Cavity-nesting birds

Nesting in a cavity—a hole in a tree, a burrow in a bank, or another enclosed space—gives a bird significant advantages. Cavity nests are generally safer than open-cup nests. The nest contents are concealed from predators (to some degree), although some predators can sniff out the nests anyway (think of snakes, monkeys, squirrels, marten and weasels, and so on). The cavity opening may be easier to defend than a wide-open cup. The eggs and chicks, and an incubating or brooding adult, get some protection from inclement weather, be it cold or hot or wet or whatever. For birds raising chicks in a nest (not applicable to ducks), nest-tending and food-delivery activities of the adults makes the nest a focus for observant predators; reducing the length of time needed for such activities reduces the risk of being observed and attacked. So open-cup nesters are in a hurry to get their chicks out of the nest, while cavity-nesting birds generally have longer incubation and nestling periods. 

If you put a dot for every cavity-nesting bird species on the evolutionary tree, the dots would be scattered all over the many branches of the tree. Clearly, cavity-nesting has evolved many times, appearing in such disparate groups as ducks, falcons, woodpeckers, owls, puffins, petrels, chickadees, nuthatches, bluebirds, flycatchers, and others.

In many cases, these birds depend on finding an existing cavity, perhaps in a rotting or storm-damaged tree, perhaps in a rock crevice, or perhaps in a hole made by another animal such as a rabbit or a woodpecker. Interestingly, sometimes closely related species differ in their use of nesting cavities: for example, common mergansers nest in existing cavities but red-breasted mergansers (in the same genus) make open nests on the ground. Birds that are dependent on existing cavities often face intense competition for suitable, available spaces, which can be limited in supply. For example, there are many observations of European starlings displacing bluebirds or tree swallows from cavities. Likewise, tree swallows and chickadees sometimes contest ownership of a nice cavity.

There are two ways around that problem. One is for each bird to excavate its own cavity. The most familiar excavators (to most of us) would be woodpeckers. All of the birds we call woodpeckers excavate holes in trees or tall cacti, but some of their relatives in the southern hemisphere or the Old World do not.

Hairy woodpecker. Photo by Bob Armstrong

Another excavator familiar to us is the belted kingfisher, which digs tunnels in earthen banks. There are many kinds of kingfishers; not all eat fish, but all are reported to excavate their nesting tunnels in earthen banks or termite mounds. A related group of many species, called bee-eaters, are also burrowers, often nesting colonially in earthen banks.

Other examples of excavators are found among the penguins, shearwaters, and petrels. The three species of puffin are usually dig burrows, the Atlantic and tufted puffins in soil and the horned puffin commonly in rocky places. All parrots make cavities for nesting, usually in trees, but one species stands out: the burrowing parakeet of Chile and Argentina regularly digs nesting burrows in limestone or sandstone cliffs. Three of the four species of North American nuthatches can excavate their tree nests from scratch, although all four more commonly use existing holes, modifying the cavity as needed. Our chickadees can excavate holes in soft rotten wood but often use existing holes.

The swallows, some of which are cavity nesters, provide an example of marked variation of nest type within one taxonomic group: Bank swallows (called sand martins in Europe) excavate tunnels in earthen banks, but rough-winged swallows generally depend on finding old holes in the banks. Tree swallows are not known to excavate but need cavities; cave swallows and barn swallows make open-cup nests. Cliff swallows, however, make covered nests of mud, stuck onto a vertical surface—which leads me to the second way of avoiding competition for nesting cavities:

Some birds aren’t cavity nesters (strictly speaking), but achieve some of the advantages of cavities by constructing a covered nest. The cover may be made in different ways, but it serves to conceal the nest-contents to some degree. Here are some examples: In Latin America, a large set of species typically constructs covered nests, often of clay; the resemblance to old-fashioned adobe bake-ovens earned them the sobriquet of ovenbirds. They are not, however, related to the North American ovenbird (a warbler), which builds a ground nest woven of plant material, with an entrance on the side. Marsh wrens and sedge wrens weave semi-globular nests of grasses and reeds; dippers, too, make bulky, more-or-less spherical nests of moss and plant parts, with a side entrance. Then there are the oropendulas and caciques of Latin America, the penduline tit of Europe, the orioles of North America, some weaver birds of Africa, among others, who weave bag-shaped nests, suspended from branches.

American dipper. Photo by Bob Armstrong

It would be wonderful if it were possible to unravel the genetic and ecological events that led to each evolutionary twig or branchlet with one or more cavity-nesting species, diverging from their open-cup relatives.

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Mid-June

As June progressed, there was an assortment of interesting observations in my yard. Chickadees and red-breasted nuthatch juveniles, fully volant, pursued their parents energetically. The number of hummingbirds at my feeder increased dramatically, as the juveniles from (probably) two nests figured out there were goodies there. As they became more numerous, so did the conflicts among them; a male appeared more often than before, adding to the fuss. One brood of juncos had big, streak-breasted juveniles, while another one had fluffy new fledglings that didn’t yet have their juvenile stripes.

One brood of just two mallard ducklings had passed through my pond, never to be seen again. There were lots of empty days with no ducklings, and then a surprise! Just after Solstice, a late brood of eight small ducklings came by. But they, too, disappeared. In many other years, there have been several broods that spend lots of time on my pond, but this year is different.

Mallard brood. Photo by Helen Unruh

Hummingbird feeders sometimes attract opportunist foragers in addition to their intended guests. A feeder in Wisconsin occasionally draws orioles and house finches. Here, yellow warblers and orange-crowned warblers come occasionally, and one local observer even recorded a downy woodpecker that regularly hung on a feeder while sipping sugar water.

At Kingfisher Pond, the red-winged blackbirds had fledglings of different ages, from different nests. And, to my surprise, I found a male yellowthroat singing and foraging in the brush and sedges on the edge of the pond. This warbler is not common here: very few spring sightings are noted on e-bird for Juneau. I think I recall that my field crew and I found one nesting in a Valley meadow, many years ago.

Red-winged blackbird fledgling. Photo by Helen Unruh

Common yellowthroats breed all across much of North American, wintering in southern U.S. and Central America. They often nest in wetlands, but sometimes also use various other habitats offering thick vegetation near the ground, such as burned-over oak forest and shrub thickets in pine forest. Male yellowthroats are territorial. Social monogamy is the rule, but both sexes frequently look for extra-pair matings (as is common among songbirds). The female builds the nest and incubates a clutch of about four eggs; the male may bring her occasional snacks. Both parents feed the chicks, which can leave the nest when about eight days old, reaching complete independence in four or five weeks. They typically mature at when they are a year old.

Common yellowthroat. Photo by Helen Unruh

Yellowthroats here seem to be at the very fringe of their geographic range. So I hope the fellow at Kingfisher Pond found a mate and raises some chicks. 

Along the trails, chunky young varied thrushes, startled by our passing by, thrashed their way up from the bushes into the trees; they already looked a lot like adults but for the short tail. Two fledgling Pacific wrens, more curious than frightened, peered out of the bushes, ignoring the warning chip-notes of a nearby parent. The pair of greater yellowlegs in the lower Spaulding meadows vigorously and unrelentingly protested our presence, so it was clear that they still had chicks running around somewhere, but this time I didn’t see them. A hermit thrush perched next to the trail while giving an odd whining sort of callfor several minutes, so I suspect it had young ones nearby.

On the Lake Creek trail, a tall snag showed lots of signs of use by woodpeckers—over a dozen holes along its length. One of them was occupied this year, as evidenced by the scuff marks below the hole and shrill calls emanating from within. The chicks were still too small to stick their heads out, so the attentive adults were not met at the door. That would soon change, as the older nestlings begin to anticipate arrivals of the adults. 

Out at Eagle Beach on a sunny day not long after Solstice, a family for four ravens foraged together in a tide pool close to the upper beach. As I settled down on a log to watch them, another bunch of five ravens came out from the forest, making a big racket, and landed on the beach, still yelling. After standing around for a few minutes, one (which turned out to be an adult) ambled down to a patch of low intertidal vegetation to forage, eventually moving back up-beach to pick up and swallow invisibly tiny items from clumps of drying algae. Presently, two of the young, squawking ravens ventured down to a stand of sea milkwort and goose-tongue and desultorily pecked here and there for small things. A third one continued to yell as it followed the adult along the beach; the adult went on, setting a good example for the juvenile. This young one occasionally jabbed its bill at something but was clearly more interested in yelling than in feeding for itself. After a little while, the adult and these three juveniles all took off in a hurry and sailed way down the beach somewhere. That left one juvenile in the original landing spot, where it had hunkered down and apparently gone to sleep. But when the others took off, this one stood up and began to squawk—quite softly. And there it still was, many minutes later, all by itself. I didn’t stay to see if there was a reunion.

Red-winged blackbirds

Red-wings are found all over North America, barring the very far north and some patches of unlivable habitat. They are here in Juneau, too; two easy places to see them are Kingfisher Pond in the Lemon Creek area and the Duck Creek Greenway near Nancy Street in the Valley. I’ve had fun this spring, hanging out near these little wetlands and watching their behavior. And they used to nest in the wet meadow area between Sunny Point and Fred’s, but I haven’t seen them there for years.

My ties to Red-wings go back decades, to the beginning of my career, which began in the marshes of eastern Washington, where I studied yellow-headed blackbirds that nested in the same marshes as red-wings. The two species are interspecifically territorial, defending territory borders against each other as well as against members of their own species. Both species are often polygynous; a male’s territory commonly included the sub-territories of several females.

Red-wing males are fiercely territorial, defending their space by singing and displaying their showy red epaulets, flying from perch to perch to cover all their borders. A really intense confrontation between two males involves perching next to each other, standing upright and very sleek, pointing their bills to the skies. Males after their first winter are often called first-year males; their plumage is not fully black but some of the back and chest feathers have brownish edges. In addition, their epaulets are not as intensively red as those of full adults. First-year males are capable of breeding and sometimes invade the territories of adults or attempt to set up territories of their own but they are usually subordinate to full adults. They mature fully by the second year and then can hold their own quite well. Incidentally, those ‘epaulets’ appear to be on the shoulder (hence the name)when the wing is folded, but they are not close to the shoulder joint–they are really closer to the wrist joint.

Females hold sub-territories within a male’s territory and defend their borders against other females. They choose where to settle using many kinds of clues, including especially habitat quality, but also characteristics of the males, and perhaps an assessment of space-available on a male’s territory. In some cases, a preference for unmated males occurs, but it is commonlyovercome by the other considerations. Although it is common for a male’s harem to have three or four females, the record number seems to be fifteen females on the territory of one male.As is true for many other songbirds, it is not uncommon for a nest to harbor chicks with two or even more fathers, the result of so-called extra-pair sexual activity.

Females build the nest, usually suspended from wetland vegetation, and do the incubating of eggs. Males help feed their chicks in some populations but not in others. So far, I haven’t seen the males at Kingfisher Pond attend to active nests at all.

Blackbird female and chick. Photo by Bob Armstrong

Red-wings generally prefer to nest in wetlands, although they sometimes use agricultural fields and other upland habitats.They eat various insects and even seeds in the breeding season. Their favorite foods include dragonflies and damselflies, which are particularly vulnerable to capture at a time of transition between aquatic living of larvae (called naiads) and aerial living of the adults. The aquatic naiads crawl up on the wetland vegetation, split open the exoskeleton, and emerge as adults; this usually takes several hours, at least. Furthermore, initially those new adults (called tenerals) are soft-bodied and their wings need some time to expand and harden so the adult can fly. And that’s when the blackbirds can nab them easily. 

Red-wings are feisty birds, ready to defend their holdings against intruders and potential predators. This they do by screaming, making diving attacks, chasing, and even physically attacking. They chase crows and ravens, hawks and even eagles (https://vimeo.com/564686399.

A blackbird chasing a northern harrier. Photo by Kerry Howard

One drizzly day, I wore a black rain jacket when I went to see what was happening at Kingfisher Pond. The male redwing whose territory includes the platform and adjacent sedges went crazy. He made the ‘check’ notes of annoyance and the whistle-like call of alarm for many minutes and swooped low just over my head several times. He’d never done this on earlier visits (sans black jacket), so I conclude, provisionally, that he really did NOT like my black jacket. Perhaps I reminded him of crows and ravens, potential predators who would be chased if they got that close.

Two species

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

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

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.

Calcium

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?