The Wisdom of Birds

1. Ornithology Evolved from Folklore and Superstition to Empirical Science

Let it not suffice to be book-learned, to read what others have written and to take on trust more falsehood than truth, but let us ourselves examine things as we have the opportunity, and converse with Nature as well as with books...

Medieval views. For centuries, understanding birds was intertwined with symbolism, mythology, and superstition, often drawing from ancient texts like Aristotle and medieval Bestiaries. Birds were seen as portents, their behaviors imbued with moral or religious significance, leading to fantastical beliefs like geese growing from barnacles or kingfishers acting as weathercocks.

Shift to observation. The 1600s marked a turning point, driven by figures like Francis Bacon and Sir Thomas Browne, who championed observation and experiment over unquestioning acceptance of tradition. Browne's simple test debunking the kingfisher myth exemplified this new empirical approach, laying the groundwork for modern natural history.

Early encyclopedias. While Renaissance encyclopedias by Gessner and Aldrovandi still mixed fact with folklore, later works like Jonston's began to focus more purely on natural history. This shift was partly fueled by the influx of new species from the New World, which lacked existing symbolic associations, forcing naturalists to rely on description.

2. John Ray Revolutionized Bird Study with Classification and Natural Theology

After a long and considerable investigation, no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed.

A new system. John Ray, a central figure in England's scientific revolution, transformed ornithology by rejecting folklore and focusing on verifiable facts and systematic classification. His collaboration with Francis Willughby resulted in the Ornithology (1676/1678), which provided the first functional definition of a species based on reproductive isolation.

Physico-theology. Ray's philosophical framework, articulated in The Wisdom of God (1691), proposed that the natural world's intricate design revealed God's wisdom. This "physico-theology" encouraged direct observation to understand the adaptive fit between organisms and their environment, identifying key biological questions that would drive future research.

Dual legacy. Ray's contributions were twofold: the Ornithology established the foundation for bird taxonomy and systematics, while The Wisdom of God inspired the field study of birds and the investigation of their ecology and behavior. He was the first to approach bird study with a truly scientific mindset, seeking certainty of knowledge.

3. Unlocking the Mysteries of Reproduction: From Ethereal Semen to Sperm Storage

All animals come from eggs, as well as those called viviparous as oviparous: For the females of viviparous have eggs within them, though they do not bring them forth.

Early confusion. The process of reproduction, particularly how male semen contributed to the egg, was a profound mystery. Early naturalists like Fabricius and Harvey were perplexed by the apparent disappearance of semen after copulation and the identical appearance of fertile and infertile eggs, leading to ideas of "ethereal" action or mistaken beliefs like chalazae being semen.

Ex ovo omnia. William Harvey's meticulous dissections led him to conclude that the egg was the primary element in reproduction, famously stating "all things from the egg." Malpighi's later discovery of a nucleus in the egg's germinal disc further supported the importance of the female contribution.

Sperm and storage. The discovery of "animalcules" (spermatozoa) in semen by Leeuwenhoek in the late 1600s, though initially met with skepticism by Ray due to their sheer number, eventually led to the understanding that a single sperm fuses with the egg. The puzzle of prolonged fertility in hens was finally solved with the discovery of sperm storage tubules in the female reproductive tract, allowing sequential fertilization.

4. Bird Behavior: A Complex Interplay of Instinct and Intelligence

To the uninitiated it is often surprising (even incredible) that a bird does not recognise conspecifics purely ‘instinctively’ in all situations...

Nature or nurture. The debate over whether bird behaviors are innate ("instinctive") or learned has a long history. Early observations noted young birds following humans (imprinting) and hand-reared birds building typical nests, suggesting both innate tendencies and learning capacity.

Experimental ethology. Douglas Spalding's pioneering experiments in the 1870s, particularly with hooded chicks, provided the first scientific evidence for sensitive periods in imprinting and the existence of innate predispositions. Later ethologists like Konrad Lorenz and Niko Tinbergen built on this, professionalizing the study of animal behavior and defining key questions about its causes, development, evolution, and function.

Cognitive abilities. While Descartes viewed animals as soulless automata, Ray argued they possessed a "lower degree of reason." Modern research confirms birds exhibit sophisticated cognitive skills, far beyond simple instinct. Examples include:

• Imprinting (filial and sexual)
• Song learning and mimicry
• Tool use (New Caledonian crows, woodpecker finches)
• Spatial memory and planning (corvids)
• Individual recognition and personality differences

5. Migration: Dispelling Myths and Discovering Internal Navigation

But how come they to be directed to the same place yearly, though sometimes but a little island... which they could not possibly see, and so it could have no influence upon them that way?

Ancient knowledge and persistent myths. While ancient Greeks like Aristotle and Anacreon knew some birds migrated, the disappearance of others, particularly swallows and swifts, fueled the persistent myth of hibernation or submersion under water, perpetuated by figures like Olaus Magnus and even initially accepted by Ray and Linnaeus.

Evidence emerges. Sightings of birds far out at sea by explorers and sailors provided compelling evidence against hibernation. Early attempts at marking birds, like Frisch's colored threads on swallows, further supported migration and the ability of birds to return to the same places.

Unlocking the mechanism. The observation of "migratory restlessness" (Zugunruhe) in captive birds provided the first clues to an internal drive. Experimental studies by Gwinner and Emlen demonstrated that the duration and direction of this restlessness were linked to migratory distance and direction, suggesting an innate program. Perdeck's displacement experiments with starlings revealed that while young birds rely on an innate compass, adults use experience for true navigation, combining internal clocks, celestial cues, magnetic fields, and even smell.

6. Light Orchestrates the Bird's Annual Cycle of Breeding and Migration

Sweetness of voice and melody of song are qualities, which in birds are partly natural, partly acquired.

Timing is crucial. Birds' lives are governed by a precise annual cycle of breeding, moulting, and migration. Getting the timing right is critical for reproductive success, ensuring young hatch when food is abundant.

Bird-keepers' secret. For centuries, bird-keepers used techniques like "stopping" (manipulating light levels) to induce premature moult and song in captive birds, effectively shifting their annual cycle, though they didn't understand the underlying mechanism.

Photoperiodism discovered. The link between light and seasonal events was scientifically established in the early 20th century. Studies on plants revealed that day length (photoperiod) triggered flowering. Bill Rowan's experiments with juncos demonstrated that increasing day length stimulated gonad growth and migratory readiness in birds, regardless of temperature.

Proximate vs. Ultimate. Jacques Benoit showed that birds perceive light through their skull, directly affecting the brain and hormone release. This led to the understanding of proximate factors (how the bird knows when to breed/migrate - e.g., light, hormones) and ultimate factors (why it breeds/migrates at that time - e.g., food availability, survival). David Lack later championed the study of these ultimate factors, linking breeding seasons to peak food resources for young.

7. Territory: A Fundamental Concept in Bird Life, Long Observed but Late Formalized

It is proper to this bird at his first coming (saith Olina) to occupy or seize upon one place as its freehold, into which it will not admit any other nightingale but its mate.

Ancient observations. The idea that birds defend a specific area has roots in antiquity, noted by Aristotle for eagles and ravens, and later by bird-catchers for nightingales ("a stone's throw") and swan wardens. These early observers recognized that defense occurred against members of the same species and was often linked to food or nesting.

Formalization by Howard. Despite these scattered observations, the concept of territory was not formally developed until Eliot Howard's work in the early 20th century. Through meticulous field studies of warblers, Howard argued that territory was a fundamental aspect of bird life, with males competing for space rather than directly for females.

Debate over function. Howard's ideas sparked significant debate among ornithologists, including Margaret Morse Nice and David Lack. Key questions revolved around the function of territory:

• Does it ensure a food supply for young?
• Does it facilitate pair formation and maintenance?
• Does it regulate population density (a controversial idea championed by Moffat and Wynne-Edwards but refuted by Lack)?

Defining territory. The lack of a clear definition fueled controversy. Ernst Mayr and Gladwyn Noble later helped clarify the concept, defining territory broadly as "any defended area" and proposing different types based on the activities they encompassed (all-purpose, mating, nesting).

8. Birdsong: More Than Music, a Product of Learning and Sexual Selection

The natural tones of birds ... express the various modifications of passion... The females are much more silent than the males...

Learning vs. Instinct. Early bird-keepers knew many species could learn tunes, suggesting song was not purely innate. Pernau and Barrington conducted experiments showing that young birds adopted the songs of their foster parents, emphasizing the role of learning, though later work showed innate components also exist.

The vocal apparatus. The source of birdsong was debated, initially thought to be the tongue (leading to cruel mutilation practices), but later correctly located in the syrinx, the bird's voice box. Studies revealed the syrinx's complex structure and how air passing over membranes or tissues creates sound.

Function of song. The purpose of birdsong has long fascinated observers. Ideas ranged from emotional expression or pleasing humans to entertaining mates. Darwin proposed song evolved through sexual selection, serving two main functions:

• Competition between males (as observed by bird-catchers)
• Attracting and captivating females (female choice)

Modern insights. Research using sound spectrographs (Thorpe) and experiments manipulating song complexity (Kroodsma, Vallet) confirmed that females prefer males with larger repertoires or specific "sexy syllables." Studies also demonstrated song's role as a territorial "keep-out" signal (Krebs). The link between song, hormones (testosterone), and brain structure has revealed remarkable neurobiological plasticity in songbirds.

9. Sex and Conflict: Unveiling Promiscuity and Sperm Competition

It is shown by various facts, given hereafter, and by the results fairly attributable to sexual selection, that the female, though comparatively passive, generally exerts some choice and accepts one male in preference to the others.

Sexual anomalies. Throughout history, unusual sexual phenomena like hermaphrodites ("half-siders"), sex change (females adopting male traits), and virgin birth (parthenogenesis) have puzzled and sometimes frightened observers, leading to superstitions like egg-laying cockerels hatching basilisks.

Gonads and hormones. Early naturalists noted the dramatic effects of castration on appearance and behavior. Later research revealed that sex determination is genetic (ZW chromosomes in birds, with the female being the heterogametic sex) and that sex hormones (oestrogen in females, absence of oestrogen in males) play a crucial role in developing secondary sexual characteristics. Disease affecting ovaries can cause females to adopt male traits.

Darwin's blind spot. Despite evidence from domestic animals and observations of "thief pigeons" luring females, Darwin largely denied female promiscuity, assuming females were coy and monogamous after pairing. This limited his understanding of sexual selection, focusing primarily on male competition for mates.

Sperm competition. The recognition that females are often sexually promiscuous led to the concept of sperm competition (Parker), where sperm from multiple males compete to fertilize a female's eggs. This provided a framework to explain previously puzzling traits:

• Relatively large testes in males (producing more sperm)
• Frequent copulation and mate guarding by males
• The evolution of penises in some species

10. Life Histories: The Adaptive Balance Between Reproduction and Longevity

For they being kept only for profit, and within a few years ... becoming unfit for generation, who is there that without all hope of gain will keep them only to make experiment how long they will live?

Early longevity estimates. Ancient and early modern naturalists recorded maximum lifespans for captive birds, noting differences between species and signs of senescence. Aristotle speculated that larger animals lived longer and that excessive reproduction shortened life, ideas echoed by Ray and others.

Cost of reproduction. The belief that reproduction is costly to an individual's health and lifespan was supported by observations of castrated animals living longer and hens becoming "effete" after laying many eggs. This suggested a trade-off between investing energy in breeding now versus surviving to breed later.

Average lifespan and population dynamics. Reliable estimates of average lifespan in wild birds became possible only with the advent of bird ringing in the early 20th century (Burkitt, Nice, Lack). David Lack used this data to understand bird population ecology, proposing that numbers were regulated by density-dependent factors like food availability, balancing birth and death rates.

Life-history strategies. Lack's work, and subsequent studies, revealed that species exhibit different "life-history strategies" – suites of traits like clutch size, age at maturity, and lifespan – that are adapted to their specific environments. For example, short-lived species like the zebra finch invest heavily in rapid reproduction, while long-lived species like albatrosses reproduce slowly and conserve energy, maximizing lifetime reproductive success in different ecological contexts.

Last updated: May 14, 2025