How to carry a lunch

A human day-hiker usually carries a lunch in a backpack and may require a canine companion to do likewise. However, other animals carry lunch internally, in special storage compartments. Here are some examples.

Some birds have gular pouches that start under the tongue and extend down the throat. Clark’s nutcracker reportedly can carry a hundred or more pine seeds in their gular pouches, to be cached for future lunches. Blue jays can carry several acorns in the pouch, and Steller’s jays can pack many sunflower seeds into theirs. Townsend’s solitaires, both male and female, have pouches just during the breeding season, presumably to carry bigger loads to the chicks; several caterpillars can be stored there for delivery to the nest. Pinyon jays have, not exactly a pouch, but a distensible esophagus that can carry several dozen pine seeds for the jays to cache. A bird with a full gular pouch shows a noticeable bulge on the throat. (Note: gular pouches have other functions too, but those are not part of this story.)

Many birds have crops (or craws)—an enlarged, thin-walled, distensible part of the lower esophagus that can accumulate considerable amounts of food. The size and shape of bird crops varies greatly among species, from just a stretchy portion of the esophagus (e.g., in fish-eaters such as cormorants) to a bag-like expansion. Most omnivorous and seed-eating birds have storage crops. For example, parrots, as well as canaries and other finches, store food in their crops to regurgitate for their chicks. Chickens and turkeys have crops, but not geese. Even many raptors are said to have crops, but not owls. Scavengers such as turkey vultures can store a lot of carrion in a bulging crop. A foraging hummingbird may fill up its crop with nectar and then sit quietly for several minutes. It’s not doing ‘nothing’; it takes a few minutes for the nectar to move on to the stomach and make room for more. Hummers can store nectar, and perhaps insects, in their crops, to deliver to chicks in the nest.

Bohemian waxwings eat a lot of fruit of many species; mountain ash is reported to be a favorite. They can pack several mountain ash berries into their crops at a time. A little flock of these waxwings was perched in an alder tree outside a window, where an observant friend watched them. A waxwing with nothing in its bill suddenly held a fruit that was quickly swallowed. Up came another fruit; it was dropped, probably accidentally. Then another one was regurgitated and swallowed. As one fruit after another emerged, the size of the lump in the throat area got smaller and smaller.

One day on my pond this fall, I saw two mallards that looked most peculiar. Viewed from the front, the lower neck and upper chest were totally lopsided, bagging way out to the right. Both birds had clearly been foraging very successfully, storing lots of goodies in their crops. And they were back to get more!

The strange hoatzin of South America eats mostly leaves; its crop has been modified to do double-duty, becoming used for digestion as well as storage. Hoatzins are the only birds known to have foregut fermentation (as cows do): The modified crop houses special bacteria, which ferment the ingested leaves. When they load up this crop with big meals to be processed, hoatzins would bulge so much they’d tip over but, conveniently, they have a big callus on the breastbone that braces them upright.

Lots of insects have crops too, near the beginning of the digestive tract, just after the junction of thorax and abdomen. As is the case for birds, in some insects, the crop is like a bag, but in others it’s just a wide place in the esophagus. It may be used only for storage, but in some insects digestion may begin there, before the food passes on the stomach. In honeybees, the crop is sometimes called the ‘honey stomach’. A foraging honeybee gathers nectar from flowers, stores it in the crop, and the loaded bee flies to the hive, perhaps digesting a bit of the nectar to fuel its own activity. In the crop, an enzyme breaks down some of the sugar into simpler sugars and compounds that resist invasion by microbes. (Sucrose breaks down to glucose and fructose, and glucose is further broken down to make protective compounds). In the hive, the load of nectar is passed from the forager to worker bees, who move the liquid around in their mouths, making bubbles that allow water to evaporate. They may also place water droplets on the honeycomb and fan them with their wings, evaporating more water. So, at each step, water is removed from the nectar, leaving thicker liquid with a high concentration of sugars and some protective compounds, to be stored in the cells of the honeycomb. That’s how they make honey to feed the larvae.

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Bird song

One day in early November, after a morning of plonking about on snowshoes for the first time this year, I sat myself on a snowbank and leaned against an old alder tree—time out for a snack. My companion perched at the base of another alder a few feet away. We’d spent the morning looking at animal tracks in a meadow. As soon as we opened our packs to dig out our lunches, we were visited by a raven, who called and attracted another one. But they ignored our (admittedly) small offerings and departed.

Thinking about the raven calls reminded me of some recent reading about the singing behavior of birds and a lingering question. First, some background: ravens, crows, and jays are classified as songbirds on the basis of both morphology and genetics. But their singing behavior differs from that of other songbirds in an interesting way.

Most songbirds sing during the nesting season, sometimes all day long, sometimes mostly in the morning. Males may use one song before dawn and another after sunrise, and there is usually a dawn chorus of many species all vocalizing at once. Morning bird songs are something that lots of folks, not just birders, look forward to in temperate-zone springtime and even sometimes in the tropics.

Bird song has many functions. Males advertise their territory ownership, telling others of that species to stay away. Sometimes, two males face each other aggressively at a territory border and have song duels, each one trying to out-shout the other. Males’ songs also attract females to their territories, the vigor of their songs indicating their state of health and motivation. Female songbirds may sing too—telling other females to stay away from that territory. Or, in some cases, a female might sing to tell her mate to bring her a snack.

The ‘voice box’ of birds is totally different from that of humans, other mammals, and reptiles. We have a cartilaginous larynx at the upper end of the trachea (windpipe). The larynx is thought to have evolved from a valve involved in swallowing (and when we swallow, the larynx moves). We sing or talk by moving air out of the trachea and through the larynx, causing vocal membranes to vibrate; muscles linked to the cartilages control the sound. The closest living relatives of birds are crocodiles and they too use a larynx for vocalization. However, although birds have a larynx, it’s not used for making sounds; they use another structure, called a syrinx. The avian syrinx is located at the lower end of the trachea where it branches into two passage-ways (bronchii) going to the lungs. The syrinx apparently evolved, not from a valve, but from structural supports for the branching airway, and very different muscles are involved in control of its vibrating membranes.

Because the syrinx is in two parts, at the beginnings of the two bronchii, songbirds can sing two distinct notes at the same time, something a larynx cannot do. This is not the same as Mongolian throat singing, in which singers learn to exploit the harmonics (overtones) of the fundamental note. The two distinct notes sung by a songbird are not harmonically related. Air is moved from the lungs into the trachea, as most song is produced by exhaling; but birds can also produce some song when they inhale. Loudness and pitch are changed by altering air pressure and by changing the tension of the muscles that control the elastic, vibrating membranes of the syrinx.

Jays, crows, and ravens have the avian syrinx, but they don’t use it in the same way as other songbirds. They can make a huge variety of sounds; the loud, raucous ones are most familiar to us. But they are perfectly capable of making musical sounds and do so, quietly, upon occasion. If you listen carefully, you might hear them! But these songbirds have no dawn choruses, no song-battles for territory, no lovely singing to attract mates. That begs the question: Why not?

The underground ecosystem

The ground: we stomp all over it, dig holes in it for buildings and alien plants, scrape it off and discard it for access to minerals. That’s the meaning of being ‘treated like dirt’.

Nevertheless, a lot goes on underground in the layers of soil. It’s a very complex ecosystem. Sometimes called the rhizosphere, it’s the realm of roots.

This essay includes several topics that have appeared here before, but this time I’ll try to bring them together, if briefly, in one context, while expanding on some of them.

Vascular plants have roots that help stabilize the upper parts of the plant and carry soil nutrients and water to the rest of the plant. Thus, roots make possible the forests and grasslands in which we and other terrestrial critters walk or crawl around.

In some species, roots are a means of vegetative propagation, spreading away from a parent plant and sending up new shoots that look like separate individuals but that are all part of one. Aspen is a classical example among the trees, making groves of tree trunks that are all one individual, genetically. Perennial grasses, such as beach rye, can spread rapidly in this way by means of underground stems called rhizomes.

Tree roots sometimes graft to each other, linking their vascular systems and sharing nutrients (and, potentially, pathogens). Grafting is relatively common among neighboring individuals of the same species but is also known to occur between individuals of different species (e.g., pine and spruce).

Neighboring plants of all sorts are also often connected by mycorrhizae—fungi that carry nutrients from plant to plant and from soil to plant. Their networks can be very extensive, connecting plants of different species. As many as eighty or ninety percent of all plants may use mycorrhizal connections. Mycorrhizae are said to be especially dense in perennial grasslands.

Mycorrhizae even make connections to mosses and other rootless plants. Mycorrhizae contribute significantly to growth, survival, and reproductive success of many plants (some orchids even need them for seed germination). However, these useful fungi are inhibited by agricultural fertilizers containing inorganic nitrogen and water-soluble phosphorus. 

There’s plenty of nitrogen (N) in the atmosphere but it is, nevertheless, often a limiting factor for plant growth and development. Nitrogen is also used in plant defenses against pathogens and herbivores. Plants can’t use atmospheric N directly, but N becomes available via a process called N-fixation; this process is catalyzed by enzymes that transform atmospheric N to ammonia and thence to various amino acids that are usable by the plants. This work is done only by various kinds of bacteria and some other one-celled organisms, which have been around, somewhere, since long before plants evolved. Some of these N-fixers live freely in the soil, others associate with the outsides or the insides of roots, and still others are housed in special nodules on plant roots. Within these nodules, the process of N-fixation uses energy provided by the plant’s photosynthesis.

The legumes (peas, peanuts, etc.) are well-known for their nodules inhabited by bacteria called Rhizobia. These nodules are induced by the bacteria in response to chemical triggers from the plant. These bacteria can only fix nitrogen when with a suitable host. Some of these symbioses between plant and bacteria are highly specific—each partner associating only with the other one; and if a different type of bacteria gets into the nodule, it can usurp energy and cause the plant to die. But some Rhizobia are much less fussy, associating with several plant species. In our area, beach pea and lupines (for example) have Rhizobia nodules.

However, many other kinds of plants form symbiotic relationships with N-fixing bacteria of a different type, mostly known as Frankia. These filamentous ‘actinorhizal’ bacteria produce hormones that mimic certain plant hormones and force the root cells to proliferate rapidly, causing swellings (nodules) to form. Local examples of this symbiosis include alders and sweetgale.

Plants with N-fixing bacteria use that nitrogen for better growth and defense. In addition, plants with enhanced nitrogen levels may provide nitrogen to other plants via grafts or mycorrhizae. Many plants with N-fixing bacteria are also mycorrhizal—a three-way symbiosis! In agricultural settings, nitrogen-enhanced plants are plowed down into the ground to improve the next crop. A little N-fixing water-fern is sometimes cultivated to be used as green manure for other crops. Plants with symbiotic N-fixers are sometimes also planted near human-favored trees (e.g., Douglas fir), so that the products of leaf and rootlet decomposition enter the soil and enhance the growth of the favored neighbor.

Roots have all those helpers (so to speak) that contribute to supporting the plants. They also have enemies. Roots are eaten by some above-ground consumers; geese dig the roots of silverweed, bears dig the roots of lovage, angelica, hemlock parsley and the bulb-like tubers of northern ground cone. Some invertebrates (including nematodes, insect larvae) nibble roots or suck their fluids. And there are root-rot fungi that can demolish a root system and cause a tree to collapse.

That’s just the root-y part of the complex soil ecosystem. Fungal hyphae pervade the soil, sometimes reaching high densities. There’s a teeming community of invertebrates: mites, earthworms, millipedes, nematodes, springtails, spiders, isopods, insects….as well as astronomical numbers of bacteria and other one-celled organisms creating complex food webs. It’s an ecosystem that’s not as well-studied as some others but surely has many interesting stories.