Ecological engineers

builders, excavators, burrowers, and nest-constructors

We use the word ‘engineer’ in a confusing variety of ways and contexts, but here I mean to refer to organisms that create physical structures or changes in the environment—physical changes that affect other kinds of organisms. The concept is still very broad—one could say (and some researchers do so) that a forest of trees or large kelp, or a tallgrass prairie or an eelgrass bed, produces an environment in which temperature, humidity, air or water currents, precipitation patterns, or soils may be altered, thus affecting many other organisms by providing habitat or access to resources.

However, here I want to consider other ‘engineers’—those that deliberately, intentionally make or modify physical structures for their own purposes, with collateral consequences for other organisms.

The most well-known ecological engineers in the natural world are beavers. By building dams, they impound water, raising the water table, creating ponds, sometimes preventing floods, but also flooding low-lying areas. Although they may instinctively respond to the sound and feel of running water by trying to build a dam, they make deliberate choices about the size and shape of a dam and its component parts; they also maintain their structures continually. Beaver ponds provide good habitat for fish, especially juveniles, aquatic insects, various birds, and certain plants, although they obviously destroy portions of the adjacent area by flooding it. Some dams are hundreds of yards long and some are many feet high, depending on the terrain. A well-constructed, well-maintained beaver dam can last for many years, and its effects on the landscape may persist long after the beavers have moved on: the pond gradually fills with sediment and dead vegetation and eventually turns into a meadow.

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Hairy woodpecker at nest–notice the beak full of grubs! Photo by Bob Armstrong

Woodpeckers are engineers too. They excavate nesting cavities in trees; the holes persist for years and are used by other birds, such as chickadees, bluebirds, small owls, and certain ducks, flying squirrels, as well as insects and fungi. Experiments have shown that the density of cavity-using critters in an area depends on the availability of woodpecker-made holes. Woodpeckers known as sapsuckers are doubly active as engineers: their nest cavities provide essential nest sites for swallows in parts of Colorado and the swallows are reported to be absent in forests that lack the sapsuckers. The second engineering function of sapsuckers is their characteristic drilling of sap wells in the bark of selected species of trees, a physical change that alters the distribution and abundance of food resources by other birds (e.g., hummingbirds), squirrels, and insects (e.g., wasps and butterflies). The trees eventually heal the wounds in the bark, so the effects of this engineering feat are less long-lasting than that of the nest cavities.

Terrestrial burrowing critters modify drainage patterns and create micro-terrain that alters plant communities. And there are many burrowers (e.g., ants and termites, earthworms, pocket gophers, moles, badgers, mice, and so on). Prairie dog colonies (before humans plowed up most of the prairies) change soil condition over large areas, which then supported plant communities different from surrounding areas. Their burrows also make nest sites for burrowing owls and refuges for snakes. Kingfisher nesting burrows may sometimes alter riverbank stability, but they also provide nesting holes for rough-winged swallows. Marine burrowers, such as certain molluscs or worms, stir up sediments, dig holes in submerged wood and rock, and change erosion patterns.

Even bird nests can be considered to be engineering feats; carefully selected materials are woven together, creating structures that provide habitat for certain insects and microbes and sometimes for other birds. Most nests are quite ephemeral, disintegrating in a year or two, but that’s not the case for the nests of eagles, for instance, nor for the Australasian birds known as megapodes or incubator birds, which build huge mounds of decaying vegetation in which the heat of decay incubates the eggs.

It has been said that ‘humans are tool-using organisms that specialize in engineering’. We seem to be compulsive engineers. People have been and still are the main agents of environmental change virtually everywhere on the planet. Think of all the dam-building, forest clearing, prairie burning and plowing, mining and quarrying, channel dredging and straightening, water diversions, and whatnot. Historically, much of that engineering has been done without any serious consideration of the ecological consequences, but those consequences have been enormous, long-lasting, and mostly negative for the environment. In recent years, there has developed a small impetus to reduce ecological damage resulting from our engineering, but human gratification usually wins the day.

Where to put freshly-laid eggs

…a panoply of nest types

Most people would answer: In a nest, of course! But it is a tad more complicated than that. For instance, there are some mammals in Australia and New Guinea that produce milk to nourish their young, but they lay eggs and care for them until they hatch. The duck-billed platypus lays its eggs in a burrow, where the two eggs stick to each other and to the female’s fur while the female incubates with her tail wrapped around the eggs. In a sense, the female herself is the nest, and the burrow houses the female. In contrast, a spiny echidna puts her single egg into a pouch, like that in which marsupials (e.g., kangaroos) keep their offspring, until it hatches. So echidna females carry their eggs with them as they forage.

But birds typically don’t carry their eggs around; they put them in special places. It would be easy to guess that birds don’t lug their eggs around because they are basically flyers (although some have reverted to walking) and it could be very awkward to tote eggs while flying. But sungrebes in Latin America are able to fly while carrying chicks in pockets under their wings, so why not eggs too? Furthermore, even though they did not fly, lots of dinosaurs made nests: fossil eggs have been found in ancient nests, sometimes with a presumed parental dinosaur fossilized nearby. That leaves open the question of why birds usually have nests for eggs.

One possibility is that it would be hard to fly with more than about two eggs in some pocket and having a nest would allow the production of larger clutches of eggs. Some birds lay large clutches of six or eight eggs, or even more. Having more offspring could mean that at least some might survive to adulthood and produce the next generation.

Bird nests come in a huge array of forms. Some birds, such as most shorebirds, just make a little scrape in the soil or sand, sometimes adding bits of stone or grass. Others build substantial piles of vegetation in marshes or in trees. Some weave complex chambers or hanging bags, some make a sketchy latticework of sticks, and others build cup-shaped nests of various fibers. Some birds build nests of mud, shaped in various ways, and even of their own saliva (think: bird nest soup!). Some birds nest in cavities made by other creatures, and others carve their own cavity from a tree or earthen bank. And all that just begins to indicate the variety of nests used by birds.

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Barn swallow nest. Photo by Bob Armstrong

Even among our local birds, the variety of nests is impressive. Some examples: Barn swallows stick a little ledge of mud pellets to a wall and line it with feathers. Cliff swallows take it one step farther and build a gourd-shaped mud nest with a narrow entrance. Juncos make a grass-lined cup on the ground, while hermit thrushes make a mossy nest in a shrub or small tree. Killdeer and spotted sandpipers carve out shallow saucers in the sand. Ducks generally make downy bowls on the ground, but some have taken to tree cavities. Woodpeckers chisel their own nesting cavities in trees, but tree swallows, nuthatches, and chickadees depend on old woodpecker holes for their nests. Solitary sandpipers take over abandoned robin nests and great horned owls often use old nests of red-tailed hawks.

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Ruby-crowned kinglet nest. Photo by Katherine Hocker

Although some kind of nest is the norm for breeding birds, there are some oddball birds that don’t make nests. For example, emperor penguins put the single egg on the feet of the incubating male, and a fold of feathery belly skin drapes down to cover the egg; the male can actually shuffle around on the ice while holding his egg safely on his feet. Fairy terns in the South Pacific perch their single eggs on a bare branch, somehow keeping them from falling off. Cliff-nesting seabirds often just balance their eggs on a ledge.

Once the eggs are in a basket, so to speak, there is an inevitable constraint on the behavior of the parents. They (or at least one of them) are now tied to a specific place for a certain period of time while the eggs are incubated and, in many cases, while the chicks are growing up. Being tied to a central place that is the focus of activity gives human nest-searchers many clues about finding scattered nests; we commonly just try to follow songbird adults as they carry nesting material or food to the central place. Nest predators are likely to do the same thing: jays and ravens and crows are strongly suspected to do so, for example, and weasels and foxes surely can do the same.

A few birds avoid the central-place focus by dumping eggs in the nests of other birds, often other species. American cowbirds, Eurasian cuckoos, African indigobirds and honeyguides, and some ducks, for example, have become brood parasites, letting other birds rear their chicks. The foster parents still have the central-place focus, of course, but the brood parasites have dispersed their eggs among several foster parents and at least diffused the potential problem.

When parent birds are central-place foragers during incubation and chick rearing, the nest needs to be located within range of a sufficient food supply. Few songbirds fly more than a few hundred yards from the nest to find food, although American Dippers sometimes fly a mile or more along a stream. An extreme contrast is seen in some seabirds, such as albatrosses. For example, the Waved Albatross of the Galapagos Islands may fly more than sixty miles from its nesting site to find food; other species may fly even farther. The interval between chick-feeding trips may be several days, for some kinds of shearwaters, because the parents are ranging so widely over the ocean in search of food.