Cavity-nesting birds

…seekers of snug places to rear their young

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.

Birds underwater

a variety of avian submersion strategies

Many kinds of bird regularly forage for prey underwater. These birds have a variety of ways of doing so and adaptations to match. Life in the water is very different from life in the air.

The first hurdle to overcome is simply getting there. Some species start from the water surface. A few are able to just sink below the surface by decreasing their buoyancy: small grebes and anhingas do this by compressing the plumage (thus pushing air out) and exhaling. Others tuck their heads and kick with their webbed or lobed feet (e.g., mergansers, goldeneyes, buffleheads, most cormorants, loons, and some grebes) or flip their wings (murres, long-tailed ducks, dippers). Those that surface-dive a lot (e.g., loons) typically have legs set well back on the body, making them awkward on land. 

Another way to get underwater is from above the surface.Dippers often dive into a stream from a rock or low-hanging branches not far from the water surface. Kingfishers may plungefrom several meters above the surface, folding the wings closer to the sides. Brown pelicans can dive from a height of twenty meters, extending the neck and angling the wings back, making a more streamlined shape. The grand champion divers may be seabirds called gannets and boobies; they can start a dive from almost a hundred meters up, turning the body into a sleek dart, with the neck well-extended and the wings held back close to the body. The dives can reach a speed of sixty mph; to protect the bird from the resulting high impact, the skull is reinforced and subcutaneous air sacs on the chest and sides cushion the jolt.(https://vimeo.com/319325491. By Bob Armstrong, in Loreto, Mexico.)

Belted kingfisher diving and rising with fish. Photo by Bob Armstrong

Most of these dives are quite shallow, but some species are adapted for deeper ventures, with heavier, stronger bones than other birds, to resist water pressure and decrease floatation. Gannets are quite deep divers, sometimes going on down to twenty meters. Loons may dive as deep as seventy-five meters and some of the murres and their relatives go down over a hundred meters; the common murre is said to be the deepest diver (sometimes down to 180 meters) in Alaska. Penguins often launch from ice-ledges; small ones make fairly shallow dives, but the emperor penguin can dive down more than five hundred meters!

The second hurdle to underwater foraging is locomotion in a medium that is denser than air. Most aquatic birds have webbed feet, often set far back on the body for good propulsion and steering; grebes have broadly lobed toes instead. But fancy feet are not always sufficient—some of these birds use their wings to swim in pursuit of prey. Gannets and cormorants can wing-it underwater; murres and puffins have narrow, stiff wings adapted to underwater ‘flight’ (without forsaking aerial flight); penguins swim with their flipper-like wings (and cannot fly) and steer with their webbed feet (some of them are very fast swimmers, clocked at over twenty mph).

Murre underwater. Photo by Bob Armstrong

Kingfishers and dippers don’t have webbed feet, so they have their own ways of moving in water. Kingfishers seem to rise buoyantly to the surface after a dive, wing-fluttering as they lift back into the air. Dippers have strong toes for clinging to rocks and walking even in fast currents, and they swim with their wings for short distances in pursuit of prey; they are the only songbird known to do so and do not have the same adaptions of bones and wings as other, more aquatic, birds do.

Plumages of birds that forage underwater are generally dense and well-waterproofed with oils from the preen gland. Penguinplumage has unusually many tiny filaments that hold air bubbles; when the bird swims, the bubbles are released, which decreases the density of water around the body, allowing faster swimming. Birds that decrease buoyancy by compressing the feathers might get a little of this effect, but penguin plumage can hold more bubbles and release them more gradually.

Diving birds hold their breath underwater, storing oxygen in their lungs. But they can also store extra oxygen in their muscles, in a compound called myoglobin–which, like hemoglobin, is a specialized protein with iron-containing compounds that hold oxygen. Species that engage in long dives and underwater pursuits have more myoglobin than those that spend shorter times without access to air. Emperor penguins can stay underwater for twelve minutes or more (for comparison, humans can normally manage to hold breath for less than two minutes).

Solstice at Cowee Meadows

surrounded by abundance

Around the time of the summer solstice, I spent a couple of nights in the Cowee Meadow cabin, along with some friends and my visiting niece. We like the flower show in the meadows. This year, everything was a bit early (after the sunny, hot month of May) and flowering was somewhat past its best, but nevertheless we noted as many species of plants in flower or just past flowering as we found last year (over 75 species, barring grasses and sedges).

The cabin needed some attention, so we swept it out and washed the windows with plain water, having neglected to bring window cleaner. We brought in window screening and duct tape, so we were able to cobble together screens to exclude the flying bugs and thus keep the windows open. As it happened, however, there were astonishingly few mosquitoes and such (what a difference from last year!). Sitting outside for breakfast, lunch, and dinner was very pleasant.

Every day we cruised around the trails, seeing what we could see. A major highlight was the discovery of a sizable patch of RIPE strawberries—just one big patch; the plants all around this site held only green, unripe berries. This discovery caused a significant delay in our progress but there were big smiles all ‘round!

strawberry-kgh
Photo by Katherine Hocker

These coastal strawberries have a very interesting distribution: they occur naturally along our coasts but different subspecies or varieties also occur on the coasts of Chile and Argentina and, reportedly, in the mountains of Hawaii. The species is thought to have originated in North America, but it was probably carried to South America by migrating birds. There is good evidence that migratory shorebirds can carry seeds and spores of a variety of plants on their long-distance seasonal journeys. Some seeds may stick to feathers and escape being preened off. Strawberries (and other fleshy fruits) are adapted to be eaten by vertebrates, which snack on the fruits and pass the seeds through their guts. Most small birds pass seeds rather quickly, in just a few hours, so viable strawberry seeds travelling all the way from the northern hemisphere across the equator to the southern would require a really quick flight or a long residence time in the gut. I observe, with interest, that the coastal strawberry, Fragaria chiloensis, bears the name of the Chilean island where I spent many happy austral field seasons. Historically, this strawberry was widely used and cultivated in Chile and reportedly was hybridized with another species to create the domestic strawberry.

A small botanical mystery confounded us. We noted that the oysterplants (a.k. oysterleaf) on the upper beach fringe made smaller leaves on the parts of the stems that bore flowers. But on saltbush (a.k.a. orache) plants, flowers were borne on stems with both small and large leaves. Why the difference?

We had a report from later hikers of a sickly bear cub beside the main trail and eventually learned that ADF&G had picked it up. Their investigations failed to reveal any obvious cause of the distress, but clearly the little cub was malnourished and in a very bad way. Nothing could be done to save it, and it was a public safety concern to leave the cub where it was as long as the mother was nearby, so it was euthanized. Some observers later saw a female with one cub and thought that the family of the dying cub had moved on.

Along Cowee Creek a doe with two fawns come out of the woods to the sand bars. The doe was limping from a wound on a hind leg, but she was able to lead her fawns back into the thickets when she detected us a hundred yards upstream.

The meadows were popping with savanna sparrows, including lots of recent fledglings. Lincoln sparrows burbled their complex song along the shrubby edges, maybe contemplating a second brood. As we sat on the beach for a while, a male belted kingfisher came over our heads and landed on a big rock partly exposed by a low tide. Presently, he dove and came up with a long, thin fish. Then he headed overland toward an upstream part of the creek. The next day, we were walking up along the creek and heard a group of kingfishers raising a ruckus. We surmised that the chicks had fledged and were begging, but they all went around a bend of the creek and out of sight before we could be sure.

We watched dragonflies laying eggs: the females repeatedly dipped down to touch the end of the abdomen to the chosen substrate. One species dropped her eggs into open water, while another, larger one (a blue darner, I think) chose to put her eggs in decaying wood.

The wonderfully long days of solstice time gave us lots of time to wander about, including a stroll out to the beach in the evening to watch the sun setting—and anything else that came our way.

Stories in the snow

a snowy ramble reveals winter action

I love to go a-wandering along a snowy trail, looking for signs left by others who’ve been out on their business of living. A recent prolonged cold spell had kept the snow soft, preserving evidence of a very busy wildlife community along a local creek.

Mink tracks rambled along the creek-side, dipping down to the stream and curving up into the forest. The footprints were bigger than those of a second mink that traveled part of the same route, so my naturalist friend and I guessed that the first mink was a male. His trackway led a long way upstream on one side of the creek and seemed to circle back down on the other side—at least the footprints were the same size there. This might have been a male patrolling his territory.

Everywhere, we found the delicate, stitchery trackways of small rodents. According to the books at hand, mice are likely to drag their long tails, flipping them to the side as a counter-balance during sharp turns, but voles don’t usually show tail-drag marks. If that’s right, we had both mice and voles, especially on one side of the creek. The tiny trackways of shrews were less numerous.

Snowshoe hares had been busy, especially on the other side of the creek. Trackways led up to the streambank, then away, then back to creekside, then away. It was as if the hares wanted to cross the fragile ice but, lacking the nerve to do so, just dithered along the bank.

A bird had hopped about extensively in and out of some brushy areas. The tracks seemed too small to be those of a junco. Then we found wing-prints where the bird had flitted a short distance to a new site, and the length of the wing was clearly too long to belong to a junco. My guess was possibly a varied thrush, some of which overwinter here.

The only actual bird we saw was a brown creeper, hitching its way up a tree trunk and flying down to go up the next tree—their typical foraging pattern as they search for tiny bugs in the bark. According to the literature, creepers commonly concentrate their efforts on trees with ridged bark, the deeper the ridges the better; this kind of bark harbors more insects than smoother bark.

A few deer tracks, both large and small, appeared as we walked along. But there was much less deer traffic here than, say, in Gastineau Meadows, where peripatetic deer had cruised all over the place.

My friend called to me: Come look at this! I saw a shallow groove in the snow on the streambank and, without thinking, said: Oh, a shrew trail. Look again, said my friend. Ah—there’s a faint yellow stain at the bottom of the groove. And here, where I had casually supposed my ’shrew’ had dived under the snow, was—not a burrow at all, but just a deep dimple. My friend, who is smarter than I am, said: I think a bird, maybe a kingfisher, perched on that branch near the edge of the stream and projectile-defecated a jet of hot poop, melting the groove in the snow. So we said: Well, if that’s so, then in the dimple at the end of groove there should be a little wad of solid waste. And yes, indeed there was! Good detective work, friend!

A final little treasure on this walk was a dead red alder that sported a beautiful array of conks (or shelf-fungi). The living conks all had a slightly soft pile of white stuff at their lower edges. This stuff had occasionally smeared sideways over the bark, showing that it had been soft when the temperatures were above freezing. What is this stuff?

Phellinus-conks-kathy
Phellinus conks. Photo by Katherine Hocker

I took a sample to a local forest pathologist, who put it under his microscope. He said that the white material was certainly fungal mycelium (the technical word for the mass of filaments that grow through the wood before producing the spore-bearing conk). However, without DNA work, there’s no way to know if it belongs to a parasitic fungus growing on the conk or to the conk species itself, because this kind of conk (of the genus Phellinus) often grows some of its own filaments right down through the conk itself. So we ended our walk with one more mystery.

Streamsides in winter

some rewards of getting out and about

I take a walk on one of Juneau’s many trails almost every day, alone or with friends. Sometimes it’s a bit hard to get myself out the door, because there’s a deluge or big wind, or I’m just feeling lazy. So I remind myself that sitting inside my house practically guarantees that I won’t see much of interest—so get out there and look around, something may turn up. And something always does.

Here’s a sampling of small pleasures that turned up along Juneau streams in January:

–Fish Creek: Huge, thick plates of ice had washed far over low banks on the small, upstream floodplain and into the forest, and also into the meadows around the combat-fishing pond. It was fun to speculate what it would have been like to actually see the ice cakes pushed out of the creek and into the forest (from a safe distance, of course).

–Eagle River: A dipper was foraging along the edge of the river, occasionally disappearing under the ledges of ice that lined the shores. It searched diligently in the riffles and sometimes brought up something that required some work before swallowing—maybe extracting a caddisfly larva from its case.

–Mendenhall River: I checked out the ‘gooseneck’ peninsula where a breakthrough seems imminent. The narrow neck of land is thinner every time I look, and it seems as if one more good jökulhlaup might be enough to make an island of the peninsula tip. I have to guess that hydrologists have determined the large buildings just downstream to be safe from such events.

–Fish Creek: Winter-active beavers had dragged brush from recent cuttings over to their home pond, leaving trails in the old snow. These beavers, and others in Juneau, have obviously not read the books that report beavers holing up in their lodges for the winter.

–Mendenhall River: A pair of hooded mergansers, the snazzy, gorgeous male with a more-demurely -plumaged female, sailed sedately downstream. Hooded mergansers are the smallest of the mergansers; they eat a more varied diet that includes not only fish but also lots of invertebrates. Males and females commonly pair up in late fall and hang out together through the winter until nesting time in spring. Then the female choses a nesting cavity in a tree or nest box, usually not too far from water, lays her fertilized eggs, and incubates the clutch of eggs, while the male, having done his studly task, goes off and leaves her to do the work. When the eggs hatch, the tiny ducklings almost immediately jump out of the nest cavity, fluttering down to land with a little bounce, and follow mama to feeding areas. This species nests in some places in Southeast, but not commonly. In winter, it favors coastal waters such as shallow bays, estuaries, and tidal rivers, so we see it occasionally.

male-hooded-merganser-by-bob
Photo by Bob Armstrong

–Eagle River: Small insects were flying, possibly midges, looking rather like miniature mosquitoes. Some other insects, such as certain stoneflies, regularly fly in winter, but I’d like to know more about midges (if that’s what they were).

–Cowee Creek: A kingfisher winged upstream and perched over a pool. I hoped to see it catch a fish, but apparently it saw nothing worth pursuing. Kingfishers and many other birds have two small areas, called fovea, on the retina of the eye (humans have just one); foveas have a high density of visual cells and provide good acuity. One fovea, near the bill, is used for monocular, sideways vision; this fovea has especially numerous visual cells and is used for finding prey (as well as keeping track of other birds and predators). When a kingfisher dives and enters the water, its vision switches from that fovea to the other one, located away from the bill; using these lateral fovea in both eyes gives the kingfisher binocular vision and better depth perception as it gets close to an elusive small fish that may try to dart away. When the bird dives, its eyes are protected by a nictitating membrane. As some of us have found out when we try to grab something underwater, refraction often causes us to misjudge the depth of that object; kingfishers can avoid much of that problem by diving vertically after prey that is deeper than a couple of inches.

Kingfishers nest in burrows alongside or at least near streams. I have found nest burrows by several Juneau streams. The lower Mendenhall River and Cowee Creek, where it flows through the meadows, have cut steep mud banks that are perfect places for kingfisher nests. But there’s a problem: in both areas the streams are rapidly eroding those banks; what remains is still potentially suitable for nest burrows, but the stability of the banks is obviously uncertain, and high water in spring and summer could wipe out a kingfisher nest.

–Peterson Creek: A little light snow had fallen on top of old, crusty snow. Shrews had traveled far and wide over the top of the snow. One shrew had plunged, presumably deliberately, over a small mud cliff at the edge of the stream. A few feet downstream, its tracks continued on a flat shelf of shore-bound ice. It sure looked like the critter swam from the base of the cliff to the ice. There is a shrew that is adapted for swimming, but it is not as common here as ordinary shrews, which apparently can swim if necessary.

What did I get out of those little walks? Some really fresh air (Juneau is good for that!), mild exercise (followed by a comfy cup of tea), sometimes companionship, sometimes a solitary meditation, and some observations to think about. Not bad for a small investment.