…a resource for its producers

Long ago, my first big research project necessitated much wading around in marshes. My task was checking the nests of blackbirds: counting eggs and chicks, measuring growth of chicks, sampling food brought to chicks, and keeping track of chick mortality. The blackbirds weren’t too fond of me, so they often yelled and fussed, but what really made the job more interesting were the black terns that also nested in the marshes.

Even though I seldom came very close to the tern nests, the watchful parents were on the alert. They commonly attacked from behind—swooping down, occasionally actually hitting my head, and usually dropping fecal bombs. A few well-directed splats persuaded me to wear a hat. (Strictly speaking, bird excrement is a combination of feces and uric acid from the kidneys; birds save weight by voiding nitrogenous waste not as heavy, watery urine but as lighter, semisolid uric acid.)

The Arctic terns that nest on the wetlands and near the Mendenhall Glacier would defend their young ‘uns in the same way. But they are not the only creatures to use their own fecal matter as a weapon. Howler monkeys in the American tropics hurl their dung at unwelcome intruders near their arboreal camps and feeding trees.

When common eider ducks are flushed from their nests by foxes or weasels, they defecate a noxious fluid over their eggs. The stink and perhaps an initial taste repel the would-be egg predator.

One of the rangers at the Mendenhall Glacier Visitor Center watched a bear high in a cottonwood void the contents of its intestines and bladder on another bear that attempted to climb up the same tree. This response may have been engendered by fear (which tends to loosen the relevant sphincter muscles), but the effect was certainly to deter the second climber.

Sometimes dung is used in construction. For instance, certain caterpillars build portable cases of fecal pellets, glued or silked together. An Australian caterpillar goes a step farther, sticking several fecal pellets together, end to end, making structural rods that support a silken tent. In fall, marmots close the entrance of their hibernation burrow with a mixture of feces and dirt. Burrowing owls are reported to pile up cow dung near the entrance to the nest burrow; this may help control the microclimate within and attract edible insects.

Dung (and urine) can be used for markers, signaling the borders of a territory or, more simply, the passing-by of identifiable individuals. A sort of “Kilroy was here.” For example, foxes may deposit a pile of dung, and then anoint it with urine and anal secretions, letting other foxes know just who was present (and probably when). Stallions with harems of mares often create groups of dung piles where they defecate repeatedly, sometimes urinating on them as well. These apparently signal other stallions that this one is the boss of the harem, and it may signal something to the mares also.

Many rodents (including beavers) and rabbits produce two kinds of feces: the usual kind, which is just deposited, and a soft, nutrient-rich kind, which they eat. It’s a way of recycling the food and extracting more nourishment. This process is called ‘refection.’ Interestingly, that term (from the Latin for refreshing or repairing) is also used to mean a lunch or refreshment for humans (and a refectory is a dining hall). How’s that for a reflection on refection!

Just after the Old River Channel froze over this year, another retired biologist and I found a place where swans had walked around on the ice. There were remnants of green feces on the ice, but only remnants, suggesting that the swans were really hungry, because their usual green plant food was under the ice, and so they recycled their own feces.

A wonderful example is provided by the nesting habits of hornbills, large birds of Africa and Asia that nest in tree cavities. A female picks a nesting cavity, in which she will stay for several weeks and even months, while she incubates the eggs and broods the chicks. She seals herself into the treehole by closing the opening with her feces, wood chips, and leftover food scraps, a mixture that hardens, leaving just a narrow slit through which the male feeds her and, eventually, the chicks. What a life!

Humans have often used their own waste products, euphemistically called ‘night soil’ to fertilize agricultural fields. This practice tends to foster the distribution of certain parasites, from one human to another. Synthetic fertilizers and manure from domestic animals have replaced this source of soil nutrients in many areas.

Weapons, building material, signals, fertilizer, and food. What a versatile product! Of course, dung turns out to be useful for creatures other than its producers, but that’s a topic for another essay.

Bits and pieces

observations of mallard chicks, mosses, and the raven-eagle interface

Sometimes my local peregrinations don’t yield a suggestion for an essay on just one thing. Instead, I get a little scrapbook of unrelated vignettes on various topics. Here is one such scrapbook:

A friend and I were watching a brood of young mallard ducklings on one of the Dredge Lakes. No attendant female was evident for quite some time. Then in flew a female, calling as she descended. Every duckling immediately perked up and, in a bunch, they all rushed over to her, peeping all the way, as if to say, Mama, where were you? We missed you!

This same mallard family was peacefully foraging along the shore a few days later. We heard a sudden ruckus, as the female was loudly protesting something. We peered through the brush and saw a bald eagle swooping down over the ducklings. At each pass, the fuzzy youngsters dove frantically beneath the surface of the pond. The buoyant ducklings popped back up to the surface as soon as the eagle went by. One, two, three passes, and the empty-fisted eagle retired to a tall spruce to rest and reconnoiter. After a few minutes, the eagle tried again, three or four passes, with no luck. Then it gave up. These mallard ducklings were more fortunate than merganser families that can often be seen in Mendenhall Lake. Sometimes eagles wipe out entire broods of unlucky mergansers.

I always enjoy watching ravens in the grocery store parking lots. Perched up on the lamp-posts or along the edge of the roof, they flirt and gossip and groom and wait for a likely-looking pickup truck that might have something interesting and edible in the back. This strategy must pay off fairly often, because the birds keep on watching and investigating. One raven was seen near a local thrift store, where food was not likely to be discovered. But this hopeful (or desperate?) bird was hopping into the open trunk of a car that was merely delivering donations to the store. No rewards there!

Recently I watched a raven, yelling and fussing, chasing an eagle for several minutes. I did not see the outcome of that encounter, but a friend saw a similar interaction, with a visually pleasing (to the observer) outcome. In this case, when the chase was over and the eagle departed, the raven indulged in several barrel rolls and half-twists, in which it was soon joined by another raven, probably its mate. I have to think that the aerobatics were a kind of celebration. Ravens are known to raid eagle nests for the eggs, and now I wonder if eagles reciprocate, trying to snatch raven chicks, causing parental ravens to defend their broods. After all, they were recorded, a few years ago, grabbing heron chicks from the nest.

An uneasy truce? Photo by Jos Bakker

I was reminded, just the other day, of some very interesting and pretty mosses. Most mosses produce dry spores in small capsules that open to let the breezes waft away the spores, to land more or less at random. But some mosses disperse their spores differently, in a way that greatly improves the chances of a spore landing on a patchily distributed substrate that is suitable for germination and growth. These specialized mosses produce sticky spores that do not fly on the wind. Instead, these mosses produce scents from the expanded base of the spore capsule, and the aroma of carrion and decay attracts certain flies that commonly feed on dung or, in some cases, rotting flesh and bones, usually of mammals but sometimes of birds. The flies land, the spores stick to them, and the flies carry the spores to another dung pile, which provides the necessary conditions for the germination of the spores. I’m told that some of these mosses carry specialization one more step: some depend on herbivore dung while others focus on carnivore scat. Making sure that the spores arrive in a good spot is a nifty way to increase the success of your offspring!

Finally, here’s a little mystery. I have observed that robins rototill moss beds, in search of small invertebrates. Ravens and crows sometimes pluck hundreds of tufts of grass and moss from broad reaches of lawns. Mallards grub up square yards of moss and debris, in search of spilled seeds from a bird feeder in early spring. And bears rototill more deeply, in their quest for the roots and stem bases of northern ground cone. But deep in the forest this spring, a friend and I found many places where some creature had scuffled up big patches of leaf litter. Most of these places seemed to be where snow had stayed a bit late and few green shoots were present. This is not good habitat for robins or mallards, and the scuffled places seemed too shallow for bears. The question is Who did it and what were they searching for?

Bizarre February

porcupines, squirrles, and a peripatetic beaver

This had been the winter that wasn’t! On at least one day, the temperatures soared into the fifties. Many days saw temperatures in the forties. The snow alternatively got crusty (at night) and soggy. It became difficult for Parks and Rec hikers to choose a trail for their twice-a-week outings.

We tried the Herbert Glacier trail. It worked fine for skiers, but snowshoers were in for serious ankle-twisting on the unevenly packed snow. Not fun. The Auke Nu trail was pretty good in most places, with only occasional sections of frozen, deep bootprints that are so difficult to walk on; the descent was facilitated by snowshoes, after the sun had softened the snow.

When the group decided to go to Peterson Lake, a few hikers rebelled, thinking that snowshoeing this trail would be as miserable as the Herbert trail. Walkers that had passed when the snow was soft left deep bootprints, and now these had frozen. Some of the would-be hikers, weary of lurching over the frozen bootprints, turned back and went home. The rebellious ones went, instead, to the Eaglecrest area, on a day when the lifts were not running, so the ski-runs were not full of fast traffic.

We wandered around, on and off the trail, enjoying sunshine and peace. Mount Ben Stuart was spectacular: the slanting sun brought the lateral ridges into high relief and framed the snowy twin summits against a backdrop of purple-black cloud.

The snow was firm enough to walk on but soft enough to show the tracks of a busy animal community. Peripatetic porcupines had ambled here and there, leaving a web of tracks over a wide area. A skinny, flimsy spruce branch whose tip was well-buried in snow had been recently de-barked by a hungry porcupine. It was surprising that a hefty porcupine was evidently able to climb several feet up the spindly, wobbly branch, gnawing all the way.

Red squirrels had made little highways from tree to tree. One was actively moving spruce cones from one hole to another. Snowshoe hares left evidence of their passage. A weasel had bounded down a long meadow, leaping several feet with each bound, neatly placing its hind feet just on top of the prints left by the front feet.

The snow was very deep but in a few places there were openings down to running water. At a couple of these water-holes, we found concentrations of the distinctive tracks of a beaver. We couldn’t believe our eyes, so we tried to make those tracks belong to almost anything else; but in reality, there was no mistaking those prints.

The big mystery is why a beaver would be up there in February. Beavers usually hunker down in their lodges in winter, but this one was unseasonably active. This isn’t the first time we’ve seen beaver activity at Eaglecrest; a few years ago, one burrowed into a streambank and tried to build a little dam. The divide between the Fish Creek drainage and the Hilda Creek drainage lies at Eaglecrest, and when young-adult beavers from lower down in those drainages leave their natal ponds in search of a new home, they may sometimes go uphill. But to find one in February was really weird; perhaps our bizarre weather caused a young adult to start its dispersal earlier than usual.

Dung fungus

…and the extinction of megafaunal species

Animal dung has many uses, both to its producers and to other organisms. I’ve written about some of these before: Beavers and many other rodents recycle some of their dung to extract more nutrients; terns sometimes defend their nests by dive-bombing intruders with messy fecal bombs; otters establish latrines that notify all comers that the place is occupied, and so on. Bears eat salmon and distribute the nutrients over the landscape, feeding the vegetation and some insects that become prey to birds; whale dung nourishes whole communities of marine invertebrates. When flying squirrels defecate after eating truffles, they distribute truffle spores for the next generation of truffles.

In summer, hikers find blue splats on boardwalks, indicating that a thrush (probably) has eaten some blueberries and defecated the seeds. Birds distribute seeds all over the place, usually just a few in any one spot. In contrast, bears sometimes have whole gut-loads of berries, and deposit thousands of seeds in a single place. If a junco or a mouse doesn’t come to the bear scat and tear it apart to extract seeds, there will be what we called a ‘bear garden’ of seedlings, germinating in dense stands in which the intense competition permits only a few individuals to survive [photo]. But those few may get a head start from the nutritious medium they grow in.

Recently, however, I have read about another use for animal dung—this one by paleontologists that are interested in the well-known extinction of large mammals such as mammoths and mastodons when the Pleistocene glaciers retreated, beginning about 20,000 years ago. By about 10,000 years ago, most of the North American mammals that weighed more than about one hundred pounds, and all of the species that weighed more than a ton or so, had disappeared (34 genera in all). There has been intensive, sometimes vitriolic, debate about the reasons for this geologically rapid loss of all these spectacular creatures. Maybe it was driven by climate change (either directly or through vegetation and habitat changes), or the crash-landing of a meteorite that set off massive fires, or human predation by the early humans that populated North America.

Fossilized dung (called coprolites) of ancient herbivores such as mammoths and mastodons may offer a means of sorting out the major causes of these extinctions. There is a fungus known by the tongue-tangling moniker of Sporormiella that requires passage through the digestive tract of an herbivore to complete its life cycle, producing spores in the dung of mammals and some birds that eat vegetation. The spores are known to occur in the gut contents and coprolites of mammoths; when they were deposited on the landscape, they got washed down into lakes, where they accumulated in the sediments—and researchers can find them. Along with the spores, pollen accumulated, allowing researchers to track changes in the composition of the surrounding vegetation. The material in the sediments can be dated quite accurately, allowing researchers to compare times of vegetation change with the demise of the large mammals as indicated by a drastically lowered level of Sporormiella spores.

Sediments from a lake in Indiana revealed that the megafaunal (and fungal spore) decline began about 14,800 years ago and took about a thousand years before final extinction. The principal changes of vegetation and habitat, from savanna to various woodlands, happened after the fungal spores diminished. Following the decline of the mammals, along with the vegetation changes, came an increase of fire and the frequency of charcoal in the deposits, reflecting the more wooded habitats. Thus, it is not reasonable to argue that vegetation changes and fire caused the megafaunal extinction, but it remains possible that the absence of large herbivores contributed to the vegetation changes.

All of that happened well before the arrival of extraterrestrial objects that struck the earth about 12,900 years ago, so that suggested cause can be discarded. What about the human factor? Could prehistorical hunters have eliminated the herds of large mammals, as they are known to have done in many other areas, especially islands? Maybe, but it would have been some very early people; the demise of the big mammals occurred before the arrival of the famous Clovis culture that was previously thought to be responsible.

So by eliminating some of the disputed factors involved in the megafaunal extinction, the little spore-bearing fungus has helped to clarify the arguments and focus attention on the remaining possibilities. That’s how science progresses.

It has been useful in other locations as well. A precipitous decline in Sporormiella spores in cave deposits in western North America also marked the demise of ancient herbivores, and its resurgence marked the arrival of domestic livestock on the landscape. In the Great Plains, the abundance of these spores in sediments provides a record of grazing intensity by bison, allowing researchers to interpret the past effects of bison on grasslands, as registered by datable fossil pollen deposits.

In New Zealand, the fungal spores have been found in the dung of native herbivorous birds such as the takahe and kakapo parrots, and the frequency of the spores tracks the decline of these bird populations. The spores also occurred in the dung of the now-extinct moas, large herbivorous birds that were exterminated by human settlers. When red deer were introduced to New Zealand, the spores tracked the spread of the deer populations across the countryside.

In Madagascar, giant lemurs, elephant birds, pygmy hippos, and giant tortoises all were virtually wiped out by human settlement, hunting, and habitat destruction. Sporormiella spores in sediment cores have been used to reconstruct the pre-human distribution of their habitats and plot the time-course of the animals’ demise in various regions of the island. As in other regions, the eventual introduction of domestic livestock brought a rise in the occurrence of the spores.

April is the cruelest month

the poet was right

The poet had it right! Although April has often been a benign month here, with lots of sun and rapid warming, this year’s April has (so far) offered us lots of rain and temperatures parked in the forties. Not living up to expectations! Nevertheless, Mother Nature has not forgotten Spring, and things are happening.

The yellow hoods of skunk cabbage are now conspicuous in many damp places, with both male-phase and female-phase flowers available. A little experiment in Washington indicated that the sweet fragrance of the flowering display initially stimulates insect pollinators to search for the flowers, where pollen on male-phase flowers is the chief reward. A more local experiment found that the searching insects land preferentially on displays with the bright yellow hood, rather than those that are still green. The little brown beetles that are the principal pollinators are still scarce (in mid April). But eventually they will appear and come first to male-phase flowers, to feed on pollen and use the inflorescence as a mating rendezvous, and then carrying pollen to female-phase flowers. I have observed that, at any one time, there are usually many more beetles on male-phase than female-phase inflorescences, but on some occasions, there are crowds of beetles on the females too. That pattern suggests that perhaps the females are only fully attractive at certain times, possibly drawing in the beetles by air-borne chemical signals.

Photo by Bob Armstrong

The rufous hummers arrived a few weeks ago, one of the earliest arrivals on record. There are rumors that Anna’s hummers, usually just vagrant visitors later in summer, may have overwintered here. If they start to nest here commonly, it will be interesting to see if there is evidence of competition between the two species.

Ruby-crowned kinglets are now cheering human listeners with their rollicking song, even in the rain. I watched a female white-winged crossbill poking about on the ground, selectively choosing certain wisps of grass for a nest lining. In mid April, I heard my first fox sparrow, singing from an alder thicket.

Salmonberry canes with new pink flowers decorated a south-facing upper beach at Auke Rec, and my favorite yellow streamside violets shone against the still-drab forest floor.

Several observations in the Eagle River/Eagle Beach area piqued the interest of a couple of curious naturalists:

–Crows foraged on a mudflat at low tide, finding very small items and gobbling them down. Later, we saw crows exploring the wrack left by a very high tide, extracting mussels and maybe amphipods, and trying vainly to crack the mussels by flying up and dropping them on the too-soft sand.

–An immature herring gull was foraging at the edge of a sand flat, rapidly paddling its feet up and down on the wet sand. This technique was successful in stirring up small organisms, and the gull nabbed one after another. At what age do they learn this mode of foraging?

–There was goose scat that contained seeds of (I think) Canada mayflower, reminding me that geese up on the tundra (and, as I saw, in Tierra del Fuego) commonly eat fruit and disperse the seeds. Geese are generally known as grazers, so this is an added ecological role, shared with bears, thrushes, and some other songbirds.

–A burrow under some tree roots in the sediment bank at the edge of the river had been occupied for some time by a porcupine, which deposited some long white hairs and the usual oval winter pellets (reflecting a diet of bark and needles), as well as more recent, small, dark, round spring pellets (reflecting a shift to soft, fresh, green vegetation). It seems unlikely that the porcupine made this burrow, but it provided a very nice retreat.

–Deer of all sizes had danced on the river sandbars exposed by low-water conditions. We wondered why they spent so much time in that habitat, which offers nothing to eat.

–A little promontory in the river was liberally strewn with the marks of ownership by an otter. There were dozens of small piles of debris, each one topped by a dark, slimy mass. We failed to find a den in this area, although the nearby forest held a number of old, now-unoccupied burrow systems under tree roots.

–As we basked at the river’s edge in some momentary rays, we saw lots of small insects fluttering about. A few landed where we could inspect them, and so we could see that they were stoneflies. Some of them regularly dipped down to touch the water surface, no doubt laying eggs. We wondered how they choose the sites for placing their eggs—what are the cues that indicate a potentially good place?

Golden-crowned sparrows

and what they have in common with kangaroos and giant pandas

Photo by Bob Armstrong

Mid-May, and golden-crowned sparrows are everywhere, dodging about in the brush, and feeding on their northward migration. I’ve seen them pecking at seeds, and there’s a report from Ketchikan of over two hundred and fifty of them pecking away in one small patch of backyard. Then a friend told me that someone in Tenakee had watched their young broccoli and cauliflower plants being demolished by these sparrows; lettuce and carrots seem to be less popular. I’ve seen them gobble up the spore-producing ‘cones’ of horsetails too—would that they‘d eat more of ‘em!

I think of sparrows as eating chiefly insects during the breeding season and mostly seeds in the off-season. So it came as a surprise (to me) that, in addition to eating some insects and seeds, golden-crowns might be serious leaf-eaters. However, the literature is full of decades-old reports of intensive leaf-eating by golden-crowns. They love newly sprouted lawn grass, weed seedlings, and—gardeners beware—leaves of annual flowers such as primulas and pansies, and young plants of the cabbage family (broccoli, cauliflower, and so on), beets, and peas. Evidently they have a very different diet than that of their closest relatives, such as the white-crowned sparrow.

Leaves are generally a difficult food for vertebrates to process, compared to, say, meat. The cell walls of plants are composed of complex molecules such as cellulose and lignin, but vertebrates lack the enzymes needed to digest them well. Therefore, much of the material ingested by an herbivore is not readily accessible to the consumer, which then relies on the contents of the cells. Yet there are many kinds of vertebrate herbivores—and they have solved the digestibility problem in a variety of ways. Many of them have symbiotic bacteria and protozoans, commonly housed in special chambers in the digestive tract. The microbes can break down the complex molecules, and may also synthesize certain vitamins and amino acids. Some mammals house the helpful microbes in complex stomachs (for example, cows, sheep, kangaroos, deer) and typically regurgitate a cud of partially digested material for a second chewing. Others house the microbes in pouches (called ceca) attached to the gut or in the large intestine itself (for example, horses, elephants, howler monkeys, beavers). Still others rely simply on eating large quantities of plant material, being quite selective of high-quality forage when possible, and passing it through the digestive tract quickly, thus making room for more (for example, the giant panda).

A similar diversity of adaptations is found among birds. Grouse and ptarmigan have large microbe-inhabited ceca and, in addition, their intestines elongate in winter, when their diet includes quantities of difficult-to-digest plant fiber. Geese have smaller ceca, but ingest great amounts of plant material, passing it through the gut fairly quickly; when feasible, they are also quite selective of high-nitrogen plants. In the late 1900s, it was discovered that a peculiar South American bird known as the hoatzin has a modified crop and esophagus in which symbiotic microbes help break down plant tissues (this is the only known bird with such modifications at the front end of its digestive system, in contrast to the many mammals with a similar arrangement).

For a long time, it was thought that only relatively large birds could be plant-eaters, because they commonly need to process so much material every day. However, in South America there are three species of leaf-eating birds called plantcutters, which are quite small, weighing roughly forty-two grams (about an ounce and a half). They don’t seem to have special symbiotic microbes in their system at all. How do they manage?? Recent studies show that, like many other avian herbivores, plantcutters tend to select high-protein leaves and pass the material quickly through the gut. In addition, they chew their leaves, using a serrated bill, which may help break down the plant tissue. Perhaps most importantly, they have unusually high levels of enzyme activity in the intestine, so they can extract the soluble nutrients very effectively.

That brings us to back to golden-crowned sparrows, which weigh only about twenty-nine grams (about one ounce). They are highly herbivorous, especially outside of the nesting season, but how do they manage to do this? There doesn’t seem to be any information in the literature on this subject! There is a report that they chop up fresh greens between the edges of their bills, in effect chewing their food a bit. But that’s all we seem to know about how this small leaf-eater manages its difficult diet.

Jays and seed dispersal

when a predator doubles as a disperser

On a recent hike, I heard a volley of high-pitched screams coming from a thick stand of small spruces just beside the trail. They sounded very much like the cries of a red-tailed hawk, but that bird would be highly improbable in such a place and at this time of year (February). Surely it was a Steller’s jay, which is well-known to mimic redtails and some other birds as well.

That small incident set off some musings about our Steller’s jay and jays in general. Steller’s jays are omnivorous, eating all sorts of things, including bird eggs and nestlings, carrion, insects, seeds, and fruits. I was thinking in particular about their role in seed dispersal; when they eat fruits, the seeds pass through the digestive tract and get deposited, sometimes in a place where they can germinate and grow. The jays share this important ecological task of seed dispersal with thrushes, waxwings, crows and ravens, bears, coyotes, marten, and other animals.

They are also seed predators, along with sparrows and finches, chickadees, squirrels, mice, and others. In this capacity, they raid bird feeders and train humans to offer peanuts. Sometimes they cache their seedy prizes, under a bit of moss or a stick. Peanuts and most seeds offered in seed feeders don’t grow well here, but cached sunflower seeds sometimes produce seedlings in improbable places.

This seed probably won’t grow. Photo by Bob Armstrong

I don’t know how often our jays consume the seeds of our trees (spruce, hemlock, pine, cottonwood, alder, willow); all of these seeds are small and typically dispersed by wind; they probably don’t offer much nutrition to a relatively large bird such as a jay. However, in more southerly portions of their geographic range, Steller’s jays are known to harvest and cache the seeds of several conifers. Not all of these caches are retrieved, and the seeds germinate, so there the Steller’s jays are contributing (along with other kinds of jays and Clark’s nutcracker) to seed dispersal, a critical portion of a plant’s life cycle.

Steller’s jays are closely related to the blue jays that live in wooded areas of eastern North America. The blue jay is also an omnivore; among its varied dietary items are acorns and beechnuts. Blue jays harvest and cache these items, sometimes several kilometers from the mother trees. They are much better dispersal agents than squirrels, which cache nuts but don’t travel as widely. Researchers think that blue jays were important in the development of northern forests as the Pleistocene glaciers retreated, by carrying nuts northward to ice-free zones and stashing them.

Neither of these jays is as specialized to a diet of seeds as the pinyon jay of the southwestern U. S. or the more distantly related Clark’s nutcracker of the mountain west. Both of these species depend on conifer seeds year-round, even feeding cached seeds to their chicks early in the summer. Both species travel long distances, sometimes many kilometers, to cache their harvested seeds, and both have excellent spatial memories for retrieving those seeds. But, as usual, not all seeds are retrieved and, because they are often cached in good sites for germination, they become important for forest regeneration. As our climate changes, they and other seed-caching birds could facilitate altitudinal shifts in tree distribution.

Without the seed-caching jays and their relatives, some forests cannot regenerate. For example, pinyon pines are dispersed by western scrub jays (along with nutcrackers and pinyon jays). A study in New Mexico, where scrub jays were the main pine-seed dispersers, showed that near a very noisy industrial area, the jays became extremely rare and, correspondingly, there were many fewer pinyon pine seedlings in the forest, while at the same time, in a relatively quiet area, both jays and seedlings were common. Noise pollution drove out the jays and severely reduced pine forest regeneration.

There isn’t room here to sketch out the whole story of jays and nutcrackers, their adaptations for seed harvesting and caching, the adaptations of trees that facilitate seed dispersal by these birds, and the sometimes complex interactions with other seed-eaters, such as mice and squirrels. For the time being, suffice it to note that the interdependence of these birds and certain trees means that if one side of the mutualism fades away, the other side declines too.