Chapter 2: A Story of Species Success

“Relicts” are species that tend to become more and more restricted both geographically and ecologically, because they are unable to compete successfully with other species. At the other extreme are those dominant or ultra-successful species that are expanding geographically and also, in many cases, in ecological tolerance. The American Robin (Turdus migratorius) is an example.

Without doubt, robins are much more widespread and numerous today than they were five hundred years ago. Robins thus tell a story of species success that has seldom been attained in the animal kingdom, at least within the past couple millennia. To be sure, a few other organisms have also achieved great and recent success; humans and rats, for instance, can both boast representatives almost planetwide. But when considered against the countless species that are having difficulty holding their own in today’s world, the countless others that are quickly becoming extinct, and the still countless others that have already passed out of existence, the American Robin is surely among the biological elite. Indeed—forgetting for the moment the indestructible insects, the ubiquitous microorganisms, and the unapproachable fish of the sea—Mother Earth may one day look at herself and, despite a most thorough search, find little more than robins, rats and people scurrying about her surface.

Robin Haunts

Robins can be found in an impressive variety of environments, including mountains and valleys, forests and seashores, cities and farms; at times they even visit deserts and the treeless arctic tundra. The robin’s preferred breeding habitat, however, contains open, grassy ground for feeding, and sturdy, low-limbed trees for nesting and shelter. Modern suburban areas, with their fine-trimmed lawns and scattered orchard and shade trees, are thus ideal breeding forums for redbreasts.

Interestingly, the large stretches of heavily cluttered forest that covered much of pre-colonial America did not provide a very conducive habitat for the ground-feeding robin. Instead, the birds probably gathered most densely around sporadically occurring forest clearings or near forest edges (that is, the borders between woods and open fields). Such areas were then tolerably common, at least in the northeastern part of the continent, and the robin was accordingly a tolerably common bird—populous here and there, scarce more generally. But when humans axed forests to form fields and lawns, and later when they irrigated sand and sagebrush to grow trees and grass, robins mushroomed in both range and number.

Today, robins live across all of North America, from the Atlantic to the Pacific, and from the middle of Mexico to the upper regions of Canada. This impressive distribution, however, needs to be qualified in several ways. First, since robins are a migratory species, the areas in which they breed are not necessarily identical to the areas in which they—during the different seasons—regularly appear. Although virtually no region of the continent fails to see robins sometime during the year, the Far North hosts few redbreasts during the winter and, until relatively recently, the Far South saw few during the summer.

Secondly, the fact that robins breed in a given habitat does not imply that they prosper or even hold their own there. Robins can be found in most urban areas, for example, despite mortality rates that may be twice as high as that suffered by redbreasts living in rural areas where cats and dogs—major enemies of breeding robins—are more scarce. In fact, some urban areas may retain robin populations even though more birds die in the city than are born there! A city’s population can continue to survive under such circumstances because of regular influxes by surplus robins from surrounding suburban areas.

A third qualifying note to the robin’s wide distribution is that the birds may be extremely numerous within a restricted area and yet be quite scarce or even absent throughout immediately surrounding regions. This island-of-robins effect can occur in many different contexts. In frontier regions (for example, the Far North, or in mountain areas), robins may be absent except near the isolated villages that spot the area, and in these they will be common. In swamps and bogs where redbreasts don’t usually hold extended residence, flocks may congregate around small patches of berries that have suddenly ripened, while surrounding areas have no robins at all. After a springtime snowstorm, when nearly all the ground is cloaked in white, robins may sardine themselves onto one forageable green patch that somehow escaped the covering. And in a comparable manner, a source of water in an otherwise desert region will attract robins in great numbers while neighboring regions are completely robinless.

Incidentally, although robins are inhabitants of North America almost exclusively, redbreasts have occasionally popped up in such distant realms as Bermuda, Cuba, Greenland, Britain, Ireland, and Austria. Once, an American Robin was even sighted on Helgoland, a tiny mile-long island whose 200-foot cliffs project from the North Sea some forty miles off the mouth of Germany’s River Elbe. Redbreasts have apparently never ventured to Australia, probably in deference to the giant seven-foot earthworms that tunnel through that strange continent.

Robins are quite able to reach the nearer of these foreign lands (Bermuda and Cuba) under their own birdpower, but some of the European sightings probably involved individuals who were released, or who escaped, from captivity in those lands. The possibility remains, however, that a stray robin or two has actually managed to cross the Atlantic Ocean in some spirited Lindberghian adventure. While robins generally try to avoid flying over the open sea, they will travel over long stretches of water to reach numerous islands on which they regularly breed (for example, the Magdalen Islands which lie off Quebec in the Gulf of St. Lawrence, sixty to seventy miles from any major land mass). There is also one report of a redbreast who cruised by a ship that was chugging four hundred miles off the eastern coast.

Regional Variations among Robins

One interesting sidelight of the robin’s extensive distribution is the existence of differences among groups of redbreasts residing in different localities. A blatant behavioral dissimilarity, for example, is shown by those robins who breed in areas where human beings are scarce (for example, near the arctic limits) and those who breed in suburban backyards. Usually the latter are relatively trusting animals who parade about in plain view of people, while the former are wild, wary, and extremely difficult to approach.

The slight color and size variations displayed by the numerous races of redbreasts provide another illustration of group differences among robins. As previously noted, these subspecies distinctions are generally associated with various geographic regions; nevertheless, a single region can sometimes contain two separate races, with each restricted to a relatively exclusive ecological niche. As we shall see shortly, there are areas of the southern United States where Eastern Robins inhabit the mountains while Southern Robins live throughout the surrounding lowlands. Another example has, at least in the past, characterized the Washington-Oregon area where both the Western and the Northwestern Robin have been found to breed. Again, the two races prefer slightly different habitats, for while the Western Robin has lived primarily within human-inhabited districts, the Northwestern Robin has instead resided in nearby tracts of coniferous forest.

Numerous reproductive variations exist within the robin population. First, due to the obvious differences in the schedule of spring’s arrival, the early-nesting southern populations enjoy a much longer breeding season than the later-nesting northern groups; as a result, the former can frequently rear three broods in a single season while the latter, especially in the Far North, may manage only one. There also seems to be a tendency for the offspring of robins in the Far North (for example, at Umiat, which is at an arctic latitude of 69°N) to leave their nests a few days sooner than the offspring of robins living in the contiguous United States.

A third reproductive variation involves nest-building materials: grass and mud are the most general ingredients to a robin’s nest, but redbreasts in northern Maine may instead use twigs and leaf-mold while birds even farther north may build with moss and lichen. Even clutch-size (the number of eggs laid) varies with place of residence, for robins tend to lay slightly smaller clutches at lower latitudes (two or three eggs at Mexico City’s 20°N latitude) than at higher ones (three to five eggs in the northern United States). Interestingly, clutches also tend to become somewhat smaller with increasing nearness to the coastline, at least across the northeastern part of the continent.

A final distribution-related difference exists between mountain-living robins and lowland-living robins; the former usually have heavier hearts and lungs than do the latter. Why? Because the mountain birds, flying as they do at such lofty altitudes, must somehow cope with respiratory and circulatory problems engendered by the mountain air’s decreased density, poorer oxygen content, and lower temperature. Larger lungs and heavier hearts do the trick.

Altitude Equals Latitude

According to a basic principle of ecology, many of the differences between southern regions and northern regions—in terms of climate, vegetation and wildlife—are paralleled by comparable differences between lowlands and mountains. In other words, the same sort of ecological changes that a person would observe in traveling from a southern latitude toward a northern latitude can also be observed as one gains altitude by climbing up a tall mountain.

Climatically, this altitude-equals-latitude principle is reflected in the dropping of both temperature and air pressure as one approaches either the arctic (from the south) or a mountain peak (from the lowlands). With vegetation, the same succession of plant life that occurs as one moves from the equator to the arctic—namely, from tropical forest to deciduous forest, to coniferous forest, to mosses and lichens, and finally to bare ice and snow—also tends to occur with increasing altitude along an equatorial mountain (the mountain would have tropical forests surrounding its base, coniferous forests halfway up, and unvegetated ice and snow at its peak). Even the changes occurring in animal life across increasing latitudes can find parallels in the species living at progressively increasing heights along a mountain slope; fur thickens (for better insulation), tails and ears shorten (to minimize loss of body heat by decreasing the body’s surface area), and gestation periods increase (so that babies are more developed at birth, and therefore better able to face harsh arctic or mountaintop climates).

Robins follow this altitude-equals-latitude principle in a number of different ways. First, while the majority of redbreasts use twigs, grass, and mud to make their nests, both mountain-dwelling robins and robins of the Canadian north frequently use lichen and moss as major nest materials. A second example involves molting (changing of plumage), because both mountain robins and northern robins molt as much as a month later than do birds of the lowlands and birds of the South. Thus, in terms of both nesting materials and time of molting, robins who live at high altitudes show marked similarity to robins who live at high latitudes.

Another demonstration of the seeming equivalency of altitude and latitude is provided by the distribution of the Eastern Robin who, in parts of Virginia, the Carolinas and even Georgia, infringes upon what is normally the domain of the Southern Robin. Frequently, Eastern Robins can be found living high in the mountains while their southern relatives reside in the surrounding lowlands. The high altitudes of the South, the Eastern Robin seems to be telling us, are just as ecologically satisfactory as the high latitudes of the North.

A final illustration involves vertical migration, which is the descent of mountain-breeding animals into nearby lowlands for the winter (vertical migration thus substitutes a decrease in altitude for a decrease in latitude as a way to circumvent winter’s severity). The best example of vertical migration among redbreasts is provided by the San Lucas Robin. Unlike most of its relatives, this beautiful but little-known member of the robin family is strictly nonmigratory—at least in the traditional sense. Rather than journeying southward like most redbreasts, the San Lucas Robin simply descends from breeding grounds in the mountains of Lower California and winters in surrounding lowlands. Similar vertical migrations are performed by some robins living in Mexico (of subspecies phillipsi), as well as some robins breeding in the mountainous pine-belt regions of Washington.

These vertical migrations, incidentally, are similar to the more conventional north-south migrations in several ways. After arriving at their northern breeding grounds in the spring, normally migrating robins typically remain in flocks for a few days before dispersing to claim individual nesting areas. Comparable behavior is shown by mountain-breeding robins who also remain in flocks for a short time upon first returning to their alpine abodes. Another similarity involves the fact that northern breeding robins usually begin nesting several weeks later than do more southern breeding individuals; in like manner, mountain-breeding robins usually start nesting long after those redbreasts breeding in the surrounding lowlands. In short, robins who make vertical migrations show behavioral parallels to those who make latitudinal migrations.

Recent Expansions of Range

As of 1890, the only robins who ever visited Fort Reno, Oklahoma, were a few migrating transients en route to someplace else. By 1921, however, redbreasts were well settled and breeding with vigor throughout the entire district. Similar nesting invasions of areas heretofore unoccupied by Turdus migratorius have occurred time and again in North America. And almost always, these extensions of the species’ distribution have been the direct result of human modification of the environment.

Of course, acts of Nature do occasionally extend the robin’s range. Redbreasts frequently pour into the wake of lightning-sparked forest fires where denuded soil and charred logs readily yield an abundance of worms and insects. More long-termed effects have apparently been occurring as a result of recent climatic changes; specifically, there has in past decades been a trend toward increasing winter mildness within the continent’s higher latitudes, and this seems to be encouraging a gradual, northward extension in the robin’s winter distribution.

Nevertheless, natural forces have generally been less striking than human activities in their effects upon the robin’s range. As noted earlier, the suburban setting—now so widespread across North America—has provided a nearly optimal breeding environment for robins. Indeed, robins today are probably far more populous in these “artificial” habitats than anywhere else. Let us now, however, consider additional and more specific cases of human-wrought ecological changes that have proved beneficial to the species.

One localized example was recently provided by a forest in Pennsylvania which, in its natural state of heavy underbrush, originally hosted few robins (less than five pairs per hundred acres). Most of the undergrowth was eventually cleared away in a campsite-making project, however, and before long robins had increased to an average density of fourteen pairs per hundred acres—a climb of more than 180 percent within a couple of years. Another quick increase in robins occurred during World War II near Churchill, Manitoba, an area which until then was largely uncivilized wilderness. Construction of an airport as well as several other war-inspired building projects brought great changes to the environment—changes of which robins evidently approved, for the birds were soon several times more numerous in Churchill than ever before.

On a grander scale, robins have succeeded in colonizing whole continental regions—specifically, the midwestern, western, and southern parts of the United States—where they were previously absent or scarce during the breeding season if not year-round. The midwestern invasion centered upon the Great Plains where, historically, robins had been discouraged from foraging by tall grasses, and from nesting by the virtual absence of trees. As told by one author:

Before the prairies of the Middle West were settled, and when bison roamed in vast herds over the boundless grassy plains, robins bred only in the northern woods of Michigan, Wisconsin and Minnesota. But as civilization moved westward and trees were planted about the ranches, the robins adapted themselves to the new and welcome conditions and made their summer homes near human dwellings in regions they had formerly passed over on migrations.

Incidentally, the gardens that settlers dug around their homes were perhaps as important as the trees they planted. The tough dirt of the plains, it seems, was initially quite wormless; when people imported potted plants for their newly tilled gardens, earthworms were inadvertently introduced into the region’s soil. Before long, the prolific annelids were burrowing wherever gardens and lawns were cultivated, much to the robin’s dietary delight.

The spread of robins in the Far West was typified by the invasion of southern California that occurred from about 1915 to 1925. Prior to that time, robins nested only in northern regions of the state where persistent fog-belts and trickling mountain streams watered significant stretches of forgeable, short-grassed meadows throughout the breeding season. In contrast, the dry lowlands of southern California simply did not provide robins with sufficient food for raising their families. Some of these regions—little more than wind-tossed sand and sagebrush—were completely barren of worms as well as most insects. And even those areas that managed to support grass remained so parched under the scorching sun that earthworms summered in the deep, cool underground where robins couldn’t reach them, while most insects persisted in their hard-bodied adult stages (rather than in their soft-bodied larval stages) and were thus much less vulnerable to predation as well as to desiccation.

But then humans grew grass to grasp the sand, sowed saplings and shrubbery to stop the wind, and whooshed in water to wet the lawns. As a result, earthworms tunneled to the top, insects languished in larval life, and robins nurtured numerous nestlings. This instance of range extension, by the way, clearly illustrates that the robin’s pre-colonial distribution was restricted largely by requirements of food supply and, to a lesser extent, of nesting facilities; temperature per se was not a restraining force for the population. Of course, this is not to say that robins have no inherent limitations regarding temperature but simply that the species has always been able to tolerate the great thermo-variations found across present-day North America.

The most recent example of robin expansion into a major continental region was the species’ colonization of the Far South. Robins have always spent their winters in southern locales but until relatively recently never remained for the breeding season. Early southern rural areas did not provide conducive habitat for robins inasmuch as most of the land was covered by cotton and corn crops, tall grass, and heavily undergrown forests; even the farmhouses and small towns did not have much in the way of lawns or nestworthy shrubbery. The larger cities, on the other hand, had both lawns and shade trees, and it was in these areas that breeding robins first appeared.

Two aspects of the robin’s southern expansion deserve special note. First, in reaching the widely scattered big cities, robins initially bypassed large stretches of unsuitable rural areas. It was only after becoming safely entrenched in urban centers that the birds began to filter into the less desirable outlying districts. Secondly, suitable big-city environments had existed in the South several decades before robins actually arrived. Presumably it was a sudden surplus of redbreasts in areas farther north that finally spurred the southern campaign. Having begun their invasion, however, robins conquered the region relatively quickly; for example, even though robins did not reach the northern border of Georgia until after the turn of this century, redbreasts were nesting just above Florida (at Waycross) by 1937.

As a final example of the effects of human actions upon the robin’s distribution, let us consider the introduction of grazing cattle into a forested area which, because of heavy underbrush, has previously been an unsatisfactory environment for the species. Such a situation, which has occurred in many regions of North America, is particularly interesting because it involves a continuously changing habitat that is suitable to robins only during its middle stages. More specifically, the ensuing ecological succession typically occurs as follows:

The most immediate effect of the grazing is to clear out the underbrush—most importantly of all, the seedling trees. Then over the years, as the larger trees die but are not replaced, the forest canopy begins to yawn in this place and that, and short-grassed clearings begin to emerge. Each of these clearings, of course, is an attractive breeding area for robins, and the birds soon move in. Next, as further grazing continues to prevent tree replacement, the forest shrinks more and more until eventually the trees form mere islands while the clearings become the sea. Nevertheless, the area probably still offers sufficient nest-sites for robins, and so the birds remain. Finally, however, the few surviving woodland patches die out, and the redbreasts accordingly abandon the region for more arboreal pastures. All of these changes, of course, happen only very slowly over many, many years.

One obvious question raised by all of the above examples of range extension is, where did the initial pioneer breeders come from in the first place? In general, there seem to be at least three different possibilities, each of which has occurred in one invasion or another. First, the pioneers might have been birds who had regularly wintered in the frontier region but, until the year of the invasion, had always deserted it at the first sign of spring. Alternatively, the pioneers could have been migrants who—while traveling to breeding grounds farther north—were enticed out of their flocks by an attractive and as-yet unclaimed new region. The third possibility is that the pioneers were birds who had been crowded out of their traditional breeding grounds by a local superabundancy of robins, and had subsequently wandered into virgin territory.

In any event, we might in closing emphasize that the robin’s repeated successes in extending its range over pre-colonial times offer a marked contrast to many other native North American populations. Bison, prairie dogs, antelopes and beavers are all far less numerous today than before the arrival of Europeans. Among American birds suffering even worse fates are the Passenger Pigeon, the Heath Hen, the Carolina Parakeet and the Ivory-Billed Woodpecker, to name a few. Against the sad theme that these mammals and other birds exemplify, the American Robin thus provides an especially sweet story of species success.