open areas.

Just over our heads, a chestnut-backed chickadee perched on an alder, pecking and pulling furiously at something for several minutes. Finally, it began to extract and eat some bright green bits. After it flew off, our further inspection revealed that the chickadee had found a cocoon stuck to the alder twig. The cocoon was very tough—not easy for us to tear open even with forceps (we had to use scissors), but the persevering chickadee had won the prize inside and eaten all the juicy bits except for the very end of the pupa, leaving a fragment of pupal skin. That was one happy chickadee! We wondered how they learn to recognize insect cocoons as potential food sources.

When the tide went out, we strolled along a snow-free beach; what a relief from floundering in the knee-deep white stuff, too soft for those little snowshoes, typical of our area and never meant for powder snow, to do much good. Here the crows were plucking cockles from the silty sand, flying up a few feet, and dropping them. This is a common behavior by which crows crack open a shell to get at the edibles inside, but it depends on the shell landing on something hard enough to crack it. On this beach, there weren’t many rocks, and the chance of dropping a cockle and having it hit a rock was small. One crow tried two different locations and dropped its cockle sixteen times (!!) before it could eat its prey.

There was a stiff on-shore breeze that buffeted the foraging crows. So, instead of their usual walking gait, they often faced into the wind and side-stepped—just as I used to do when wading a fast-moving stream.

Barn swallows and thixotropy

dabbing and daubing for stable nests

One day in early September, I was fascinated by a pair of barn swallows still feeding big chicks in the pavilion by the visitor center. All the other pairs there had long since fledged their chicks, and those chicks would be experienced foragers by the time of migrations to southern climes. The September chicks would leave the nest soon, but they would have a lot to learn about catching insects on the wing, and it seemed unlikely that they’d be proficient foragers by migration time. An uncertain fate!

As I looked at that nest, I began to wonder about how barn swallows manage to build such a nest. Their nests are shallow cups made of little mud balls stuck together, and lined with feathers. Their relatives, the cliff swallows, build a more elaborate, gourd-shaped nest with a narrow entrance, but they use the same basic technique—mud pellets stuck together and the whole edifice stuck to the side of a building or cliff.

But what makes the pellets stick to each other? In a nest under construction, the first pellets to be placed have had a chance to dry a little, and when they dry, they shrink a bit. If a swallow just plopped new, wet pellets into place, the shrinkage rates of the new and old pellets would differ, and this creates weakness and cracks in the structure (a result well known to human potters). Not a good result.

That’s where thixotropy comes in. It’s a fancy but concise way of describing what happens to some seemingly stable materials when they are mechanically agitated. They become fluid, temporarily, and a little later become stable again, often in a new configuration. It turns out that thixotropy (from the Greek words for ‘touch’ and ‘change’) is characteristic of many materials and situations. It’s involved with some metal casting, certain printing processes, and some foods, for example. Perhaps most famously, it can happen during earthquakes, which shake and liquefy wet soils, causing buildings and trees and everything else to sink or tip or slide, sometimes catastrophically.

Animals use this curious phenomenon too. When bald eagles dance up and down on wet sand in order to capture buried sand lance, one effect of their prancing is liquefying the sand, making the fish more accessible. Gulls can use the same trick.

Mud dauber wasps build little cells of mud, stuck to walls. When they add new pellets to the cells, they add a bit of water from their crop, and they buzz. The vibrations liquefy the mud, letting it spread into the earlier, drier pellets. Then old and new pellets vibrate together, achieve the same consistency, and are stable when vibration stops.

barn-swallow-with-mud-by-bob-armstrong
Gathering mud. Photo by Bob Armstrong

That brings us back to barn swallows. They collect mud pellets from puddles and gradually add several rows of pellets to form the nest cup. But they apparently don’t just whack each new pellet into place. Instead, when they add pellets to the growing base, they use a dabbing or dabbling motion. This jiggles the old (drier) and new (wetter) pellets until the water content is similar and their consistency is equalized. As soon as the dabbling stops, the junction of new and old pellets becomes stable. Wouldn’t it be fun to find out if young adult barn swallows know to do this automatically or if they have to learn the hard way (if their first nest-building attempts collapse)!