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ABOUT THE AUTHOR
Richard F. Johnston is Professor of Ecology and Evolutionary Biology Emeritus at The University of Kansas, Lawrence, KS 66047-2906. His professional area of study is the ecology of birds with a focus on English sparrows and feral pigeons. Bob Gress and Mike Blair, along with Richard Johnston and John Richard Schrock, provided the photographs for this issue. Robert Clarke, editor emeritus of the KSN, provided the line illustration.
Rock doves are also called street pigeons, barnyard pigeons, or feral pigeons, and are known by the scientific name Columba livia. They arrived in North America with European settlers in the 17th-century. They came as domestic birds, caged and maintained for their use at the kitchen table. Individuals periodically escaped confinement, and although most escapes could not survive in the wild, some did. Their descendants today are called feral ("wild") pigeons, and they are among the most familiar of birds in Kansas, North America, or indeed the world. Ferals have a complicated ancestry that includes not only domestic pigeons but the wild ancestors of domestics as well. The natural history of today's feral pigeons involves an enormous range of biology stemming from this ancient and complex genealogy. This account examines the various aspects of feral pigeon biology.
ORIGIN OF FERAL PIGEONS
Old World rock doves were domesticated at least 5,000 years ago. This makes them the first bird species to have been so treated by humans; even the jungle fowl has a shorter history. Rock doves were brought into captivity initially for culinary and religious purposes, but artificial selection by humans soon produced variants that could be employed in other ways. For instance, by 3,000 years ago pigeons were being bred not only for the table but for delivering messages and for racing. The basis of selective breeding was genetic variation resulting from mutations concerning size, shape, color, behavior, physiology, and just about any other characteristic that can be imagined. Rock doves thus became domestic pigeons, and since then their care and breeding have been a focus of human attention.
What is not generally recognized, however, is that when domestic pigeons escaped from captivity they formed a new, free-living biological entity, the feral pigeon. Such birds are often called "domestic pigeons" but they are not domestics; neither are they wild rock doves. Ferals are distinctly different, with their own characteristics that set them apart from their domestic and wild ancestors. We still call them Columba livia, but they are special pigeons.
Feral pigeon colonies possibly could have been formed as early as 5,000 years ago. Most such enclaves are younger, however, and some are very recent. The first feral colonies in North America developed only 400 years ago, after European settlers brought domestic pigeons with them to Nova Scotia in 1606. A later introduction in 1796 brought pigeons to the Hawaiian Islands, and countless others have allowed feral populations to develop in most places in the world. Some populations are evidently distinct - for example, 80% of the birds in Honolulu, Hawaii, are in white plumage - but all can be shown to be in some degree different from both their wild and domestic ancestors. Some of those differences are examined here.
Figure 1. Blue-bar or wild plumage. Photo by Richard Johnston.
One conspicuous characteristic of feral pigeons is that they have many more plumage colors and patterns than rock doves, and many fewer than domestics. Wild rock doves have one plumage, the so-called blue-bar plumage (Figure 1). This consists of a pale gray back and wing coverts, two black bars across the trailing edge of the secondary converts, a white rump, gray tail with black subterminal bar, pale gray belly, and a darker head and neck that also has iridescent blues, greens, yellows, and reds. Females are slightly less iridescent than males.
Figure 2. Checker plumage. Photo by Richard Johnston
Domestic and feral pigeons may also wear the blue bar plumage, but most are in some other feather coat. A common alternative plumage is checker (Figure 2), which is similar to blue bar but features a graded series of few to many black spots, "checks," on the wing coverts. Also common is theT-pattern plumage (Figure 4), in which the wing coverts are almost entirely dark, with small, pale t-shaped flecks; the remainder of the plumage is also darker, except for the white rump. Bars, checkers and T's may also be in ash red plumage (cover and Figure 3), in which the blue is replaced by a pale red, so that we speak of blue bar or red bar, blue checker or red checker, and so on. Additionally, all of the above varieties sometimes appear with a gray rump.
The darkest plumages are also called spread, in which black pigment is spread all over all feathers. Checker, T and spread individuals are commonly referred to as "melanics," that is, birds that are darker than blue bar pigeons.
White plumages, not true albinos, are common among domestics and uncommon in ferals, although many birds show a few or even several white feathers in an otherwise normal blue or red plumage or any other pattern. Additionally, breeders have established a large number of other colors that breed true - milky, indigo, almond and smoky are examples.
|Figure 4. T-pattern plumage.
Photo by Richard Johnston
Figure 5. Spread plumage.
Photo by Richard Johnston
Wild rock doves are almost wholly blue bar in color and pattern; therefore, plumage variation in domestics must have been the result of human preservation of plumage mutants by selective breeding. The range of patterns and colors in free-living ferals, however, is considerably reduced over that in domestics. Such reduction is independent of human selective breeding, and suggests that natural selection has reasserted itself. Consistent with such a suggestion, some of the colors and patterns of domestics come with survival advantages or reproductive penalties and are accordingly either favored or culled from pigeons living in the wild. This should be expected because ferals live under demanding wild conditions.
The most regular consequence of life in the wild is that unusual plumage color and pattern combinations are removed by predators. Many of these, such as house cats, Cooper's hawks, peregrine falcons, or goshawks organize their hunting on the basis of recognizing odd individuals. Successful hunting begins with focusing on one individual, which allows a single bird to be chased from a flock of dozens. One result is that 95% or more of the birds in feral groups will be in blue bar, blue checker, blue T, or spread plumages, with a few o father other possible plumages irregularly represented in the remaining birds.
The proportion of a pigeon population that is in melanic plumage varies geographically: northerly localities have high melanic frequencies, which decline smoothly to the south. For 21 Eurasian localities between latitudes 41 to 62 degrees north, the percent of melanic birds in sample populations shows a correlation coefficient of r = 0.75 with latitude, which means the relationship is not a result of chance sampling.
MATE CHOICE AND PLUMAGES
The common plumage colors and patterns occur at stable frequencies in most feral populations. In Kansas, these proportions are maintained by the way in which choices of mates occur. Mate choice is a serious decision in feral pigeons, perhaps because pigeons remain paired for life. We find that pigeons employ at least seven different characteristics in judging other individuals as possible lifetime partners. The birds use age, previous breeding experience, relative dominance status, body size, feather condition, plumage color, and plumage pattern when looking at possible partners. Some of these characteristics overlap - body size and dominance status are examples - so that the birds often use redundant information. Additionally, most of the characters, even feather color and pattern, can be related to performance in reproduction. This emphasized how important mate choice is for birds that rarely divorce one another.
Females are usually the most discriminating of the sexes, and it is they who choose mates based on their plumages. Males apparently look more for large size or previous experience. As a group, mates of males have plumages in direct proportion to their availability in the population at large - this means that males do not select certain plumages over others. Females, however, pay attention to plumages and tend to choose males different from themselves. Researchers find fewer pairs in which both individuals are alike, and more that are unlike, than would be expected if the birds formed pairs at random. The only way in which unlikes in plumage can disproportionately pair with each other is by deliberate nonrandom, or disassortive, mate choices. This is an unusual pattern to find in nature, where positive assortative mating [or preferring a similar mate], such as is found in humans, is more common.
ADVANTAGES OF DIFFERENT PLUMAGES
Aside from selective mating, special advantages are associated with some pigeon colors and patterns. These advantages are held only by ferals. As shown in an exhaustive review of the literature by my colleague Marian Janiga, the advantages are superior reproduction and lengthened lifespan, depending on the environment. Perhaps as a consequence, under essentially wild conditions, ferals are far superior in survival or reproduction to domestic stocks, and at certain European study sites, also to wild rock doves. Ferals may be considered "superdoves."
One of the most important advantages associated with plumage variation in ferals is increased reproductive output. This is a complex relationship and depends on environmental conditions. In suburban or rural environments, blue bar individuals produce more offspring per pair than the melanics. Conversely, in central city environments, melanics outproduce blue bar birds. Blue bar individuals usually occur in small breeding and feeding aggregations, melanics in large ones. Blue bar birds commute greater distances to outlying districts for daily feeding than melanics. Very few inner city pigeons are blue bars, but they are much more common at city margins. Some big cities show a smooth gradient of increase in frequency of blue bar birds from the inner core to the suburbs (Figure 6).
Figure 6. Map of Moscow showing variations in a feral pigeon plumage.
Reprinted with permission of Oxford University Press.
Such gradients in plumage distribution are apparently due to gradients in reproductive characterisitcs. In Britain it has been shown that melanic males have larger testes and more spermatozoa in their tubules than blue bar males. And large melanic females in Kansas wait less time between clutches and thus have a high annual reproductive, exceeding that of blue bar females of any size. Melanic birds of both sexes have a much longer period of annual sexual activity than pigeons in other plumages, and they are responsible for most of the wintertime reproduction of ferals. Melanic squabs, or "baby" birds, have superior early development - they hatch from larger eggs and have more rapid growth than blue bar squabs. We also know that blue bar males are especially successful in defending their breeding sites from intrusion and disturbance by other pigeons. Both sexes of blue bars are more attentive at nests and provide better parental care than birds in other plumages.
Advantages and disadvantages are thus distributed irregularly among the plumages, but sort out in such a way that melanic pigeons do well in cities and blue bar pigeons do well in the suburbs and on farms. The differences are to some extent tied to differences in population densities between inner urban and suburban sites. Even so, why the superior parental care of blue bar birds is not an advantage under high density is an unsolved mystery. However, mate choice of females will maintain a wide range of colors and patterns regardless of location.
ADVANTAGES OF CHOOSING DIFFERENT MATES
Since random mate choice would select for similar neighbors, nonrandom mate choice in pigeons causes outcrossing. Among other things, it tends to maintain a diverse set of plumages. Perhaps as a result, feral pigeon populations show reasonably stable plumage variation. This disassortative mate choice would effectively maintain plumage variation whether or not escapes from captivity brought a variety of colors and patterns into a population. If for any reason the frequency of one form should decline, its value in mate choice would increase, resulting in frequency-dependent selection. This generates slowly fluctuating frequencies of forms in populations, creating a balance in variation through time.
Outbreeding or outcrossing resulting from disassortative mating generates what has been called hybrid vigor or "heterosis." The term "hybrid vigor" was in use by animal breeders as long ago as the 18th-century. Outbreeding causing multilocus heterosis [a variety of genes at many chromosomal locations] is known as a source of vigorous domestic stock having rapid embryonic growth with development generally free from structural defects. There is no reason to suppose feral populations are dissimilar in any fundamental way; they too should benefit from this genetic variation.
A simple test for multilocus heterosis results from the prediction that character variability will be low in genetically variable (heterozygous) versus more nearly invariable (homozygous) individuals from a population. In a study at the University of Kansas, sixty-four North American feral pigeons examined at 49 enzyme loci were separated into groups of high and low heterozygosity and the lengths of eight appendicular skeletal elements were taken for analysis. Bones proved to be larger in the birds with high genetic heterozygosity and, most importantly, all eight elements of these birds were significantly less variable than those from the group of low genetic heterozygosity. A permissible conclusion is that the developmental pathways of the heterozygous birds were better buffered against outside environmental influences, thus avoiding possible induced developmental problems, such as left-right asymmetry.
A dramatic example of the effect of single-locus (one gene) heterozygosity or reproductive and survival fitness is drawn from J. Felinger's work on pigeon transferrins. Transferrins are bacteriostatic and fungistatic proteins important in vertebrate immune systems. Young pigeons, while in the egg and for more than a week after hatching, are incapable of producing their own transferrins. But transferrins of their mothers are provided in egg yolk and egg albumin. Sixty-six percent of the eggs of transferrin heterozygous mothers hatched, while mothers of one transferrin homozygote hatched 52 percent, and of the other 48 percent, showing a highly significant reproductive advantage for heterozygous females. Thus, prospects for survival of squabs and reproduction by adult females were obviously affected by genetic variability.
Figure 7. Pair of pigeons. Photo by Bob Gress
Feral pigeon populations have year-long breeding seasons in most localities. Individual pairs maintain nests, lay eggs and rear young for from six to ten months each year, taking time off in or near winter. Because individuals are not of synchrony with each other, some nesting occurs in populations every month of the year, even in localities at high latitudes.
Wild rock doves have a relatively long breeding season, but it is clearly set off from a period of inactivity during which the annual molt occurs. When rock doves were first held in captivity, environmental pressures dictating a limited breeding season were to some extent relaxed. At the same time, the most vigorous birds were selected for their high reproductive output. Those that remained active for the longest periods would have been the most valuable. Consequently, captive populations gradually would have assumed a breeding season longer than that of their wild relatives. For such reasons, pigeons assumed a position of importance in early human religions, and became sacred to fertility figures such as Aphrodite and Astarte.
The yearlong season of ferals overlaps with the molt schedule and reflects both the innate capabilities of wild rock doves and the modifications wrought by human selective breeding in domestic pigeons. Relaxation of natural selection and assumption of characteristics thought to be desirable by humans is a pattern found in a number of aspects of pigeon biology (such as plumage color and pattern) as well as in other species (such as house cats) that have become feral after having been domesticated.
Figure 8. Western feedlots attract pigeons that line up shoulder-to-shoulder on overhead lines,
awaiting grain feed. Photo by Richard Schrock.
Clutch size in feral pigeons is two eggs, which is unchanged from that of rock doves. Parents continuously incubate eggs for about 18 days, females from late afternoon to mid-morning, males from mid-morning to late afternoon. Hatching rate is variable, from 44 percent to 95 percent, averaging perhaps 70 percent.
Squabs initially are fed "crop milk," a cheese-like substance secreted by crops of both sexes, high in proteins and fats, on which the young birds grow rapidly. A crop is a food storage area in the throat of birds. Parents begin feeding a few seeds with the milk at about day 4, and by day 8 or 10, the squabs are almost wholly on seeds. Squabs are ready to leave the nest at day 30, and most are gone by day 35. Parents will have produced another pair of eggs at perhaps day 20, overlapping the first squabs with their second clutch of eggs - "clutch overlap." Males assume the greater share of feeding the squabs at this time. Squabs gradually become responsible for their own maintenance beyond day 30. About 55 percent of all eggs result in fledged young, ranging from 24 percent to 79 percent in different populations over different years.
Figure 9. In times of scarcity, the later second hatching may die,
leaving enough food for its older sibling to survive. Illustration by Robert Clarke.
Since pigeons spend around fifty days caring for eggs and squabs, each young bird represents a major investment of time and energy. What should pigeons do if food shortages occur when they are breeding? Should they continue to feed both young, and risk both being underfed? Or should they concentrate on feeding just one of them? The answer is that feeding one is better biology. However, pigeons do not do this consciously and it is not a matter of simple choice, as the following discussion will show.
The two eggs of pigeons are laid about 40 hours apart. The first egg is partly incubated before the second appears, and it hatches about 24 hours earlier than the second. First eggs tend to be male, second eggs female. Thus, just after hatching, a pair of squabs usually consists of a day-old male and a recently-hatched female. Males are larger than females at hatching, and they have been fed for a whole day prior to the female's having hatched, so the size difference is pronounced.
Second hatchlings therefore need to compete with an older, more capable sibling for food and parental care. But second hatchlings have a faster rate of growth, and if food is abundant they can make up much of the size difference at around day 15. If food is scarce, the first squab is always fed more than the second, and the latter often dies. The surviving squab is fledged at a greater than average body weight, at the usual age of 30 days.
Pigeon reproduction thus includes this automatic brood reduction mechanism. It enables parents to rear at least one squab if food scarcity makes it difficult to rear two. Indeed, such brood reduction enables parents to raise one squab when trying to raise two would lead to death for both squabs. This is a remarkable adaptation for efficient reproduction.
The first hatchling does not have to be a male for this automatic mechanism to be brought into play. Brood reduction in pigeons would function even if the first egg were female. It would not function quite as efficiently because the size disparities of the sexes would, at least sometimes, inactivate the mechanism. Having the first egg male is a kind of insurance that the brood reduction strategy will work, and that probabilities of loss of both squabs will be reduced.
In birds the chance that the first egg will be male is ordinarily 50 percent, which makes great sense since the way in which eggs are made results in half of them being male and half female. But in pigeons, the first egg is male 70 percent of the time. No positively effective way for dictating sex of eggs is known, so that a certain mystery attends how pigeons manage such control 70 percent of the time. It is a mystery that should attract attention of students of biology.
LIVING IN COLONIES
Pigeons are considered to be highly social creatures that live in colonies. However this is not wholly accurate because many pigeons live in partial isolation from others. In a Kansas town that has feral pigeons, you almost always will be able to find birds nesting in a colony, that is, their nests will be built close to one another. At the same time, many pairs will nest away from the colony and apart from other individuals nesting alone.
Pigeons therefore are not required by their nature to nest in colonies or for that matter to nest singly. Pairs of pigeons readily adjust to the social variables of colonial life or to living alone, which means that they are reasonably complex birds in their social behavior. When pigeons nest colonially inside city limits, they do so because humans have provided a suitable environment. The most important element is safe nesting sites. If such sites are distributed singly and unpredictably around town, the birds find and use them. But aggregations of sites that are secure from predators such as house cats or crows, and that can be defended against intruder pigeons, are usually more desirable to the birds.
Food that is locally available around a breeding colony, while welcome if it exists, is not as important as nest sites to the formation of a colony. This is because pigeons are able to commute daily from city centers to outlying districts for feeding. Commuter pigeons usually gather into flocks, numbering from perhaps a dozen to thousands of birds, and follow established routes to and from feeding sites. Wintertime commuting to feeding sites begins in early morning, is followed by feeding and loafing, by feeding again, and then by return to the roosting sites before dusk. Summer schedules are less synchronized, owing to breeding activities that include nest exchanges by pairs in mid-morning and late afternoon. Return flights tend to have more birds in them than those going to the feeding grounds in the morning.
RELATIONSHIPS WITH PEOPLE
When we think about the 5,000 or more years over which people and pigeons have coexisted, we might well expect that both groups have experienced problems living with one another. People consistently complain that feral pigeons are problematic to human interests - they eat foodstuffs of consequence, they are untidy in nesting habits, and they are potential sources of zoonotic infections. There is no question that it is ferals that people view with alarm, for the domestic stocks are tractable and user-friendly, and wild rock doves are uncommon enough that they are ignored.
Humans are responsible for creating domestic pigeons, and by extension also for the existence of feral populations. Humans have an obligation to treat all these pigeons in a humane manner. This is easy to do for domestic birds, but can be a problem when ferals are involved.
Ferals do what they do just as they please, and human interests are sometimes very far from theirs. They frequently build nests where we do not want them. This leads to littering of our buildings and monuments with nesting material and dung. When these materials accumulate, they can block rain drainage systems and cause water damage to building walls and ceilings. Stonework and metalwork, whether commemorative or decorative, can be eroded by errant nesting and excretory materials. In some parts of the world feral pigeons are serious agricultural pests, accounting for appreciable loss of growing and stored grain. And we know of a great range of infectious microorganisms that live on or around pigeons, and which are capable of infecting humans. However, the perception of risk is greater than the true risk.
Political pressure from humans who are affected by unrestrained pigeons is sometimes important, leading to attempts to control the numbers of pigeons in cities and farmsteads. The range of control measures used is considerable and expensive.
City officials who are requested to reduce pigeon populations are often in a no win situation. When they use humane control methods, it costs a small fortune, and if a public official is in charge, the taxpayers complain. If an official of an agribusiness is involved, the stockholders complain. If chemicals or firearms - which are perceived as inhumane - are used to kill pigeons, the public generally is outraged. In the long run, everyone is likely to be disturbed.
The most effective way to control pigeon populations is to first remove foodstuffs from where pigeons can find them and then block off potential nesting sites from use by pigeons. Such a program is very easy to describe, and is unbelievably difficult to practice. The problem is that people are careless about how they themselves affect the ecology and behavior or pigeons. Grain frequently is not handled properly, and much is spilled and left to rot - or to feed pigeons. Moreover, our domestic and public architecture invites pigeons to build their nests really close to people. The "pigeon problem" is therefore a people problem - "we have seen the enemy and it is us." We have to obtain public cooperation to modify pigeon populations and it is costly to implement and control, with or without cooperation.
People in many cities have decided to let well enough alone and peacefully coexist with pigeons. This is an excellent procedure, and it reflects the good aspects of having pigeons live close to us. These birds are some of the big winners in the lottery of life; they are real survivors and know how to live in a world dominated by humans. Pigeons are also elegant creatures of style and grace aloft, and are otherwise beautiful to watch. Our world is brightened by them.
Johnston, R.F. and M. Janiga. 1995. Feral Pigeons. Oxford University Press, New York. This book is the source for everything included in this Kansas School Naturalist. Full citation to original sources is available in this book's bibliography.
Figure 10. Statue of diana bespeckled with pigeon droppings; Old Town Square, Warsaw, Poland.
Photo by Richard Johnston.
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