Introduction

THE object of this book is to describe the biology of nest-building behavior among birds. A nest is a special construction forming a bed or receptacle in which the eggs and young develop. Nests occur throughout the animal kingdom (N. Collias and Collias 1976), and are designed by evolution to help the parents meet the needs of their young. The type of nest built gives important insights into the life of each species, since nests focus the essential requirements of animals for reproduction. Nests are therefore very relevant to the science of ecology if we define ecology as the study of the relationship of living organisms to their environment.

F. H. Herrick (1911), one of the early pioneers in the scientific study of nests, or "caliology" as it was then called (C. Dixon 1902), stated that "it would be difficult indeed from the standpoint of the student of instinct and behavior, to find a more unsatisfactory class of scientific literature than that which deals with the nests of birds." The picture is now changing with the growing realization that nests provide clues to real and significant ecological relationships. Particularly during the last ten or fifteen years a large and important body of literature has grown up.

Bird nests include the best and the most highly evolved nests known among vertebrate animals. In relation to the nest there are two main types of birds, nidifugous ("nest fleeing") and nidicolous ("nest dwelling") birds (Nice 1962, Welty 1975, Skutch 1976). Nidifugous species have precocial young, which leave the nest soon after hatching. Examples are chickens and ducks. The young hatch covered with down, their eyes are open and alert, they can walk the day they hatch, and they are soon able to feed themselves. Nidicolous species generally have altricial young, which hatch in a helpless state — unable to stand, naked or nearly so, and with eyes closed. The young are completely dependent on the parents for food, comfort, and protection, and are unable to leave the nest for days, weeks, or even months, depending on the species. Nidicolous birds build the best nests among birds, and include the great majority of species. The most important examples are the passerine or perching birds, which comprise the largest order (Passeriformes) of birds and include more than half the living species.

Nests are of particular interest to the student of animal behavior. Nest-building behavior has traditionally been accepted as a classic example of instinct, a vague term that has served as a convenient label for species-typical behavior, without implying necessarily anything as to the causation of this behavior. The type of nest built is as much a property of a species, as is its behavior, appearance, morphology, or physiology. When a bird builds a nest it makes a more or less permanent record ("frozen behavior") of its behavior as related to its evolution.

Plan of Treatment

One can relate some important problems in the life of birds to the types of nests they build. These problems are the subjects of the various chapters and determine the general organization of this book. Comparisons of different kinds of nests (chap. 2), particularly within a group of related species or genera (chap. 3), give clues about the evolution of nest-building behavior. Sex differences in nest-building behavior are related to the type of mating system characteristic of the species and to the ability of a bird to attract and keep a mate (chap. 4). In male birds of some species the ability to manipulate materials has even become specialized in evolution and has been redirected solely to obtaining mates instead of to the task of building a receptacle for the eggs and the young (chap. 5).

Adaptation to the stress of the physical environment helps explain the nature of the nest site (chap. 6) and the type of nest (chap. 7). The diversity of competitors also helps explain the diversity of nest sites and construction in different species (chap. 8), while the form and strength of nests is often closely related to the need for security from different predators (chap. 9).

Just how a bird builds its nest (chap. 10) can be analyzed according to various sign-stimuli governing the acts and sequences of building behavior (chap. 11). The "play nests" built by immature birds of some species differ from the finished products turned out by the adults, and a comparison of the building behavior of young birds with that of mature birds gives an insight into the development of the ability to build (chap. 12). Changes in internal factors, particularly in the levels of secretion of different hormones, interacting systematically with behavioral factors, lead to the onset, increase, decline, and termination of nest building and of nesting motivation (chap. 13).

Finally, there are problems related to gregarious nesting (chap. 14). Many birds breed in groups or colonies. The degree of crowding of nests can be related to such problems as the distribution of food resources, predation, and stresses imposed by the physical environment. Gregarious nesting also increases the competition within the species. These negative aspects are balanced against the advantages of gregarious nesting, which include social facilitation of nest building and of breeding behavior.

There are about 9,000 known species of living and recently extinct birds in the world, variously classified in some 20 to 29 orders and 148 to 171 families (Mayr and Amadon 1951, Storer 1971, Welty 1975, Van Tyne and Berger 1976, Bock and Farrand 1980, Cracraft 1981). Appendix One gives a list of orders and families of the world's living birds with the general types of nests they build. Scientific and common names of birds referred to in the text are listed in the index.

Most birds are small, and in general small birds build the finest nests. In the Old World, the weaverbird family (Ploceidae) and the Old World warblers (Sylviidae), and in the New World, the tyrant flycatchers (Tyrannidae), ovenbirds (Furnariidae), and the orioles, caciques, and oropendolas (Icteridae) are noted for the great variety of complex nests they build.

Appendix Two lists some references that have excellent photographs of many kinds of nests. A selection of photographs showing some of the main types of nests is presented in the next chapter, especially in relation to the evolution of nest building. Other aspects of nest building are illustrated with photographs or drawings in subsequent chapters.

Application of Scientific Methods to the Study of Nest Building

In general, we try to explain nest-building behavior among birds by applying the principle of evolution; Darwin's theories of natural selection and sexual selection; the principle of dynamic homeostasis; the basic engineering principle that function determines structure; as well as general ecological, ethological, and sociobiological principles.

We follow the common-sense, dictionary (Webster's) definition of scientific methods: "principles and procedures for the systematic pursuit of knowledge involving the recognition and formulation of a problem, the collection of data through observation and experiment, and the formulation and testing of hypotheses." We use all elements of this methodology in our book, but because this field of study has not been synthesized before in any really comprehensive way, we have had to place considerable emphasis on describing the facts of nest-building behavior and in formulating the basic problems. Nevertheless, hypotheses are not absent, as can be seen by inspecting particularly chapters 6 and 7 on the physical environment and the energetics of nest building, or the more experimental chapters 11, 12, and 13 on the analysis, development, and hormonal basis of nest building, or chapter 14 on gregarious nesting.

Our general method has been to analyze nest-building behavior by asking and attempting to answer a series of questions: What kinds of nests do birds build and why? What are the functions, causes, developmental and evolutionary aspects of the diverse types of building behavior? Hypotheses are really sophisticated guesses as to possible answers in a series of questions. Chapter 11 illustrates this process in some detail with one thoroughly analyzed case of nest-building behavior.

One must have some tolerance for the different ways in which scientists work. The results of a 1979 mail ballot to members of the Division of Ecology of the American Society of Zoologists indicated that "they thought the time was overdue for some significant effort at reopening lines of communication between theoretical and empirical ecologists." The response led to a symposium on "Theoretical Ecology" (Gordon 1981). The concluding address by Robert M. May on "The role of theory in ecology" develops the importance of theory to ecology, and May also states (1981, p. 909): "Naively simple formulations of The Way To Do Science — be they the Baconian Method of the Victorians or the extreme logical positivism of Popper today — are harmless in themselves, but have unfortunate consequences when they inspire doctrinaire vigilantes to ride the boundaries of a discipline, culling the sinners. The scrabbling, nonlinear way Darwin pursued his ends is typical of most good science. Writing about him, Ghiselin (1969, p. 236) says: 'Viewed from without, science appears to be a body of answers; from within it is a way of asking questions.'"

The preceding discussion emphasizes that in science the proper balance between inductive and deductive methods, or empirical and theoretical approaches, will vary with the stage of development and with the complexity of a particular discipline. There is presently a great need for the accurate description and synthesis of the widely scattered facts concerning nest building by birds, and we have tried to meet this need. The initial step in a scientific discipline is the precise description and classification of the phenomena of the subject, which are in fact the problems for investigation and which set the stage for further analysis. A good, basic inventory of reliable facts is the best source of scientific hypotheses for further analysis. This book includes some observations from our original research to help round out the picture, but the primary objective is synthesis of the extensive and scattered literature.

Bonner (1980) and Bartholomew (1982) have recently emphasized the importance to scientific progress of a proper balance between analysis and synthesis, that is, between reductionist and holistic approaches to scientific progress. As Bonner states (p. 9) "a holistic approach ... is a necessary stage without which the reductionist progress could not be made," and "is the only way of describing the problems and grouping the facts." The first part of our book emphasizes synthesis — the evolution and functions or the significance of nest-building behavior. The second part emphasizes the analysis of nest-building behavior into its component mechanisms and into its physiological basis, in so far as they are known. The last chapter considers the relationship between individual and group behavior with reference to gregarious nesting.

Conceptual Approach

This comprehensive review of the biology of nest building in birds, the only one presently available, is written from a conceptual point of view based on principles of evolution, ethology, ecology, sociobiology, and physiology. Much of the conceptual contribution is as a synthesis — the insights provided by new ways in which facts and ideas related to nest-building behavior are grouped together. The most general conclusion of the book is that nests of birds play a key role in avian biology — far more than has generally been appreciated. In recent years, investigators have developed many relatively new concepts of nest-building behavior, of varying degrees of novelty, which are probably not very familiar to most ornithologists, ethologists, or general biologists; these we have presented in the summary at the end of each chapter.

An important feature of the work is the presentation of more adequate documentation to strengthen earlier tentative generalizations (N. Collias and Collias 1964). For example, table 2.1 gives the first comprehensive and adequate evidence from various parts of the world supporting the important assertion that domed and roofed nests are built by a much higher proportion of tropical species of passerine birds than of temperate-zone passerine birds. Another example is the concept that for birds the type of nest built may often be typical of the genus. Appendix One gives broad generalizations based for the first time on all the bird families of the world, for the type of nest built and the sex building it.

In our detailed application of evolutionary theory to the nest-building behavior of birds, we have relied for general orientation in modern evolutionary theory particularly on books by Mayr (1970), Wilson (1975), and Wright (1978).

Diversity of Nests and Major Evolutionary Trends in Nest Building

THE following classification of nests is based mainly on general trends or levels in the evolution of nest building. It is followed by a discussion of these evolutionary trends.

A Classification of Bird Nests

A. Incubation by heat from the physical environment. Eggs buried in soil; heat from sun, volcanic activity, hot springs, or decomposition of vegetable matter. Mound builders, or megapodes (Megapodiidae).

B. Incubation by heat from physical environment and parental body heat.

1. Eggs buried in sand for part of incubation period. Egyptian Plover.

2. Eggs in nest; parent does not incubate during heat of day. Many Australian grass-finches (Estrildidae).

C. Incubation primarily by parent. The great majority of birds.

1. No nest; eggs not laid in an enclosed cavity.

a. Eggs laid in suitable habitat on ground, rock, or tree. Whip-poor-will, Common Murre or Guillemot, White Tern.

b. Egg held on feet and incubated under a fold of belly skin. Emperor Penguin.

2. Eggs laid in an enclosed cavity or burrow, usually with little nest lining.

a. Bird uses a preformed cavity, as among rocks (Ashy Storm-Petrel), or in a tree (many hornbills, House Wren, Tree Swallow).

b. Bird digs nest hole in the ground (many shearwaters), in a bank (European Bee-eater, Bank Swallow), or in a tree (most woodpeckers).

c. Bird digs nest hole in ant or termite nest. Orange-fronted Parakeet, Rufous Woodpecker, and many species of kingfishers.

3. Open nest, generally a scrape, simple platform, or cup.

a. On the ground. Albatrosses, gulls, terns, many gallinaceous and grassland and passerine birds.

b. On a cliff, on the side of a house, in a cave, or in a chimney. Cliff Swallow, House Martin, cave swiftlets (Collocalia), Chimney Swift.

c. On water. Grebes, Black Tern.

d. Fastened to herbaceous vegetation above ground or water. Red-winged Blackbird, Reed Warbler, bishop birds.

e. In a shrub or a tree. Many species, with a great variety of nests, e.g., crude, simple nests (doves), mud cup (Australian Magpie-Lark), woven and suspended nests (orioles), and very small (many hummingbirds) to very large nests (Bald Eagle)

4. Domed nest, particularly common in the tropics.

a. In a tunnel in a bank or in a tree hole. Striated Diamondbird of Australia (Dicaeidae).

b. Of various plant materials, placed on or near ground. Some rails, some pittas (Pittidae).

c. Of mud (Cliff Swallow), of mud and cow dung (Rufous Ovenbird of South America).

d. Of felted plant down or wool. Penduline Tit.

e. Roofed with large leaf. Tailorbirds.

f. Nonwoven, of plant materials bound together with spider or insect silk or fungal filaments (Marasmius). Sunbirds (Nectariniidae), Bushtit, many New World flycatchers.

g. Thatched of dry grass stems or straws. Sparrow weavers (Plocepasser), East African social weavers (Pseudonigrita).

h. Woven of green, flexible plant materials. True weaverbirds (Ploceinae).

5. Compound nest, having two or more nesting compartments, genrally with a communal roof. Palmchat of Hispaniola, Monk Parakeet of Argentina, Buffalo Weaver of sub-Saharan Africa, Sociable Weaver of southwestern Africa.

D. Eggs laid in nest of other species.

1. Parents care for own eggs and young. Great Horned Owl, Chestnut Sparrow.

2. Brood parasites. Most cowbirds, viduine finches, honey-guides (Indicatoridae), many species of cuckoo.

Evolution of Nest Building

To maintain perspective and orientation in a discussion of nest evolution, it is necessary at the outset to keep certain principles in mind. Competition among species has often resulted in great differences in the habitats and nest sites occupied by related species. In turn, differences in the nature of the substrate for the nest has imposed special engineering requirements on nests with regard to materials, form, structure, and placement. Building a nest requires considerable expenditure of energy, and it is common for many birds to make a thousand or more trips to gather the necessary materials. Natural selection may therefore be expected generally to favor anything that tends to economize on effort, so long as undue sacrifice of any crucial advantage of the species is avoided.