BAERENDS, GERARD PIETER

(b. The Hague, Netherlands, 30 March 1916; d. Groningen, Netherlands, 1 September 1999),

ethology, innate releasing mechanism (IRM), heterogeneous summation of stimuli, supernormality, hierarchical structure of behavior, organismic zoology, marine science, overfishing.

Baerends, arguably the most influential pupil of biologist Nikolaas (Niko) Tinbergen, is counted among the founders of ethology. He built a world-renowned research laboratory at Groningen, Netherlands, integrating ecological and physiological themes centered on the behavior of intact organisms in their natural surroundings. Equally important, he was a major mover behind the postwar expansion of marine science in the Netherlands.

Growing Up in The Hague. Gerard Baerends, the only child of Pieter Gerardus Baerends and Adriana Johanna Baerends, née Hulstkamp, grew up in The Hague, where his father was a civil servant. As a youngster, Gerard roamed the countryside and nearby beach, at that time an ideal setting for nature study. As a high school boy he was enthralled by a lecture of Nikolaas Tinbergen from Leiden University, as a result joining a youth movement for nature study and becoming imprinted on biology for life. As an active member of this Nederlandse Jeugdbond voor Natuurstudie (NJN; Netherlands youth association for nature study), he also underwent the intensive coaching of his peers, particularly Luuk Tinbergen. Nikolaas’s younger brother and one year older than Baerends, Luuk was a budding scientist gifted with artistic abilities. This remarkable, emancipatory Dutch youth movement (founded in 1920) encouraged excursions and field studies for young people, including write-ups in their journals. Barring membership to all above the age of twenty-three, it thus broke free from interference by the older generation. As Baerends assessed it in his “Early Ethology” (1991), the NJN had a profound influence on the upcoming scientific generation by promoting independence of thought.

Formative Years in Leiden. An assiduous student at Leiden University, Baerends quickly came under the spell of Nikolaas Tinbergen and in his first year in 1934 was already helping as a “slave” (as volunteers were then called) during his holidays in the field experiments then just starting in the herring gull colony in the nearby dunes. Tinbergen, a brilliant and charismatic teacher, had returned the year before from his 1932–1933 Greenland expedition to take up his duties lecturing on animal behavior and had just initiated intensive six-week ethology practicals. As one of the pioneer generation, Baerends later described the hallmarks of these famous experimental courses for third-year students, the first ethology courses in the world:

At the beginning of the course the students were asked just to observe the behaviour of their animals and record it as carefully as possible, with pencil and paper. Following this introductory period, they were encouraged to start asking questions, and thus to wonder about the behavior they had observed. These questions were then critically discussed and where necessary corrected and refined. Finally, the students were invited to design and carry out experiments for testing their own hypotheses. (1991, p. 11)

Aside from research projects at the Leiden laboratory and local coastal dunes, Tinbergen instituted major studies of behavior by observing insects in the pinewoods and sands of Hulshorst. Staff and students camped out, and in this informal setting of self-discovery, Baerends undertook his PhD study on the provisioning behavior of the digger wasp Ammophila campestris. At the early age of twenty-five, Baerends defended his thesis (published in full in 1941), remarkable for its wealth of detail on case histories and field experiments amassed during seven summers. (He had the help of another of Tinbergen’s students, Jos van Roon, in this enterprise, and the couple married in 1942.) In Tinbergen’s autobiographical Curious Naturalists (1958), he devoted a whole chapter to Baerends’s study, stressing its originality. Conceived as an extension of his own preoccupation with homing, Tinbergen wrote that “the Baerendses went their own way ... and discovered many very remarkable things” (p. 84), somewhat to his initial discomfiture as he had intended the project to concentrate on orientation. Luckily Tinbergen gave his pupil full freedom. In his thesis, Baerends contributed the insight that the organization of behavior could best be conceived as hierarchical in structure. This conception, molded in close collaboration with Tinbergen, became one of the unifying principles of the developing field of ethology and provided a framework for the study of motivation and of the conflicts between opposing tendencies that were to take center stage in the next few decades.

Growing up in The Hague with easy access to the coast, Baerends had an early fascination for the sea, and as a young student was recruited as a volunteer during coastal cruises by Jan Verwey, the director of the Zoological Station at Den Helder. These days at sea made a profound impression and led to a lifelong friendship between the two men. Baerends developed this interest by doing part of his master’s research at the Plymouth Marine Biology Laboratory in England, also undertaking theoretical work in oceanography. Upon completion of his thesis he was promised a position at an oceanographic research institution in the Dutch East Indies. The German occupation of the Netherlands in May 1940 prevented him from taking up this job in the tropics (and indeed delayed the formal defense of his thesis, as Leiden University was closed), but fortunately, Baerends was able to undertake fisheries research near home instead, joining the government research institute at IJmuiden in 1942. During the remainder of the war years, he achieved mastery of the archives and reports bearing on the overfishing problem of North Sea fish stocks. At war’s end Baerends accompanied the fishermen at sea and learned at firsthand their problems. In 1946 he had the opportunity of verifying his opinions with colleagues in the United Kingdom, where the youthful duo of Ray Beverton and Sidney J. Holt had made great analytical strides. His official government report outlining how sustainable yields could be obtained from the North Sea fishery was judged to deserve a wider audience and was republished in translation in the United States.

A New Laboratory. In the immediate postwar years, an optimistic wave of recovery swept over the Netherlands, and the universities too tried hard to make up for the lost years. The professorship in zoology at the University of Groningen had fallen vacant due to the sudden death of the respiratory physiologist Engel Hendrik Hazelhoff in 1945, and when Baerends was approached to fill the post, he jumped at the chance. Baerends stipulated that he would make fieldwork the primary source of inspiration: academic biologists would go outdoors again. Baerends foresaw that despite all their fascination, career opportunities for scientists trained as ethologists would remain limited, and inspired by his taste for applied research in fisheries, he determined to develop both ecological and ethological themes in restructuring the Zoology Laboratory around its tradition of experimental work. His inaugural lecture was thus drawn from his work on the North Sea fisheries, and about half of the forty-three PhD students he eventually trained (ten of them in turn holding professorships) chose ecological themes.

This auspicious start (Baerends was only thirty and achieved his professorship a year before Nikolaas Tinbergen did at Leiden) should not blind one to the fact that Groningen, despite its glorious past, had little to offer at the time. Only five or six students enrolled in biology annually, the staff was tiny, the equipment was antiquated, and his laboratory was an odd amalgamation of decrepit buildings downtown. Baerends rose to the challenge and with characteristic vigor prepared for expansion. The university authorities were impressed and a new laboratory was situated in the botanic gardens in the suburb of Haren, tailored to the new concepts of teaching and research. Innovations included facilities for maintaining birds in captivity, a generous aquarium building for holding and observing fish, and a vibration-free wing designed for sophisticated neurophysiological work. As Baerends stated in his “Field Studies” entry (1981), “a full understanding of the causation, development, function, and evolution of behaviour requires studies of animals living freely in their natural environment, studies of captive animals under semi-natural conditions, and studies of animals and even parts of them in the laboratory” (p. 189). Supporting staff included a professional artist and taxider-mist; carpentry shop; glass blower; photographer; instrument makers, later supplemented with an electronics team; and, of course, animal caretakers. The new teaching labs were spacious and light, and when the laboratory was opened in 1953, it was by far the best in the country.

In these early years Baerends recruited staff by enlisting friends from Leiden. Foremost among them was Luuk Tinbergen (the younger brother of Nikolaas), chosen to fill the first chair in ecology in the country specifically to develop field-based animal ecology. At Leiden in 1946, Luuk had defended his thesis on the ecology of the sparrow hawk in interaction with its avian prey. Observational work in the laboratory was closely bound up with fieldwork of unparalleled intensity, done with the help of a growing body of enthusiastic students captivated by Baerends’s flair as lecturer. The field teams camped together at Hulshorst with Luuk (a direct continuation of the famous prewar camps of Nikolaas and his circle) or on the island Terschelling, where Baerends was able to use a World War II bunker in the dunes as his base of operations in the Herring Gull colony, starting in 1950.

Undertaking the gull work was a watershed event for Baerends and was to occupy him for the rest of his life. Nikolaas Tinbergen accepted a position at Oxford University starting in September 1949, and in a sense Baerends inherited the project he had already been associated with in his student days. Aside from the fact that he was intrinsically interested in the problem of how nesting gulls recognize their eggs, the techniques of field experimentation with dummy eggs appealed to him as a wonderful introduction to ethology for his students. With his more quantitative mind, Baerends felt he could take the study further than Tinbergen had done. The stickleback program that was the hallmark of the original ethology laboratory remained in the capable hands of Jan van Iersel, who became Tinbergen’s successor at Leiden.

International Consolidation of Ethology. Just before Tinbergen’s move to Oxford, an important symposium titled Physiological Mechanisms in Animal Behaviour was convened by the Society for Experimental Biology in Cambridge, England, in 1949. This meeting, initiated by William H. Thorpe and Tinbergen, heralded the emergence of ethology as a discipline and united observation on intact animals; sophisticated physiological laboratory work aimed at revealing central steering mechanisms; and, of course, the emerging theory.

This gathering brought the main prewar proponents of ethology together again for the first time. Konrad Z. Lorenz, only recently returned as a prisoner of war from Russia, met his colleagues again after an absence of ten momentous years. In his contribution (nearly fifty pages but now for the first time in English and available for a broader readership and including his famous “water closet model” for motivation), Lorenz reiterated his approach to the study of innate behavior by the comparative method. By contrast Baerends called into question the Lorenzian view that reactions in the form of “innate motor patterns” in reaction to “releasers” were really free from the influence of learning, thus foreshadowing his later contribution from the gull experiments. Nikolaas Tinbergen expanded on the concept of the hierarchical organization underlying instinctive behavior that he and Baerends had originated, seeking to bridge the gap with the Cambridge neurophysiologists participating at the meeting by drawing in their own work at the lower levels of coordination.

Erich von Holst, a German neurophysiologist of great originality, was prevented by illness from attending. However, he sent a letter (in German), which in its arrogant and assertive tone shocked Baerends and Tinbergen, treading deeply on their sensitivities after the long years of occupation. This dismay threatened to block communication with the group at Wilhelmshaven, where von Holst had assembled a team of brilliant young scientists (under the auspices of the Max Planck Gesellschaft). Baerends therefore persuaded Otto Koehler, the diplomatic doyen of German ethology, to visit von Holst personally on the way home after the Cambridge conference. The resulting invitation by von Holst to meet together in Wilhelmshaven in 1950 led to a reconciliation (von Holst turned out to be a charming host of open disposition) and marked the initiation of regular international ethology conferences. As one of the prime movers behind these conferences in the coming years, Baerends made a major contribution on the international scene.

The new laboratory in Haren was the venue for the International Ethology Conference in 1955, which was notable for bringing participants from North America. Chief among them was Daniel Lehrman, who as enfant terrible had published in 1953 a fundamental critique of the Lorenz theory on instinctive behavior. Baerends (together with van Iersel) had met Lehrman in Montreal in 1954 at the 14th International Congress on Psychology and quickly discovered common ground in highlighting weaknesses of the concepts as originally formulated by Lorenz. (A common interest in bird-watching surfaced as well.) The 1955 meeting at Baerends’s laboratory brought some seventy ethologists together, and appropriately, Lehrman was pictured between Lorenz and Baerends (with Tinbergen immediately behind them) in the official conference photograph. In a conversation many years later, Lehrman emphasized how vital the early contacts fostered by the rather informal ethology conferences had been in leading to a rapprochement and a new synthesis, giving equal emphasis to learned and innate components of behavior.

With his tact, linguistic abilities (fluent German, English, and French, in addition to his native Dutch), and reputation, Baerends was heavily involved in organizing these international gatherings until the early 1970s, when due to the increasing number of “converts” to ethology, they could no longer be convened by invitation. For many in the younger generation, these conferences were testing grounds for new methods and ideas and provided fascinating glimpses into how the leaders of ethology and their disciples interacted. Baerends’s international dimension was strengthened by the meticulous care he devoted to the journal Behaviour— which he founded in 1948, together with Thorpe and Tinbergen—serving as executive editor through more than one hundred volumes; he retired from the board in 1991.

Typically, it was also Baerends who took the lead in assembling seventeen essays in a book titled Function and Evolution in Behaviour (1975), issued to mark the retirement symposium of his teacher. In December 1973 Tin-bergen had just been awarded the Nobel Prize, together with Karl von Frisch and Lorenz, “for their discoveries concerning the organization and elicitation of individual and social behaviour patterns.” Baerends contributed to the book an evaluation of the conflict hypothesis originated by Tinbergen to explain the evolution of displays that was much appreciated by Tinbergen himself, and documented by some reviewers as the most important chapter.

By the early 1950s the teaching of ethology and related subjects at Groningen was far ahead of that at any other university, and new techniques of electrostimulation with miniaturized electrodes augured well for integration with neurophysiology. A major disaster overtook the team at this time, however, with the sudden death of Luuk Tin-bergen in September 1955 (at the age of only thirty-nine and recently promoted to a full professorship). This dynamic ecologist was a behavioral ecologist before the term was coined and was at the time deeply involved in defining his ethological concept of search image on the basis of observing dietary choice in free-living tits feeding their nestlings. Luuk’s colleagues and students at Groningen worked through his unpublished notes and papers, and the result was a two-hundred-page monograph, “The Dynamics of Insect and Bird Populations in Pine Woods” (1960). Although Baerends did his best to replace him, it would take decades to build a new ecology group working with birds in the field and thus linking with the ethologists.

Lasting Contributions to Ethology. Baerends made his mark with a series of papers in the 1950s. Together with his wife Jos, he assembled “An Introduction to the Study of the Ethology of Cichlid Fishes” (1950), a well-illustrated monograph on the behavior of cichlid fish based on aquarium observation and experiment; it inspired research the world over on these tropical fish and their complex parental care. With several of his new students Baerends produced a monumental study, “Ethological Studies on Lebistes reticulatus” (1955), on the guppy, another aquarium study quantifying the interaction between perception (the external stimulus situation) and motivational factors in eliciting behavioral reactions. The insights encapsulated in one of the figures have made this a classic that found its way into many textbooks. In those early years, a colony of black terns nested on a lake behind his house, and together with another student team, Baerends observed pair formation and egg care. In “Observations on the Behaviour of the Black Tern” (1956), Baerends and his collaborators speculated on the phylogenetic origins of the displays, thus complementing Nikolaas Tinbergen’s program on larids, then gathering momentum at Oxford.

Meanwhile, data from the annual campaigns in the herring gull colonies were accumulating at an alarming rate. In the 1960s Baerends faced the dilemma that his duties as director of what had become a large institute (its scientific staff numbered eighteen) with a visiting rate from North America (postdoctorals and PhD students) even exceeding that of Oxford left him little time for reflection. Temporarily, he had also taken on directorship of the Netherlands Institute for Sea Research, with its new quarters on the island of Texel, to guide this legacy of his old friend Verwey through a difficult interim phase. The invitation to spend the academic year 1964–1965 in the congenial setting of the Center for the Advanced Study of the Behavioral Sciences at Stanford University in Stanford, California, came in the nick of time. That year of unremitting analysis laid the basis for “The Herring Gull and Its Egg. Part I” (1970), by Baerends and Rudi H. Drent, the first of two monographs drawing the gull work together. In a book chapter titled “Stimulus Selection” (1973), Baerends and Jaap Pieter Kruijt also presented the essence of the finding that the external features of the egg combined in a quantitative manner to provide a compound stimulus value, thereby substantiating the concept of “heterogeneous summation.”

Alfred Seitz had pioneered this concept, contending that the animal reacted to the sum total of stimuli, and he reasoned that it must be feasible to measure the stimulus

value of an object in all possible sensory modalities (such as shape, color, texture, and size) to discover how these were combined by the animal. The challenge was to find a method to measure the contribution of each of these features of the stimulus, and Baerends perfected the technique of quantifying the response of a parent gull when presented with two competing egg models on the nest rim. Which would be rolled in first? This work, drawing on nearly one thousand field trials integrated in a gigantic titration exercise, elucidated why under certain circumstances the parent gull returning to the nest might prefer to retrieve an egg deviating strongly from normal, with exaggerated features (in color, stippling, and size) representing the “super-normal” stimulus (i.e., the stimulus value surpassing the value for the real egg).

A distraction, albeit a rewarding one, was posed by an invitation to provide scientific backing to the talented cinematographer Bert Haanstra to produce a film illustrating the status of ethological understanding at that time. The finished product, Bij de Beesten af (1972; with the English-language version, Ape and Superape, aptly following in 1973 when the Nobel Prize awarded to von Frisch, Lorenz, and Tinbergen had drawn attention to the subject), reflected Baerends’s conviction that by juxtaposition, the viewer would readily appreciate that human behavior, despite all the trappings of modern society, was but a step away from that of the wild creatures depicted. The film, viewed by millions the world over, sensitized the public to the sociobiology debate that followed, leading to a growing awareness to consider the message ethology conveyed for the human sciences. Baerends wrote a richly illustrated chapter, “De mens als produkt van de evolutie” (1972; Mankind as a product of evolution), in the popular book, Bij de Beesten af, that appeared simultaneously with the film. He also supervised the editing of excerpts of the movie for educational purposes, garnishing the many lectures he gave to students enrolled in psychology with the motto “see for yourself.”

In the late 1960s a wave of democratization swept over Dutch universities. Although very much a believer in decisions by consensus, Baerends felt frustrated by the new layers of bureaucratic interference from above. In this depressing atmosphere Baerends received an unexpected offer from the Max Planck Gesellschaft, asking if he would succeed Lorenz (who was due to retire) at the Seewiesen Institute near Munich. On the one hand, this was a tempting offer, as Baerends had a high regard for not only the scientists at Seewiesen, but also for Jürgen Aschoff’s group at nearby Erling-Andechs. On the other hand, he feared that acceptance would put him still further away from hands-on research. Opportunely, a counterproposal was launched by the Royal Netherlands Academy of Arts Sciences, Groningen University, and the Ministry of Education and Sciences. This alternative offered a personal chair, allowing Baerends to continue his work in his own surroundings abetted by a small research group, completely free of bureaucratic interference. Accepting the latter offer, Baerends devoted the years from 1973 until his official retirement in 1986 to publishing the remainder of his backlog of unfinished manuscripts, which would amount to nearly half of his lifetime writing output. Most notably, it included “The Herring Gull and Its Egg. Part 2” (1982), the second monograph on the herring gull work, coauthored with Rudi Drent. During this time he also supervised an additional seventeen PhD students. Furthermore, Baerends headed out into the gull colony again, engaging in research that incorporated the insights generated by game theory made tangible by Maynard Smith and his group in England. Baerends also found the time to put the finishing touches on a monograph, “The Morphogenesis of the Behaviour of the Domestic Cat” (1979). It concerned the development of play behavior in domestic kittens, which had been a long-term project of his wife, Jos, the coauthor.

Baerends had an enormous impact on the rise of ethology worldwide and for decades was “Mr. Ethology” in the Netherlands. It is no coincidence that the four chair holders in animal behavior in the country in 2006 were all trained in Groningen. Elected to the Royal Netherlands Academy of Sciences (1958), the Hollandsche Maatschappij van Wetenschappen in Haarlem (1968), knighted by Queen Juliana, and receiving an honorary doctorate in 1965 from the University of Rennes in France, many honors came his way. Baerends was gratified by elevation to Fellow of the American Association for the Advancement of Science in 1980, the citation reading “for foremost work in ethology and for development of the world famous institute at Groningen.” Perhaps the most fitting epitaph is a quote from the closing pages of his monumental “The Herring Gull and Its Egg,” where the determined empiricist surfaces: “Properly documented descriptions will be of more use for bridging the gap between ethological analysis and behavioural physiology than attempts at neat classifications in theoretically based categories” (Baerends and Drent, 1982, p. 357).

BIBLIOGRAPHY

A bibliography of Baerends’s published writings is in “Proceedings of the G. P. Baerends Symposium.” Netherlands Journal of Zoology 35 (1982): 1–376. A significant quantity of unpublished documents related to Baerends is held at the University of Groningen, Central Archives, Groningen.

WORKS BY BAERENDS

“Fortpflanzungsverhalten und Orientierung der Grabwespe Ammophila campestris Jur.” Tijdschrift voor Entomologie 84 (1941): 68–275. His PhD thesis.

“De rationele exploitatie van den zeevischstand.” Ministerie van Landbouw Visserij & Voedsel,Mededelingen 36 (1947): 1–99. Translated into English as “The Rational Exploitation of the Sea Fisheries with Particular Reference to the Fish Stock of the North Sea.” U.S. Department of the Interior Fish and Wildlife Service, Special Science Report: Fisheries 13 (1950): 1–102.

“Specialization in Organs and Movements with a Releasing Function.” Symposium Society for Experimental Biology 4 (1950): 337–360.

With Jos M. Baerends-van Roon. “An Introduction to the Study of the Ethology of Cichlid Fishes.” Behaviour Supplement 1 (1950): 1–242.

With R. Brouwer and H. T. Waterbolk. “Ethological Studies on Lebistes reticulatus (Peters). I. An Analysis of the Male Courtship Pattern.” Behaviour 8 (1955): 249–334.

With B. Baggerman, H. S. Heikens, and J. H. Mook. “Observations on the Behaviour of the Black Tern, Chlidonias n. niger (L.) in the Breeding Area.” Ardea 44 (1956): 1–71.

With Rudi H. Drent, et al., eds. “The Herring Gull and Its Egg. Part I.” Behaviour Supplement 17 (1970): 1–312.

“De mens als produkt van de evolutie.” In Bij de Beesten Af, edited by Anton Koolhaas, et al. Amsterdam: Uitgeverij Ploegsma, 1972.

With Jaap Pieter Kruijt. “Stimulus Selection.” In Constraints of Learning, edited by Robert A. Hinde and J. Stevenson-Hinde. London and New York: Academic Press, 1973.

“An Evaluation of the Conflict Hypothesis as an Explanatory Principle for the Evolution of Displays.” In Function and Evolution of Behaviour, edited by Gerard P. Baerends, Colin Beer, and Aubrey Manning. Oxford: Clarendon Press, 1975. With Colin Beer and Aubrey Manning, eds. Function and Evolution in Behaviour. Oxford: Clarendon Press, 1975.

With Jos M. Baerends-van Roon. The Morphogenesis of the Behaviour of the Domestic Cat, with a Special Emphasis on the Development of Prey-Catching. Verhandelingen Koninklijke Nederlandse Akademie van Wetenschappen. Amsterdam and New York: North-Holland Publishing, 1979.

“Field Studies.” In The Oxford Companion to Animal Behaviour, edited by David McFarland. Oxford: Oxford University Press, 1981.

With Rudi H. Drent. “The Herring Gull and Its Egg. Part II.” Behaviour 82 (1982): 1–415.

“Two Pillars of Wisdom.” In Studying Animal Behaviour: Autobiographies of the Founders, edited by David A. Dewsbury. Chicago: University of Chicago Press, 1985.

“Early Ethology: Growing from Dutch Roots.” In The Tinbergen Legacy, edited by Marian S. Dawkins, Tim R. Halliday, and Richard Dawkins. London and New York: Chapman and Hall, 1991.

OTHER SOURCES

Dewsbury, Donald A. “Americans in Europe: The Role of Travel in the Spread of European Ethology after World War II.” Animal Behaviour 49 (1995): 1649–1663.

Drent, Rudi H. “Dropping the Pilot: Gerard Baerends, 1916–1999.”Ardea 88 (2000): 113–118.

Hinde, Robert A. Animal Behaviour: A Synthesis of Ethology and Comparative Psychology. New York: McGraw-Hill, 1966.

———.Ethology, Its Nature and Relations with Other Sciences. Oxford and New York: Oxford University Press, 1982.

Kruuk, Hans. Niko’s Nature, a Life of Niko Tinbergen and His Science of Animal Behaviour. Oxford and New York: Oxford University Press, 2003.

Lehrman, Daniel S. “A Critique of Konrad Lorenz’s Theory of Instinctive Behavior.” Quarterly Review of Biology 28 (1953): 337–363.

Lorenz, Konrad Z. “The Comparative Method in Studying Innate Behaviour Patterns.” Symposium of the Society for Experimental Biology4 (1950): 221–268.

“Proceedings of the G. P. Baerends Symposium.” Netherlands Journal of Zoology 35 (1982): 1–376. Includes a biographical sketch.

Slater, Peter J. B. Review of Function and Evolution in Behaviour, edited by Gerard P. Baerends, Colin Beer, and Aubrey Manning. Animal Behaviour24 (1976): 720.

Thorpe, William H. The Origins and Rise of Ethology: The Science of the Natural Behavior of Animals. London: Heinemann Educational, 1979.

Tinbergen, Luuk. “The Dynamics of Insect and Bird Populations in Pine Woods.” Archives neérlandaises de Zoologie 13 (1960): 259–473.

Tinbergen, Nikolaas. “The Hierarchical Organization of Nervous Mechanisms underlying Instinctive Behaviour.” Symposium of the Society for Experimental Biology4 (1950): 305–312.

———.Curious Naturalists. London: Country Life, 1958. Baerends’s PhD dissertation is discussed in chapter 6.

Van Hooff, Jan A. R. A. M. “Levensbericht Gerard Pieter Baerends.” In Levensberichten en Herdenkingen 2005. Amsterdam: Koninklijke Nederlandse Akademie van Wetenschappen, 2005.

Verwey, Jan. “In Memoriam Luuk Tinbergen.” Ardea 43 (1955): 293–308. A necrology of the younger brother of Nikolaas, with a bibliography.

Voous, Karel H. “Baerends, Gerardus Pieter.” In In de ban van Vogels, Ornithologisch Biografisch Woordenboek van Nederland. Utrecht, Netherlands: Scheffers, 1995.

R. H. Drent