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

trailside discoveries and memories of field work

I go up into Sheep Creek Valley several times a year; it’s one of my favorite places in Juneau. I was there in mid November with Parks and Rec hikers, and we spotted several things of interest. It had snowed recently, so tracking was good. We found tracks of squirrel, deer, mountain goat, a possible weasel, a large canid that could have been a wolf or just a big dog, and lots of porcupine tracks. Two porcupines scuttled off into the brush as we walked by.

A big conifer tree had a large squirrel midden around its base; discarded cone scales and cores covered many square yards. The main cache of full cones was underground, but this red squirrel was not content with that—it had also wedged cones into every available space between the roots and in grooves of the trunk.

We found a beautiful orange and yellow fungus growing on a dead branch. It is a type of jelly fungus, possibly the one called “witches’ butter”.

Some of the more enterprising hikers went up the slope at the back of the valley, far enough that they were wading in thigh-deep snow. Other, less energetic perhaps, were content to perch at streamside for a relaxed lunch break. The creek was running crystal clear and wide open, so we had hopes that a dipper might show up. Indeed, one did, prospecting for aquatic insects along the edge of the water and moving quickly upstream.

Sheep Creek Valley is among the first places I worked when I came to Juneau over twenty years ago. My first big project was to census nesting birds in various habitats; there seemed to be no previous studies of breeding bird communities that would provide an estimate of avian diversity and abundance in different habitats around here—very basic information for future ecological studies.

So for several years, in spring and summer, my field techs and I studied bird communities in Sheep Creek Valley and elsewhere in Juneau. We counted birds, using a standard protocol, by sight and by songs and calls. We found that this valley has a very rich community of nesting birds, arguably the richest one in our area. For example, we counted several kinds of warblers, sparrows, and thrushes—more kinds than in the spruce-hemlock forest.

Along with the standard censuses by sight and sound, we regularly mist-netted birds in the understory. Our black nylon nests were twelve meters long, and we would set up an array of about ten nets in various places. Then we’d walk the array of nets every hour or so, extract and weigh the birds, and release them. Among other things, the net captures helped us detect birds that were quiet and secretive.

There were a few bears in the valley. Occasionally, we would glance up as we extracted a bird from a net and see a calm bear sitting near the end of the net and observing all of our actions! They didn’t seem to have designs on us or on the birds; apparently they were simply curious.

Our other main activity was nest-searching. This is hard work and lots of fun, rather like a continual treasure hunt. By following a bird for a while, often on several occasions, eventually one deduces the approximate nest location, and then careful searching reveals a nest. It often takes several hours of detective work, perhaps over several days, to locate a nest this way. Once a nest was found, we monitored its progress, from incubation of eggs to care of nestlings to fledging—or until the nest failed. Then, for each species, we could calculate the percentage of nests that successfully produced young. For example, about sixty-five percent of yellow warbler nests were successful but only about thirty percent of robin nests and roughly twenty-five percent of fox sparrow nests were successful.

A principal cause of nest failure was predation on eggs or chicks. By installing small cameras that were triggered by removal of an egg, for instance, we learned that predators include Steller’s Jays, red squirrels, mice, and even shrews (see accompanying photo). But Sheep Creek had fewer egg and chick predators than conifer forest.

All of that work required us to begin at dawn, because bird activity is generally greatest early in the morning. The days are wonderfully long in spring and summer, so that meant we started work by three-thirty or so (and had to get up around two a.m., to get to the study sites; this was not so wonderful!). Nevertheless, I look back on those days with much pleasure (perhaps especially because I no longer have to crawl out of bed at crazy hours).

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.

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.


…a story of acid and eggshells

Our mothers and dentists tell us to drink our milk because calcium helps build strong bones and teeth. Of course we are not the only beings for whom calcium is critically important. Rodents often chew cast-off deer or moose antlers and the bones of dead animals to obtain this essential element. Corals build their protective shells of calcium carbonate—the basic component of coral reefs. The shells of mollusks and crustaceans have high proportions of calcium; think of snails, clams, oysters, mussels, shrimp and crabs, for example.

Birds need lots of calcium for building eggshells. The calcium requirements of domestic chickens have been well studied, and commercial diets for laying hens are calcium-enriched. But until recently, virtually nothing was known about the calcium needs of wild birds – and even now there is still much to be learned.

However, it is well known that acidic conditions lower the availability of calcium in water and soil, and by the 1990s the effects of acid rain (from industrial effluents) on forest and lakes in eastern North America and western Europe were clearly evident. Then avian ecologists began to wonder if acidification also affected nesting birds, which have high calcium requirements: ninety-eight percent of the dry weight of eggshells consists of calcium carbonate, some of which is gradually absorbed by the growing embryo.

Some birds, notably species with large bodies such as ptarmigan and geese, are capable of storing extra calcium in their bones, building up their calcium supplies well before the nesting season and then depleting that storehouse during egg formation. But small-bodied birds, such as songbirds, generally can’t do this, so they have to obtain the needed calcium at the time of egg laying and chick rearing. Spiders and sawfly larvae reportedly provide more calcium than flies or butterfly and moth caterpillars, which in turn provide more than aphids and beetles.

But sometimes none of these provide enough calcium for egg-laying females. Nesting birds commonly search far and wide for sources of calcium, sometimes at a considerable distance from their nests. Many small birds consume eggshells, crushed bone, or calcareous grit to supplement the diet. Some food items can provide high levels of calcium; for instance, snails, millipedes, and woodlice (isopods), but their calcium content decreases and they also become less abundant on acidified soils.

Several studies have now shown that low calcium supplies can be limiting to avian reproduction. Negative effects can include smaller clutches, delayed egg-laying, thin eggshells, shell defects, smaller eggs, lower hatching success, and slower chick growth. Evidence of calcium limitation has been found in many kinds of small birds, such as woodpeckers, swallows, chickadees, nuthatches, flycatchers, and dippers. Experimental supplementation of calcium, by providing crushed eggshells or bone in the diet, may eliminate such negative effects. Adding lime to acid soils can lead to more abundant snails and, thus, better reproductive success and even higher densities of nesting birds.

Calcium limitation sometimes can be found in areas without acid rain; for example, the well-known effects of DDT leading to pathologically thin eggshells and poor hatching success was due, in part, to the effect of that poison on calcium metabolism. And tree swallows normally consume low-calcium prey and have improved nesting success when the diet is supplemented.

Calcium availability is even important in determining the amount of spotting or speckling on bird eggs. Historically, spots on eggs have been assumed to provide some sort of camouflage, tending to disguise temporarily untended eggs. However, another kind of explanation of egg speckles is also possible: when calcium is scarce, eggshells tend to have weak spots. Many birds reinforce these weak spots with reddish or brownish pigments (byproducts of blood synthesis), creating the speckled appearance. Evidence for this explanation includes the observation that individuals of some species nesting in areas with limestone bedrock (and therefore good calcium supplies) produce eggs with few or no speckles, while individuals of the same species nesting in nearby non-limestone areas lay very speckled eggs. There may be still other explanations for speckled eggs, but a comprehensive explanation of egg pigments across all bird species awaits future synthesis.

Is all of this relevant to Southeast? Probably, but the exact relationship remains to be determined. Here, our soils tend to be very acidic, with low calcium content. But cedar trees, especially yellow-cedar, accumulate calcium in their tissues; old, senescent foliage falls to the ground, enriching the calcium supply in the soils below these trees and reducing the acidity there, thus creating little hot spots of calcium, so to speak.

These observations raise several questions in my mind. For instance, do nesting birds in Southeast suffer signs of calcium deficiency? Or do they have access to good calcium sources—and if so, what are they? Are bird eggs in Southeast more speckled than eggs of the same species elsewhere? Do nesting birds in forest with cedar trees show fewer signs of calcium limitation that those in forests without cedars? If so, does the decline of cedar forests in Southeast have repercussions for avian nesting success and density?