The weasel family

carnivores of many shapes, sizes, and lifeways

Ermine in summer coat. Photo by Kerry Howard

This taxonomic family (Mustelidae) of carnivores is familiar to Juneau folks in the form of weasels, wolverines, martens, mink, and otters. Mustelids are a species-rich family, with fifty or sixty or seventy species, depending on what source you read. They originated over twenty million years ago from a wolf-like ancestor and have evolved into a variety of ecological niches. They are distributed naturally over all the continents except Australia and Antarctica.

Mustelids range in size from the tiny least weasel, weighing up to about 250 grams (8.8 oz) to the sea otter, weighing up to forty-five kilos (99 lbs) or occasionally more. A long-extinct mustelid is estimated to have achieved the size of a black bear or a jaguar, the biggest mustelid on record. Known as Megalictis, it once roamed the North American plains and probably preyed on many kinds of animals. 

Members of this family generally have rather elongate bodies,relatively short legs, and short muzzles. Most are highly carnivorous, typically pursuing rodents, fish, and other small critters, and most are solitary predators, but some are quite social. Certain members of the family occupy diverse, fairly specialized ecological niches—A few very social, some very arboreal, others fossorial (burrowing), some highly aquatic. It would be tidy and convenient if such specialized habits were associated with branches of the evolutionary tree of mustelid radiation (social species on one branch, fossorial ones on another, etc.). But that’s not generally the case—those habits have often arisen independently on the several known branches.

The long, slender body form of weasels is well-suited for these predators to pursue voles and mice into narrow tunnels and tightspaces. Similarly, that body shape enabled the North American black-footed ferret to chase prairie dogs down into their burrows. But that’s a sad story: thanks to habitat destruction and poisoning programs by uncaring agriculturalists, prairie dog colonies are few and scattered; the ferrets were declared extinct in the wild; captive-raised ferrets have been reintroduced in several places.

American badgers originated early in mustelid evolution, in a lineage distinct from other extant mustelids, and are quintessential burrowers. Much heftier than the weasels and ferrets, they have powerful forelegs with strong claws that enable them to dig deep and extensive burrow systems. Largely solitary, they are major predators of rodents and other small creatures of the prairies, but they too are far less common than they were.

American badger. Photo by Kerry Howard

The European badger is not closely related to the American badger, despite the shared name. A mighty burrower, it is quite social, living in multi-family, complex burrow systems. It’s a predator of worms, insects, hedgehogs and other small vertebrates, but also eats some tubers and roots.

Honey badgers (or ratels) of Africa and Asia evolved early in mustelid evolution and are not closely related to any other mustelids (including other “badgers”). They are typically solitary predators, fiercely chomping up all sorts of prey and raiding bee nests for honey, and they are good burrowers.

Arboreal species tend to be more omnivorous than the others, often eating lots of fruits in addition to small animals. The tayraof Central and South America is probably related to the martens of North America, but it’s somewhat bigger and longer-legged. It can cavort very capably in the treetops, using its big tail for balance. However, the only one I ever saw was on the ground; it burst from the brush beside a trail, giving me (not knowing that such things existed) quite a startling!

River otters. Photo by Kerry Howard

Aquatic living is typical of mink and the distantly related otters. These species eat chiefly fish and invertebrates such as crabs and crayfish, in some cases also eating birds’ eggs and nestlingswhen available. The sea otter is the most aquatic of all, seldom coming ashore. It’s the biggest extant mustelid, with large, webbed hind feet. Its dentition is much modified from the basic carnivore plan—the molars are broad and flattened, made not for cutting but for crushing shelled invertebrates.

Next in size is the giant otter of South America, which is not very closely related to the other otters. It is very social (unlike most but not all other otters), living and hunting in cohesive, extended family groups and defending the family territory with loud choruses. With webbed feet and a muscular tail, it is a strong swimmer. Group members hunt together: fish are the primary prey, but sometimes also taking snakes and small caiman. It is now endangered.

Four wintery walks

sun and shadow and snowy tracks

The thermometer at my house read eleven degrees (F) after a clear, starry night; the sky looked clear, although the sun wasn’t really up yet. Juneau had recently enjoyed about six feet of lovely snow, but many of the trails had not yet been used much. In search of a well-packed trail, a friend and I headed for the Boy Scout beach trail.

All went smoothly until we reached the junction where the trail splits three ways, and none of the splits looked good. We chose to go right out onto the goose meadow and immediately found ourselves breaking trail and post-holing through deep, crusty wind-blown drifts. Even following in the footprints of my companion, I (weighing forty pounds more) plunged and lunged, knee-deep and more. However, a hundred yards or so farther on, walking became a pleasure again, because the low vegetation was almost clear of snow, thanks to some recent super-high tides that left a few scattered cakes of frozen foam and to the wind.

Post-holing again out to the beach by the iconic cottonwood tree, we decided not to face the stiff, cold north wind that was churning up waters out in Lynn Canal. So, instead of coming back on the big, exposed beach, we turned toward the camp buildings, found a log, and had a little picnic in the sun. As soon as we got out our thermoses and lunch bags, two importunate ravens landed on the beach right in front of us—they knew the drill! Of course, we obliged them, tossing out bits of sandwich that they promptly snapped up. But they eyed a fragment of a sugar-snap pea with great suspicion and avoided going close to it—no veggies for them! One of them chose to sit next to us on the log for a while.

On this day in mid-December, the morning sun barely cleared the horizon. On the upper beach, I noticed that every isolated pebble cast a shadow much longer than itself, making a grid of conspicuous black stripes that called attention to each pebble.

The Chilkats across the canal were spectacular: the low morning sun made sharp contrasts between the sun-bright south-facing slopes and the intensely blue-shadowed north slopes. Behind us, the trees on the hillsides were individually defined by the snow they carried and on the peaks the snow delineated the minor topographical features very nicely.

Few critters were visible on this walk. A seal cruised by, just offshore, and gulls fossicked about in the tidal wrack. A wren zipped rapidly from trailside to shelter under some roots; they are so tiny, I wonder how they stay warm on frigid winter days. No midges or spiders crept on the snow surface.  But there were tracks of mink along the river, red squirrels in the woods, ermine and vole at the edge of the meadow; one or two small birds (who?) had hopped and run in the beach rye lining the meadow trail. A low-flying raven (?) left the mark of one wing-tip in the loose surface snow.

The next day was mostly sunny and, again, very cold (seven degrees at my house in the morning). I opted for snowdrift-free walk on the dike trail. A few tracks of squirrels and maybe an ermine were the only natural history notes of the morning until I had almost finished the walk. And there in the stand of willows was a female pine grosbeak, busily nibbling buds. I frequently see these grosbeak in fall and winter, as they forage on high-bush cranberry, carefully extracting the seed and dropping the red fruit pulp—the opposite treatment from that of bohemian waxwings, who eat the fruit and excrete the seed.

A couple of days later, the cold remained (just six degrees here). I had an idea to try some of the lower meadows along the Eaglecrest Road, but roadside parking was hard to find and the thought of plowing through deep snow, even on snowshoes, was daunting. So, on up the road to the Lower Loop, nicely groomed and unoccupied. The sun couldn’t make it up over the peaks, but ‘shoeing was easy.

There was not a live critter in sight but there were plenty of signs of life. A porcupine was into long-distance travel, heading straight across the meadow, not stopping to forage. Ermine had cruised all over the meadows in circuitous routes, looking for a juicy morsel. Snowshoe hare tracks were abundant, mostly under sheltering conifer branches or going from one shelter to another. Grouse or ptarmigan had been active, inspecting salmonberry and blueberry bushes for tasty buds and sometimes staying long enough to trample the snow flat. Except for a few squirrel marks, the smaller folk had left no signs on the surfacebut were no doubt active below.

A day or two later, it was still very cold. A group of friends strolled the dike trail, enjoying the bright sunshine as the sun crept over the peaks. A dusting of fluffy snow lay atop a snow crust. Several voles (I think) had made sorties out into the grassy areas, circling back to the trees or to holes under grassy tussocks; we noted at least seven of these trackways, well separated from each other. Looking through the chain-link fence and across a ditch, we saw tracks on a big snow drift that looked like Two-toes—but how could a deer walk up that crusty snow on those thin legs, without punching through, when humans (on our side of the fence) generally ended up post-holing?

Parthenogenesis in vertebrates

another form of reproduction

When we talk of ‘single parents’, we refer to a family that has only a mother (or sometimes only a father) raising the offspring. But there’s another way to be a single parent—when an individual makes the offspring all by itself, with no participation of anybody else at any stage. There are many examples of plants and invertebrates that can produce offspring by splitting off pieces of themselves, and the pieces grow into new individuals. Or, they reproduce by parthenogenesis (‘virgin birth’): afatherless embryo develops without a sperm joining an egg to start the process and the unfertilized eggs develop directly into a zygote and then an embryo. 

Among the vertebrates, however, parthenogenesis is relatively rare. There are no known cases of scientifically documented, natural parthenogenesis in mammals. However, parthenogenesis has been artificially, experimentally induced in a variety of species, although the embryo usually develops abnormally and dies.

Parthenogenesis in birds is apparently very rare. It’s known in domestic turkeys, chickens, and pigeons and in captive zebra finches, but the embryos don’t develop normally and generally die before hatching. However, two California condors in the San Diego zoo each produced a viable male chick that had no father; the chicks lived for a few years but died before reproducing. Those females made fatherless chicks even though they shared their captivity with males. 

Some reptiles are obligately parthenogenetic and unisexual (female), including some Asian geckos and North American whiptail lizards. At least some of these types arose (and are still arising, presumably) via hybridization of sexual species, leading to polyploidy (multiple sets of chromosomes per cell, not just the usual two sets). Interestingly, although the whiptail populations are all-female, if one female acts like a male (why would she do that?) by courting and mounting another female, more eggs are produced than in the absence of that behavior (because of hormones that control behavior and response). Still other reptiles are known to be parthenogenetic at least occasionally, including snakes such as pythons, boas, rattlesnakes, cottonmouths and copperheads, and monitor lizards such as the Komodo dragon. 

Among amphibians, parthenogenesis is well-known in salamanders and certain frogs, often the result of hybridization and polyploidy. Some odd variations occur, creating a mix of parthenogenesis and sexuality (sometimes called incomplete parthenogenesis): In some cases, the sperm of a sexual species is needed to start egg division and embryo development, but no male genes are transferred. In other cases, the sperm of a sexual species fertilizes the egg and an embryo develops, but when they mature and reproduce, they do not pass on the male’s genes. And sometimes, at least some male genetic material (DNA) of a sexual species mysteriously combines with the DNA of the female, and is passed on jointly, not as separate chromosomes.

Parthenogenetic, unisexual offspring have been reported for several species of shark; in these cases, the mothers (being flexible) can probably also make offspring in the usual, sexual way. Among the bony fishes, parthenogenesis and unisexuality are the regular thing in the Texas silversides, the Amazon molly, and a hybrid live-bearing topminnow in Mexico, for example, and they occur also in some populations of several others, including the pond loach of Asia and the Australian carp gudgeon.

Are there advantages to parthenogenesis? One advantage is that females can produce offspring, passing on their genes, even if males are scarce and unavailable. Another advantage is that the females don’t share parenthood with males, they pass on just their own genes; their offspring are very much like their mothers, often virtually identical. However, they generally sacrifice the advantages of sexual recombination of genes, which generates variation. Such variation is considered to be useful: by producing varied offspring, there is a higher chance that some will survive when the environment is (as usual) likewise variable. 

So, one would expect to find cases of natural parthenogenesis when and where the environments are not very variable, or males are very hard to find. Good data are needed: Theadaptiveness of parthenogenesis in the ecology of each species that reproduces this way needs further documentation.

Determination of biological sex

it’s not as simple as X and Y

The determination of biological sex turns out to be a complicated matter, even if we restrict the discussion to vertebrates. Sex determination in mammals depends on a pair of chromosomes, known as the X and Y chromosomes. An individual that is XX is female (with ovaries producing eggs); one that is XY is male (with testes producing sperm)—just a little piece of that Y chromosome is responsible for male-ness. And that’s that—or so it was thought, for a long time.

However, the reality is a bit more complex. In a very few instances, an XX individual may develop male characteristics and an XY individual may have female traits—sometimes even to the point of having sex organs (testes or ovaries) of the other sex. This can happen, despite the chromosome combinations, if some of the genes that regulate hormones turn on or off at abnormal times during development; the hormonal effects may override the sex chromosomes. If such genes are activated (or deactivated) very early in embryo formation, the sex organs may not correspond to the chromosome combinations. If the hormone switches engage sometime later in development, an XX individual could have female sex organs but the physical and physiological traits of a male…and similarly for an XY individual developing female characteristics. Such individuals may function behaviorally as hormones dictate, but have little or no reproductive success.

Sex determination in birds also depends on chromosomes, in this case called ZZ (making a male genotype) and ZW (a female). Note that this is the reverse of the mammalian system, but some of the same hormonal discrepancies probably apply to birds as well. In addition, it seems that animals with this chromosomal sex determination mechanism are, for some reason, susceptible to another developmental anomaly: some individuals develop male traits in part of the body and female traits in the rest. This arrangement is called gynandromorphy (meaning female-male-morphology). Gynandromorph individuals are well-known, apparently, among birds, including chickens and several songbirds of the eastern U.S.; these individuals have one half with male plumage and the other half with female plumage. (photos!) They also occur in reptiles, amphibians, and fishes (as well as a variety of insects and other invertebrates) that share the ZW chromosome system (but apparently not those that have the XY system). At this point, it is uncertain if most of these gynandromorphs are natural or somehow induced by anthropogenic effects (such as pesticides). These individuals may function as one sex or the other, or neither one, but not both.

Among the fishes and some amphibians and reptiles, there is the further complication of hermaphroditism. Sequential hermaphrodites function first as one sex and then as the other.Clown fish (those colorful fellows that associate with anemones) are first male, then female (protandrous). Many wrasses are first female, then male (protogynous)—if a group of wrasses has no male, the dominant female in the group becomes male. Simultaneous hermaphrodites are male and female at the same time, a very rare condition in vertebrates (but quite common among invertebrates). Examples include sea basses and hamlets, in which individuals take turns acting as male and as female during a mating session, and the mangrove killifish, which can self-fertilize!

And on top of all that, there is environmental sex determination. Some of the hormonal anomalies mentioned above may be due to environmental factors, either anthropogenic or natural. For example, exposure to the pesticide atrazine has been reported (very controversially) to reduce testosterone and sperm production in some mammals and frogs—some of the frogs were effectively castrated. The best-studied natural environmental effects on sex determination involve temperatureat which the eggs are incubated, particularly in reptiles (but also some fishes). Reptiles have sex chromosomes, like all vertebrates, and sex can be determined by those chromosomes in most (but not all) snakes and lizards. But sometimes, temperature effects can override the chromosomes readily. However, the temperature effects differ in different species: in many turtles, cool incubation temperatures lead to embryos developing as males, but warm temperatures lead to embryos developing as females. In crocodilians, both low and high incubation temperatures produce females, but intermediate temperatures produce males.

That’s a quick overview of a complex topic; all the known details would fill several volumes—and there are surely more details to be discovered. And there are the everlasting questionsabout ‘why’—why do birds have one sex-determination system and mammals another, and why do reptiles have both? Why do incubation temperatures matter to some animals but not to their close relatives? And so on…

Transition to winter

…maybe?

In early November, I walked along a gravel road in the company of frost-decorated branches. A mat of old leaves lined the roadway, each leaf fringed with crystals, making a pretty mosaic in tones of russet, old-gold, and brown. In an open area, I found a collection of more old leaves, not matted down, each one with a distinctive cover of frost crystals. Quite aside from the interesting physics that presumably accounted for the variety of crystals, every leaf was a thing of beauty. As the poet (John Keats) said: A thing of beauty is a joy forever…

Photo by Mary Willson

The Lower Loop at Eaglecrest in early-mid-November was a fine place for a walk. A bit of fresh snow lay on the meadows, and above the white blanket the tops of old seed heads and Labrador tea plants poked up –as individuals rather than part of a mass of faded greenery. Tawny-gold grasses stood tall and colorful on that background. Dainty little grasses draped their dangling seeds artistically. Tufty pine branches bore small caps of snow.

Conditions for finding evidence of several local denizens were perfect. Porcupines had traipsed hither and yon, exploring endlessly, it seemed. Every few yards, one of their trackways crossed the path, sometimes running parallel to the path for a while. One of them left a long, orange-yellow dribble as a mark of its passage.

Squirrels had made their well-beaten routes from tree to tree and across the path. A small bird, probably a junco, had hopped out of brushy area into the open. Voles had scampered about; one made a nice tunnel under the snow, revealed only in spots where the snow-roof had collapsed. A shrew left a narrow groove on top of the snow and then dove into a tiny hole under a clump of grass. Another trackway suggested that a deer mouse had been active, making quite long leaps with its sizeable hind feet.

A few days after that Lower Loop stroll, in hopes that sea-level rains meant snow had fallen at Eaglecrest, I went back up there. But even the Lower Loop had lost much of that earlier snow. So I trudged up the road, circled the Hilda Dam cabin, and came back down the Trickster run, and had some fun there. There was a thin layer of fluffy snow on top of a very thin crust. Snow covered the moss mats, but groves of moss sporophytes stood up above the snow. As usual, porcupines had wandered about, and squirrels had dared to cross some open spaces. In a little dampdrainage, a shrew had scurried over the surface, supported by that thin crust. A deer had run downhill, covering dozens of yards before jumping a log and going back into the woods.

I found a trampled cluster of indecipherable smudgy tracks near a few seed heads that had been demolished, leaving scattered fragments on the snow. On the top of the smudges, I saw what looked like ermine footprints. But, despite some hints, the smudges were a puzzle. A few minutes later, however, I found a long trackway going from one weed patch to another and noted that Smudgy was bipedal and big enough to break through the thin crust, disturbing the snow layer and disguising any toe marks. Ha! The prime suspect was probably a grouse.

Some days after that, I went with a friend to Eagle Beach State Park, to have a look at the vegetated sand flats in the river. A thin film of soft snow covered the ground. On the way, we saw that squirrels had been busy, crossing the path in several places. Along the river shore, rather recent flood waters had piled up great stacks of fallen trees and combed the herbaceous vegetation into orderly lines. A mink had run on the edge of the riverbank, but there were no bird tracks to be seen out there. Looping back through the campground, we found the trackway of a vole, scuttling across an opening and under some grasses.Near the parking lot, a raven had strolled over the lawn, but no ravens came to mooch any part of our lunches as we sat on the bench near the river.

Some nice big white flakes were falling, but they soon changed to rain (of course). Then we waited to see if the forecast snowfall actually happened…and it did! By Thanksgiving time, there had been several good snowfalls, although the streams were not yet iced-over. One day on Douglas, I saw several tiny, delicate flies flitting about and resting on the snow. According to a local expert, they were probably snow midges, whose larvae like cold-water streams and typically emerge as adults in winter.They are quite tolerant of cold temperature and may survive for many days, as they search for mates.

Caring for offspring

parenting styles in the animal world

Vertebrates have a broad spectrum of ways to care for their offspring. At one end of the spectrum are such species as herringand many other pelagic fishes that simply release their gametes into the water, where sperm meets egg, and the parents go off to do other things, providing no parental care at all. Avian brood parasites, which dump their eggs in other birds’ nests so the hosts rear the young, likewise provide no parental care (unless you count the effort of selecting the right nest to parasitize).

At the other end of the spectrum are species in which both male and female parents invest a lot of effort in caring for their young. For example, male and female of most species of penguin (except the emperor penguin male who does the incubating) and many other birds share the duties of both incubating the eggs and feeding the chicks. When the male does not incubate eggs, his care may be indirect: he usually feeds the female so she can stay on the egg-warming job longer. Among mammals, females of course nurse the offspring for a while, but in wolves and foxes, for instance, the male partners often bring food to the nursing mothers, and then both parents feed the young ones.

In between these extremes is an array of interesting arrangements, ranging from solo female care to solo male care:

–Solo female care: Many mammals leave all parental care to the females; males do no more than inject sperm to fertilize eggs. Porcupines, bears, and deer are local examples. A female salmon prepares a gravel bed for the eggs and she may guard her nest for a few days while the male she spawned with is off looking for other potential mates. Mallards and many other ducks also leave parental chores to the females.

Two species of very unusual frogs in Australian rainforest were not formally discovered by scientists until the 1970s but they were extinct before 1990. The females of these frogs brooded their eggs and tadpoles in their stomachs! Both eggs and tadpoles had ways of shutting down the digestive acids of the mother’s stomach while they were in residence. Geneticists and cell biologists are trying to resurrect a viable specimen of gastric-brooding frog from preserved tissue, but it would take more than one specimen before we could see this phenomenon in real time.

–Solo male care: This arrangement is known to occur regularly in certain birds, amphibians, and fishes. For example, jacanas are long-toed marsh birds that are customarily polyandrous; females are larger than males and defend territories that include the sub-territories of several males. The males do all the work of incubating and rearing chicks.

Spotted sandpipers have a variable mating system, sometimes monogamous but in some places they are polyandrous. A polyandrous female generally leaves her first male partner to do parental duties at his nest while she then pairs with another male, with whom she shares parental care.

The two species of Darwin’s frogs in Chile are called mouth-brooders, but actually the young are reared in the vocal sacs of the males, which ‘ingest’ the eggs and harbor them until they hatch. One of these frogs incubates the tadpoles until they can eat and then takes them to pools where he releases them to feed and grow up. The other species incubates both eggs and tadpoles; when the tadpoles become froglets, they just hop out of his mouth.

Fishes have been very inventive of ways for males to take over parental care. Stickleback males build nests and entice females to lay their eggs there, guarded by the males. Seahorse males incubate eggs in an abdominal pouch where the females inserted those eggs. The young ones emerge fully developed but extremely small. Intertidal sculpins often lay their eggs on the undersides of rocks, where they are guarded by males. Ravens know another place to look, cuing two local naturalists to finding sculpin eggs under horse-clam shells worn like helmets on the heads of males buried in the sediments.

Photo by Bob Armstrong

See Bob Armstrong’s blog for a video about this fascinating behavior.

Visiting old home ground

a visit to Wisconsin

In mid-October, I made a quick trip back to my old stomping grounds in southern Wisconsin. In addition to blue skies and sunshine, some long-delayed (by covid) good family time, a short, nostalgic trip on the little cable ferry over Lake Wisconsin at Merrimac, a fine symphony concert in Madison, visits to some excellent regional artists showing their work, and early fall foliage colors, there were some natural history highlights.

Photo by J. S. Willson

Just outside of my old home town, we found lots of sandhill cranes in the stubble fields and pastures or flying overhead and calling. They come through every year from their nesting grounds farther north; they nest mainly across northern North America and most of them migrate to Texas and northern Mexico or Florida for the winter. It’s always a big treat to see them, but there was an even better one: in one rather distant stubble field, all by themselves, were three whooping cranes, foraging, wing-lifting, and hobnobbing with each other. Whoopers formerly nested in prairie marshes all over north-central U.S., but their population declined drastically, chiefly because of habitat loss and heavy hunting. Now they nest only in three locations in North America: a natural, self-sustaining population in northern Alberta and adjacent Northwest Territories, a re-introduced non-migratory population in Florida, and a population in central Wisconsin (at the Necedah National Wildlife Refuge) that is maintained by re-introduced birds hatched and reared by captives (with human help) in facilities at the U.S. FWS Patuxent Research Refuge in Maryland and the International Crane Foundation in Wisconsin. Captive-bred juveniles are cared for by humans dressed in crane costumes and learn their proper migratory route by following older birds and special ultra-light aircraft. The Canadian contingent flies to Texas for the winter, and the Wisconsin nesters fly towardFlorida, some over-wintering along the way.

Whooping cranes. Photo by J. S. Willson

We visited the Necedah National Wildlife Refuge, about an hour’s drive northwest of town. There we saw trumpeter swans,a variety of ducks, and still more sandhill cranes, cavorting and calling. As we walked one of the wooded trails, off to the side there was a flash of black and white, flying from one tree to another. That deserved a closer look and yes!!—it was a red-headed woodpecker!! When I was a kid, many long years ago, these woodpeckers were very common. Historically, this species has undergone large fluctuations in population size, partly related to fluctuations in a major food supply (acorns and beechnuts), but also lots of habitat loss, shooting, and other factors. There have been population declines in the past several decades, and the birds may still be endangered in some parts of North America.

Red-headed woodpeckers typically nest in open oak woodlands and in forest edges and clearing, using cavities in snags and dead branches. They eat all sorts of things, including insects (they are expert fly-catchers), fruit, occasional small vertebrates and eggs, as well as acorns and beechnuts, which are especially important in winter. They are one of the few woodpeckers that store food, cramming acorns and large insects tightly into cracks and crevices and behind bark; in some cases they make a big cache of many food items, probably re-storing them later in a more scattered distribution.

My home town lies at the edge of the unglaciated Driftless Area, in a valley ringed by the remnant tops of ancient quartzite mountains that were made hundreds of millions of years ago and eroded over time. These hills now support varied habitats, including cliffs and talus slopes, oak savannas, dry hill prairies, and lots of deciduous forest. I take special pleasure in the diversity of trees in that forest; I can stand in one place and (depending just where I am) see six or eight or ten kinds of tree, each with characteristic branching pattern, leaf shape (and color, in fall), bark texture, and its own way of life. Mixed in with all the broad-leafed trees are scattered white pines.

Of course, it was also good to see old avian friends, such as white-breasted nuthatches, black-capped chickadees, cardinals, red-breasted woodpeckers, house finches and goldfinches, slate-colored juncos, and blue jays.

Why don’t bird fall off their perches?

taking a look at misconceptions

For a long time, the accepted explanation was simply that perching birds just cling with tightly curled toes. A tendon runs from the toes up the elongated foot and behind the ankle. When a bird bends the ankle a little, the tendon tightens and pulls the toes into a curl. There’s even a locking mechanism, rather like a ratchet, that keeps the toes curledwithout muscular effort.

Illustration by Katherine Hocker

It’s quite true that perching birds can grip tightly, to hold prey or perhaps hang upside down like a chickadee, although that may take some muscular effort in addition to the useful ankle tendon.But a bird perched at rest on a twig is not usually holding on tightly (photo); the toes are loosely curled, more or less draped over the twig, and the bird is just balanced there. It can perch there, stably, while twisting and turning its head to track what is going on around it or while holding the head steady as the body bobs up and down as the twig moves. How do they do that?

Birds, like most other vertebrates, have complex organs in their heads (one by each ear) that are responsible for maintaining equilibrium. The principal working parts are three fluid-filled semicircular canals (one horizontal and two vertical at right angles to each other), and two small chambers housing viscous fluid and tiny stones. When equilibrium is disturbed, the fluidsmove, triggering lots of tiny hair-like structures to send signals of disturbance to the brain. Then, if all is goes well, the brain manages to direct the body to restore its balance.

Sometimes, as we know from experience, that system is overtaxed. If we change position or direction of movement too rapidly, we can get dizzy and may start staggering or even fall down. Human gymnasts must somehow train their balance organs and their connections, so that doesn’t happen to them. Similarly, tree squirrels, which rapidly change direction of travel, must have ways of maintaining balance.

Birds can have this problem too. For them, it is a common problem. They may have to perch on a wobbly twig but keep the head steady while surveying their environment. Or, conversely, they may have to move the head around, keeping track of things, while the body is stationary. So why don’t they dizzily fall off their perches?

A growing body of evidence suggests that birds have a second organ of equilibrium, located near the hip. The lowest vertebrae (lumbar, sacral, and a few caudal ones) of a bird’s spine are fused together into a unit called the synsacrum and to the pelvis. The probable organ of equilibrium (called the lumbo-sacral organ) is an egg-shaped glycogen (a polysaccharide) body in a dorsal groove of the spinal cord, together with a set of transverse canals in the synsacrum that align with lateral lobes on the spinal cord, and that arrangement is somehow linked to the nervous system. It’s not at all clear how this might work, but it’s best developed in birds that perch.

Photo by Bob Armstrong

Having two organs of equilibrium seems to allow birds to manage head and body separately in challenging circumstances.However, more detailed research is needed to show just how this works and how it is all coordinated.

Dispersal of fungal spores

it’s not just wind!

Many fungi disperse their spores by releasing them from gills or pores into the air, for breezes to carry them away. Insects can carry some fungal spores either inside or outside their bodies, and almost any mammal that eats an occasional fungus can probably do so. Indeed, mammal-dispersal of fungi occurs in a variety of habitats ‘round the world. This mode of spore dispersal is especially important for fungi such as truffles, which are chiefly subterranean. There are thousands of species of truffle, and they apparently depend on animals that dig them up and eat them, passing viable spores through the digestive tract, and depositing them at some distance from the parent along with nutrients from digested foods and useful bacteria. This foraging habit is an essential component of many ecosystems, because truffles are mycorrhizal fungi that form mutualistic associations with many plants, providing assorted nutrients from the soil in exchange for sugars from photosynthetic plants.

In the western forests of North America, fungal spores can be dispersed by deer and mountain goats and many kinds of rodents, including chipmunks, red-backed voles, marmots, pikas, and others. But it seems that flying squirrels get the most notice (judging from the number of published reports).

A flying squirrel foraging for fungi. Photo by Bob Armstrong

Birds get into this act too; there are scattered reports in the literature of birds that eat fungi. Dozens of species eat fungi, including quail, ruffed grouse, wild turkeys, and free-range chickens, in North America, but the role in spore dispersal is not recorded. A few fungus-eating species are known to eat truffles:lyrebirds in Australia scratch up the litter and topsoil to gettruffles and other fungi; Australian eastern yellow robins often forage in such disturbed areas and eat left-over truffles; migrating birds in Kuwait eat truffles in the desert. They probably disperse the fungal spores, but good documentation is lacking.

Recently, however, there’s a fulsome report about fungus-eating birds in the south-temperate rainforest of Chile and Argentina. That rainforest is quite different from our local one in many ways; of particular relevance here is the presence of several bird species that typically and regularly forage on the ground. These include the austral thrush and several endemic species of tapaculo that are known to eat fungi. This report focused on the black-throated huet-huet and the chucao (tapaculos whose nesting biology I studied many years ago). These birds often run around on the forest floor, scarfing up bugs and fallen fruits and—it turns out–scratching up and eating truffles.  The researchers genetically screened the birds’ fecal samples and found that many species of fungus, including truffles, had been eaten. Special microscopic examination of the feces revealed large quantities of viable fungal spores. Many of those fungal species are mycorrhizal, associated with the so-called southern beech trees of that rainforest.

Although our local forests don’t feature so many forest birds that habitually forage on the ground, it would be interesting to know if any local birds sometimes contribute to fungal spore dispersal. Grouse have well-developed gizzards that might grind up spores along with seeds, but perhaps some of the tiny spores get through the gut. Robins and varied thrushes scuff up moss layers and might sometimes find fungi to be edible. And what about flickers, the woodpecker most likely to forage on the ground; we sometimes see it probing down into the mosses. Or crows and ravens, which love to poke about under moss and stones and sticks.

Infanticide

and destruction of eggs

Infanticide refers to the intentional killing of babies and sometimes very young juveniles. In general, infanticide is typically not predation or cannibalism; the principal reason for murdering babies is not for eating them, although that might occur incidentally at times. Regular infanticide occurs in many animal species, for a variety of reasons. (Humans have a long history of this practice, but that’s not part of this essay.) In addition, some birds habitually destroy the eggs of other birds, either puncturing them or tossing them out of the nest.

Some regular infanticide and egg-destruction occurs between species. Perhaps the best known for this habit are birds that are brood parasites, laying their eggs in the nests of other species, so the hosts rear the interloper’s chicks. Brood parasitism is often accompanied by destroying the host’s eggs or chicks by the adult parasite. Adult shiny cowbirds in South America puncture or remove host eggs, and adult brown-headed cowbirds in North America often remove one or two host eggs when they put their own egg into the host nest. Cowbird eggs require shorter incubation than the host eggs, so (if the timing is right) they hatch ahead of, or at least at a bigger size, than the host chicks. Not only can the larger cowbird chicks out-compete the host chicks for food deliveries, they may push unhatched host eggs out of the nest.

In some cases, it’s the parasite’s offspring that do the destruction. North American cuckoos sometimes become brood parasites, possibly in response to caterpillar outbreaks that allow well-fed female cuckoos to produce extra eggs, which they lay in other birds’ nests. These cuckoo chicks may push their nest-mates out of the nest. 

The European common cuckoos, however, specialize in brood parasitism. Their eggs have a very short incubation time (because they’ve already incubated for a day inside the mother!), so cuckoo chicks hatch at a bigger size than their nest-mates. Soon after hatching, the cuckoo chick starts evicting any other eggs or hatchlings in the nest: one by one, it backs up to each one, loads it onto a hollow on the back, and moves to the edge of the nest to dump the doomed host offspring off the edge—all of them! This reportedly can somehow even take place while the host parent is sitting in the nest! The urge to evict everyone elseapparently disappears by the time the cuckoo chick is about four days old, but by then there is nobody else left anyhow.

Honeyguides of Africa and Asia are best known for raiding bee nests and leading other consumers to the treats of honey and larvae in those nests. But they are also brood parasites with serious infanticidal behavior. For instance, the African greater honeyguide parasitizes the nests of cavity-nesting birds. In the darkness of these nests, even before the honeyguide chicks open their eyes, they murder their nest mates with deadly hooks on the tips of their bills, biting and shaking the victims until they are incapacitated and then dead.  

Other cases of regular infanticide occur between members of the same species. Infanticide by males has been reported for many species of carnivorous mammals and primates. Male lions often kill young kits in the process of taking over a pride of females; the females then become ready to mate again and the new males become fathers. Similarly, in the mating season, male bears may kill cubs and even one-year-old juveniles; removal of those maternal duties allows the erstwhile mothers to come again into heat, and the males get more mating opportunities.

Chimpanzees seem to be a rather aggressive bunch of primates. Males are known to kill infants so they can mate with the deprived females. But even female chimps get into the infanticide business. Apparently this happens when foraging is limited and neighboring females start to invade the foraging area of a resident group. The resident females may then murder the infants of the intruders, in attempts to drive out the invaders.

Infanticide also occurs among members of the same family. Especially when food is not very abundant, the older and bigger chicks in a nest may push their smaller siblings out of the nest, thus claiming more food for themselves. This happens in a variety of species including raptors, egrets, kittiwakes, boobies, and kingfishers. Also, in a few cases, at least, hard-working parents may oust a runty chick, leaving more food for the remainder.

These, of course, are just examples of what can go on; it’s a tough world out there.

But it’s not all like that: There are contrasting cases from birds (and some fishes), in which a male behaves in a very different manner toward offspring that are not his own. For example, male yellow-headed and red-winged blackbirds are often polygynous; they commonly feed the chicks of their mates, especially the chicks in the nest of the primary female in the harem. But if the male of a harem is lost or experimentally removed, replacement males mate and take care of their own nestlings and may act as step-fathers to the existing broods. They may adopt them, defending but rarely feeding them, or simply be largely indifferent to them, but these replacement males are not infanticidal. Why not? Perhaps because they apparently obtain better mating opportunities with re-nesting females or females that arrive later in the season and cannily cruise around looking for territories that have had successful nestings!