MAS Birders’ Certificate Program
I. Introduction
Flight flocks of European Starlings roil like a column of smoke, a symphony or a spirit. While this coordination in collective motion has delighted people for millennia it still defies scientific explanation.
“ The ancient Romans had their explanation: Gods, they believed, hinted at their intentions in the way birds flew. Scientists of the early 20th century, perhaps almost as credulous, groped for such mysterious and even mystical concepts as “natural telepathy” or “group soul”.4
Despite the intrigue flocks hold there is no apparent consensus on that aspect of their behavior which most fascinates - “...how they maintain cohesion, directional motion while making rapid changes in direction.”8 while in a flight flock. Birds in a tight flight flock change direction and speed together in what has been called a “manoevre wave”. “...The propagation of this ‘manoeuvre wave’ begins relatively slowly but reaches mean speeds three times higher than would be possible if birds were simply reacting [retinally] to their immediate neighbours.”10 ? How does a change in direction and speed of a flying flock get communicated to each individual within the flock in the time frame required for them to avoid crashing into each other? This then is the “authentic question”35 this paper is to address.
As background, the paper provides a survey of the current literature describing why and how birds flock in general. For the most part, research on “why” birds flock report the Darwinian cost/benefit of flocking behavior. These articles also provide a solid explanation for where and when flocks may be found. The “how” articles - how flock cohesion is maintained in flight etc. - are mostly focused on the temporal and spatial requirements that must be met by birds in the coordination of a flight flock. Theories on how birds physically collect and almost instantaneously act on the data necessary to meet those requirements are rare. Those that have been suggested either have been disproved or remain unproven.
After the survey of current literature the paper offers two speculations which, if proven correct, might explain at least some of the mystery. The first speculation is that the intent of a flight flock is established by and communicated through a social hierarchy with leaders. The second speculation is that birds are able to perceive electromagnetic energy diffracted or refracted by the flight feathers of fellow flock members and that the “light” carries the information necessary to maintain separation; to align each bird to the flight flock direction and speed and to keep the flock together.
Finally, the paper outlines some of the issues pertaining to these speculations and how they might be tested.
II. What is a flock?
Cornell’s All About Birds doesn’t list “flock” or flocking behavior in its glossary possibly because it is considered a quaint rather than scientific designation24. Merriam Webster dictionary’s definition is “a group of animals (as birds or sheep) assembled or herded together.” A number of very different kinds of bird and animal aggregations are referred to as“flocks” in the scientific literature and popular press. Bird groups that might be called flocks include those that roost (settling down to rest or sleep) together. In about 13% of species roosting includes colonial breeding9, the colonies too being considered a flock. Birds which form loose feeding aggregations might be considered a flock even where birds arrive separately to join other individuals of their species and other species at a food source. Some flocks arrive and depart together while foraging for food. Some birds live in permanent groups year round that might be called flocks while others join temporary aggregations for a migration, whether short-distance or long distance. It is fair to say then that the term “flock” is very general. And the literature presents a spectrum of thought on how bird flocks are organized. It appears that the preponderance of the existing literature questions whether flocks are organized at all.
At one end of the spectrum are those whose research is driven by the hypothesis that flocks are an expression of “self organized behavior in a social living group”27 - essentially no structure at all - purely democratic (which this paper will call the democratic school). The computer simulators generally favor this explanation for flock behavior. “This approach assumes a flock is simply the result of the interaction between the behaviors of individual birds. To simulate a flock we simulate the behavior of an individual bird (or at least that portion of the bird’s behavior that allows it to participate in a flock). To support this behavioral “control structure” we must also simulate portions of the bird’s perceptual mechanisms and aspects of the physics of aerodynamic flight. If this simulated bird model has the correct flock-member behavior, all that should be required to create a simulated flock is to create some instances of the simulated bird model and allow them to interact.”7 Pure democracy. Each bird acts autonomously but a flock results.
In the middle of the spectrum of thought are those who assume flocks adopt different social structures to fit different circumstances.31 For instance it is hypothesized that “collective opinion formation” can simply be generated by the presence of, for instance, the opportunity to feed when all birds need to feed. A flight flocks “global opinion shift”27 to land versus continuing to fly is much like a physical phase change from water to ice and that the phase change occurs when the weight of consensus reaches the critical point, however vaguely defined (no apparent effort is made to describe how that consensus is reached or communicated).
The other end of the spectrum might be characterized by those who believe that, in some species at least, flight flocks exhibit a distinct social order such as a pecking order, or dominance hierarchy.
As mentioned, the predominant view in the literature is that flight flocks are purely democratic13 22 23- that if there are leaders they are fleeting and temporary - taking their turn at the front and the burden of air resistance, for instance. “When flocks are not under attack, but instead leaving a roost site to go to a feeding area, they may also swerve back and forth apparently aimlessly, because random movements by single individuals can easily generate changes in direction. However, eventually a sort of consensus will develop based on the motivation of the majority of the flock members,and the the flock will fly off to its destination in a fairly direct manner.”11
Many researchers are pursuing the idea that flight flocks exemplify “self organized behavior in social living groups”29 . They contend that “A self-organization process can be defined as the spontaneous emergence of large-scale structure out of local interactions between the system’s subunits. Moreover, the rules specifying interactions among the system’s components are executed, using only local information, without reference to the “global” pattern (Bonabeau et al. 1997). The distributed organization implies that no internal or external agents supervising the process and that the collective pattern is not explicitly coded at the individual level. The emerging structures are in essence more complex than the addition of each agent’s contribution”.29
A prominent subset of this school of thought is a large body of engineering literature on how to create bird flock computer simulations16 - boids14. Some authors suggest that creating these simulations inform our understanding of aspects of flock behavior by presupposing that computer simulation is analogous in a meaningful way to actual bird behaviour. For instance, some computer simulations would indicate that flock behaviour can be generated by simply applying three or four simple behavioral rules to each boid in the flock (stay close to the center of the flock, don’t get too close to a neighbor and assume the average vector as the group) and starting them off from nearby but separated points in a common direction. The authors further suggest that these interaction rules may be adapted in a context dependent way such as in the case of impending danger where birds may flock more closely.29 But other than these simple rules, no other criteria need be applied for a computerized flight flock to form and be sustained. The resulting “boid” flock behavior looks a little like real bird flock behavior, at least enough for cartoons. In these simulations bird flocks act democratically - that they are the sole product of self organized collective motion of individual boids acting alone.
To make the simulations more realistic, modelers have been compelled to add an “internal variable12” to their models of individual bird behavior. These internal variables are intended to simulate an intent to eat, for example, or an intent to stay a certain distance from predators. The literature seems not to notice that, because the computer programmers assign the same intent characteristics to each boid in the flock, they have essentially assigned that intent to the flock as a whole. For instance the model will have a variable that is the intent to land, internal variable G (such that 012
It is certainly reasonable to infer that a flight flock that landed in a feeding area and began to feed had the intent to feed as long as it is recognised that two “a miracle happens here” concepts are embedded in the inference.
The first presumption is that a flock as a quanta is capable of forming intent rather than being assigned it by a researcher. How is a consensus reached within a flock, sustained and communicated within a flock? “...a key benefit [of flocking] lies in the integration of partial knowledge of the environment at the group level.”28
The second presumption is that flocks can integrate all its members knowledge at the flock level in a way that can be acted on within the physical requirements of the observed data.
The work of the “democratic” school has stimulated much constructive thought. Much bird group behavior (e.g. feeding aggregations) appears to conform to the democratic school’s concept of how flocks work. However the democratic school articles all seem to presume (a priori) a definition of flock without stating it even though, as we have seen, there are many bird group behaviors under that general heading. Though flight flocks are the focus of most of the democracy school’s work, the articles don’t provide any information to help us delineate a flight flock in the field - define the quanta (one flock). Their work doesn’t suggest when a simple gang of flying birds become a flock. Rather than gauge the Darwinian cost/benefit for an individual bird to become part of a flight flock, they presume its membership. They define the flock in question implicitly by assigning characteristics to a group of birds rather than describing the behavior of actual birds in a flock in flight. In other words, none of the surveyed articles describe a rigorous definition of “flight flock” with its particular challenges. But, surely, defining a “flight flock” precisely is a requirement if one is to assert that a flock as a discrete entity can form and sustain a collective intent however obvious. And, indeed, none of the articles surveyed describe how flight flocks establish their intent as a flock. The democratic school runs the risk of being internally inconsistent at least semantically. By dictating an intent to the “boids” and the “boid” flock the computer simulators seem to have undermined the concept that the flock is a democracy. They describe a totalitarian state with them being the dictator.
If one wants to carefully observe, describe and explain “flight flocks” specifically, a refined definition is called for.
First some criteria against which to measure the usefulness of any working definition. The proposed definition should provide a frame of reference in which at least the following questions can be addressed objectively with some precision and accuracy.
What is one flock(the quanta)?
When does a simple gang of birds become a flight flock?
What is the consequence to an individual bird from becoming part of a flock?
At what point does a bird functionally become part of a flock?
“Many birds flock, of course. But only a relative handful really fly together........:namely, highly organized lines or clusters.”4
Consider the following as a working definition of “flight flock” for the purpose of understanding flight flocks.
It should be noticed that the proposed definition defines flight flock on the basis of how it is organized socially and coordinated in its behavior.
Possibly the task of defining “flight flock” is easier for those species exhibiting flocking behavior similar to European starlings, though of course not all flocking birds do, so maybe the definition isn’t generalizable. But lets start with pigeons in the hope that some generalizable variables are teased out by the analysis.
In a group of flying birds (e.g. pigeons and possibly starlings) that exhibits a strong social order, such as a dominance hierarchy, each bird has a place in the hierarchy which can be described objectively. You can attach a GPS to each individual bird and map its flight. The individual bird is a clear “quanta” to which measurable characteristics can be attributed. Because of its clearly delineated dominance hierarchy a flight flock of pigeons is also a discrete quanta (birds are either in the hierarchy or not). Each flock has unique characteristics that can be observed and measured and assigned to that flock as a quanta. (The fact that flight flocks of other species may or may not be as coherent may be illuminating in and of itself.) We can learn if the birds and the flock exhibit an intent because it can be described by being measured, as described below.
Individual birds forego some of their autonomy in order to follow the dictates of the flock and their role in it. Individual birds neither join the flock nor work to maintain the social order of the flock without first having accepted the shared intent of the flock or at least a deference to sharing a collective intent. (see section IV, Why Birds Flock). At that point the individual bird is no longer a totally independent actor but now takes on the collective motion and other behaviours of the flock. As we will see, each bird presumably has a vested interest in maintaining its place in the hierarchy of the flock and a vested interest in maintaining the dominance hierarchy and flock as a whole. It is this shared intent and the social order which together provide the cohesive, established channels of communication necessary for instantaneous, simultaneous collective motion which is the key characteristic of a flying flock. This definition of “flying flock” facilitates the observation and quantification of many discrete variables that may help us understand the behavior of flocking birds and to compare those behaviors between flocks of different species.
Obviously it must hold that flocks in fact exhibit some form of social order if this definition of “flying flock” is to work. I have dragooned a Nature article7 into the service of this argument, though defining a flock was not the original purpose of that article.
Nagy, Akos, Biro and Vicsek17 provide a carefully wrought argument that, at least among the pigeons they studied, there is a strong dominance hierarchy within the flock. The authors established that “...birds tended to copy consistently the directional behaviour of particular individuals, while being copied in their orientational choices by others” which extended over multiple flights. Birds higher in the hierarchy were more influential in determining the direction of the flock’s movement. They expected, and found, that individuals near the front of the group were responsible for the majority of directional decisions. “Interestingly, beside the front-back distinction between leaders and followers, we also found evidence of a left-right effect. During homing, the more time a bird spent behind a particular partner, the more likely it was to be flying to that partner’s right (and would thus have been perceiving it predominantly through its left eye; Table 1). Birds visual systems are known to be lateralized, with a superiority of the left brain hemisphere (which receives input contralaterally, from the right eye) in large-scale spatial tasks, and a right-hemispheric (left-eye) specialization for social input (such as individual recognition).” They go on to show that those birds that were better getting home (flew more directly) when alone were also the ones who had leadership positions in a flock’s homing flights. They pointed out that where “most flights produced a robust hierarchical network” and that “... leadership may be related to individual navigational efficiency”, they conclude that their “...quantitative results reveal a delicate arrangement of these dynamic leader-follower relations into a hierarchical network comprising a spectrum in levels of leadership; a sophisticated system that may, from an evolutionary perspective bring benefits to individual group members over, for example a single-leader scenario, or an ancestrally (presumably) egalitarian collective.” Nagy et al observations were based on a flock of nine pigeons. Whether such a hierarchy will be found in all flying flocks is yet to be established or refuted.
If the possibility of “social order” in flight flocks has been established, then we too have to struggle with the issue of “shared intent” if we are to be able to use our working definition of “flight flock”.
“Shared intent” is easier to establish for the hierarchy school than for the democratic school. In a dominance hierarchy the question of shared intent is obviated by the existence of a leader and a social structure. The sole function of both within a flight flock is explicitly to maintain cohesion (shared intent). The leader embodies the intent of the flock. The membership demonstrates that it shares the flock’s and leaders intent by paying the price of foregoing their individual autonomy in order to join the flock in the first place. There is clear measurable evidence of shared intent, though it also remains for the hierarchical school to deduce what that intent is and how to characterise it. Is there is a short migration flock intent, a long migration flock intent, a feeding intent etc. and can we measure the relative frequency of each by species which flying flock behavior exhibits that intent? Why do birds flock, after all?
III. Why do birds flock?
Scientists have deduced that flocking behavior is often overdetermined - that it may have many advantages as well as costs rather than just one. They suggest that individual birds have a survival benefit from flocking that derives from protection from predators, more success in finding food, energy efficient flying, better mating opportunities, and communal territorial defence etc. For flying flocks the general theme is “....what a collectively moving animal group [is] trying to maximize is not order, but response.”18 whether it be to predators, food sources etc. Even so, order may serve as a major variable in the flock’s ability to respond. The importance of social structure might be inferred by the fact that the flock’s cohesion “...depends on topological rather than metric distance. In other words, two birds 5m away interact as strongly as 1m away, provided they are nearest neighbors, topologically.”18 Social order provides efficient channels of rapid communication and therefore ability to respond to the environment in a way for the group to fulfill its collective intent best.
Birds within a flock are more protected from predators. The flock collectively has more eyes to warn them of approaching predators and to warn them sooner so they might take evasive action. Once attacked, it is thought that statistically any one individual within the flock has a reduced risk of being predated. Almost certainly there is the “general confusion effect”26. A predator can be thrown off its focus simply by the overwhelming number of potential targets. There may be a startle effect. When and if the flock turns, they may simultaneously flash the lighter side of their body, and startle the attacker. The density of the flock is also thought to increase the predator’s risk of injury if it dives through the flock and it has been suggested that merlins, in particular, perceive that risk. “The high density borders that are often observed may represent a feature that enhances such anti-predatory tactics, creating a ‘wall effect to increase the predators confusion.”26 It also has been observed that in highly organized flight flocks the birds are able to launch active defenses such as mobbing behavior where a predator is attacked en masse.
There is an interesting issue buried in the hypothesis that flocking provides protection to the whole flock, by the way. That issue is whether exposure to the risk of being on the outside of the flock is shared by all, or not. One author states that “Given that, in general, animal aggregations are rather stable, it implies that group structure and dynamics must allow for a systematic redistribution of risk among its members. Individuals must be able to move through the flock and exchange positions, while at the same time maintain integrity of the group.”26 But, at least in pigeons this is not the case. At least in that case there is a particular benefit to being in a flock for those birds higher in the pecking order because it appears the more dominant individuals spend less time at the edge of the flock than do lower ranking members. This is a critical issue because the hierarchical hypothesis would allow that there may be a Darwinian benefit at the group level (the flock) from predation as the birds more likely taken when a flock is attacked are those who are less likely to breed - the weaker; the older or younger; the less dominant. If this were the case in fact there would be an additional evolutionary pressure for flight flock behavior to be inherent in a species rather than a purely cultural behavior. If flight flocking is a heritable characteristic, it is reasonable to search for physiological manifestations specific to that adaptive strategy.
Most authors concur that there is a energy benefit to all members of the flock regardless of status because with so many others taking a turn watching for predators, any one individual doesn’t have to raise its head from foraging as often to look, making its search for food more efficient.
Communal nesting in rookeries maximizes collective protection of offspring in many of the same ways listed above for general protection. But flocking also presents increased opportunities for mating of all stripes, from pair bonding to polyandrous, polygynandrous, polygynous - even extra-pair bonding - with the dicier forms (not to be moralistic) being more common in communal settings.
Birds in flocks are said to be more successful finding food. More birds are looking. Once found, more prey is likely to be flushed by the group stomping around, for instance. This is thought to partially explain why some species co-flock. If individuals from other species join a flock, and making the assumption (based on no data, but conjecture) that they assume a place lower in the pecking order of the flock and therefore at the outer edges of the flight flock, they may bring added protection to the dominant species in the flock.
There are also costs to participating in a flock for all members. The group is far more easily found by predators simply because of its size. Where the group may be better at finding food sources there is inherently more competition for the food once found. There are avian diseases which are spread within flocks more easily simply because of propinquity. This is a particular risk around bird feeders, for instance, but also in rookeries and where there is colonial breeding. There is a unique adaptive cost/benefit analysis for each bird in each rank of the social order.
By flying in flocks (not just V shaped, but it is suggested, all flocks) the birds enjoy significant energy efficiencies. The followers can surf on the waves of air created by the bird in front of them. As mentioned, there is a distinct hierarchy exhibited in flocks of flying pigeons. Such a hierarchy provides a number of efficiencies. There is evidence that homing pigeons while homing are led by the individual that is the best navigator in the flock7. Having the best navigator lead the flock has a clear advantage to the followers which is that they take a more direct and efficient way home than they would if they were flying without the leader. But there is a cost to the leader. The cost of leading can be more or less onerous depending on the purpose of the flock (migration, homing, or foraging, as examples). In flight flocks one significant cost to the leader is not being able to surf on others efforts like those behind it and, presumably, it expends energy synthesising many inputs in order to select the most energy efficient route - an effort that is greater for the leader than the followers simply following.
IV. Who flocks?
The literature suffers from the lack of specific definitions for various types of flocking behavior. We could not find even a comprehensive list of species that form flocks of any sort much less specifically those that form flying flocks - including the AOU and associated websites, or Cornell etc. The only listings found were rather poetic rather than scientific (see Appendix B).
V. How do birds flock?
This paper presents two hypotheses to explain how birds coordinate their movement once aloft in a flying flock. The first speculation is that birds in a flying flock perceive electromagnetic energy diffracted or refracted by the flight feathers of fellow flock members and that diffracted “light” carries the information necessary to maintain separation, alignment, cohesion and speed etc. I speculate that diffracted/refracted electromagnetic energy is perceived by an autonomic sensory organ in the eye rather than a retinal/cerebral one.
The second hypothesis is that the intent of the flock is established by and communicated through a led social order such as a dominance hierarchy. The question of the existence of a dominance hierarchy was discussed earlier in this paper and the techniques for testing the hypothesis are described in Nagy’s paper.7 Therefore, the remainder of this paper outlines some of the issues pertaining to the generation of and perception of diffracted/refracted electromagnetic energy.
“A key challenge in studying flocks is the understanding how they maintain cohesive directional motion while making rapid changes in direction (Couzin and Krause 2003)”21 The change of direction and speed of a flock in flight has to be communicated to each individual within the flock in the time frame required by Wayne Potts’s observation that “....the propagation of this manoeuvre wave begins relatively slowly but reaches mean speeds three times higher than would be possible if birds were simply reacting [retinally/cerebrally] to their immediate neighbours” 10
Each member of a flight flock must receive and within as little as 15 milliseconds act on data that includes:
1. Separation – avoid crowding neighbors (short range repulsion - stay at least x inches away from all of your nearest neighbors)
2. Alignment – steer towards average heading of not only your nearest neighbors but also the flock as a whole
3. Cohesion – steer towards average position of neighbors and the flock as a whole.
4. speed - individuals must attain and maintain the same speed as the flock.
Even if the required data set were that simple, the main challenge remains unaddressed in the research surveyed. The challenge to describing flying flock behavior is to understand how all members of the flock perceive a set of data that allows them to react nearly simultaneously and instantaneously with a shared intent to that data.
Flock movement, propagates through the flock in a wave radiating out from the initiation site which Wayne Potts call “maneuver waves”10. “Once one of these waves began, Potts found that it spread through the flock far more rapidly than could be explained by the reaction times of individual birds. A bird’s mean startle [retinal] reaction time to a light flash as measured in the laboratory was 38 milliseconds but maneuver waves spread through the flock between birds at a mean speed of less than 15 milliseconds. However, the first birds to respond to an initiator took 67 milliseconds to react.”3, 10 Potts’ explanation is that somehow birds do not take their clues just from the birds immediately proximate to them but instead perceive “...that birds farther away from the initiation site were able to see the wave approaching them, and could “get set” to respond before it actually reached them. He dubbed this the “chorus line hypothesis”.3, 10 Presumably Potts meant that birds perceived the chorus line retinally because he didn’t report considering any other possible sensory mechanisms. But the “chorus line” hypothesis leaves questions to be explored.
What specifically are birds seeing as the chorus line approaches etc?
Prof. “Charlotte Hemelrijk of the University of Groningen in the Netherlands found that flocks of starlings “…......could maneuver … by watching the neighboring seven birds and by flying at the same speed”.37 38 But this assertion is simply a report on a computer simulation which shows that birds could be organized if they were able to respond nearly instantaneously to seven near neighbors, not that the researchers are saying they do, or have any physical evidence that they do. Is their assertion that each member of the flock randomly targets seven fellow flock members, somehow locks onto them, and follows their behavior slavishly? Are we to assume that those seven constitute the only birds the individual will follow and for how long are we to assume they follow what then must be a tightly cohesive grouping of seven. With just the ability to observe casually, it doesn’t seem likely to me.
If the chorus line hypothesis is correct then how is the initial condition after launch, or startle, explained? Right after the birds take off, there is no chorus line to be seen and yet they form up and assume an organized pattern immediately.
Another, not completely contradictory interpretation of Pott’s research results might be that each bird’s reaction is autonomic rather than retinal/cerebral. An autonomic response to a general stimulus would fit Potts’ time constraint. In that case, whatever speculation is proposed to explain flight flock behavior must also describe the sensory organ which would initiate that retinal or autonomic response.
“Most animal species have photoreceptors that are inherently polarization-sensitive, and many species use this sensitivity in their orientation and navigation behavior. A few of these polarization-sensitive animals have evolved a set of signals that are based on the controlled reflection of polarized light33 from parts of their bodies.” 20
I speculate that one way birds in a flying flock coordinate their flight is by sensing diffracted31 and refracted light5 (and/or circular and linear polarized light34) from their flock mates’ flight feathers. Bird feather barbs are a natural diffraction grating. In fact the first description of diffraction patterns can be found in a letter to John Collins by James Gregory dated May 1673 where Gregory describes using bird feathers as the diffraction grating. In the subsequent 340 years it has been established that large quantities of information can be carried by diffracted and refracted light2. “Diffraction will produce the entire spectrum of colors as the viewing angle changes”30. Such light, if perceived by birds, could instantaneously and simultaneously convey the information necessary for the spacing, speed and direction needed to coordinate the behavior of all birds in a flock - well within Potts’ time constraints. That being said, in a survey of the scientific literature done for this paper I found no research that suggests birds can perceive light diffracted through and/or refracted from flight feathers and use that information for flocking flight. But then too, the literature didn’t provide any other suggestions for “group soul” either.
Some literature (Kreithen, M.L. and Keeton 1974)19 states that some birds perceive polarized light but recent articles equivocate 15. The literature suggests that polarized light is in fact perceived but the usual suspect organs of perception (the retina) have not been shown to be the agent of perception. J. J. Vos Hzn et al’s article convincingly points out possible errors in experiment technique in the earlier work that suggested bird retinal perception of polarized light. J.J. Vos Hzn et al’s article reports convincing evidence that the organ that is sensitive to polarized light is NOT the retina, as their title suggests. The response times required for flock flight behavior to be perceived by all of the members of the flock within Pott’s time frame would preclude a retinal, cognitive response in any case.
The information would have to be communicated in an autonomic-like response instead. The J.J. Vos Hzn article does not address, much less rule out whether other sensory organs might be involved.
If individual birds use diffracted or refracted electromagnetic energy to perceive the movement of its flock then we must test whether birds in fact perceive polarized light or other diffracted electromagnetic sources by other routes than the retina. We must establish where that energy is perceived (by what organ) by the bird. If in the eye alone, we need to identify the structure in the eye that does the perceiving and communicates it. (the lens, the cornea, the pecten
oculi? )42
I suspect that the Pecten oculi (the structure near the fovea) (see illustrations 1 - 5) has an involvement as the organ for perception of diffracted/refracted light which carries the information necessary to coordinate flocking flight. As published in 2012, “First reported (the pecten oculi) over 300 years ago, its function(s) remains a puzzle to ornithologists, ophthalmologists and anatomists.”32 The Pecten oculi is highly vascularized and highly pigmented. The former could facilitate sensing the power of electromagnetic energy entering the eye. The dense pigmentation could be useful for sensing polarized or diffracted/refracted energy waves in various ranges. The Pecten oculi’s sensitivity to both the strength and wavelength of diffracted electromagnetic energy entering the eye can be tested. We can be sure, but also test, that the polarized light signal can be propagated within a flock, (regardless of size) and sensed using this mechanism. We can establish whether it can be propagated fast enough to conform to Wayne Potts3 time requirements. It can be tested whether the Pecten oculi is capable of perceiving that energy.
We then need to test whether the Pecten oculi provide a sensory conduit (nerve path) for what is essentially an autonomic response - that the wiring is there to trigger changes in movement without cognition.40 If the Pecten oculi is shown to be exquisitely sensitive to both the intensity of energy from a particular range of the electromagnetic spectrum and to the ratio of circular polarized to linear polarity, the Pecten Oculi might be considered an autonomic response “organ” more like the lateral line down the side of a schooling fish, as such it would be all the more effective for flocking given the Potts time constraints which do not allow for a visual sensory response alone. My suspicion that the pecten oculi is the sensing organ is heightened by the location of the Pecten oculi within the eye. It is roughly at 45 degrees to the axis defined by the line between the lens to the fovea. (see illustration 1) It is roughly shaped like a radiator grating - like a diffraction grating - enhancing its sensitivity to different wavelengths. (see illustration 1-5). It is rooted at the optic nerve, giving it immediate access to the choroid.
If, for the moment, one grants that birds sensory organs perceive the information required for highly coordinated collective motion and that that sensory organ might be the Pecten oculi, the question remains as to how the signal is transmitted autonomically to instantly reorient the bird to its flock. I suggest that the choroid36 is a candidate for explaining how the signal is processed. The choroid can change the shape of the eye autonomically. As a gymnast establishes a focal point of orientation in four dimensions and maintains her orientation relative to that point, so could a bird orient itself with the interference pattern within the flock.39 That focus is maintained autonomically with changes in the shape of the eye caused by the choroid - with the bird reorienting its body to re-establish stasis in the eye by a reorientation of the body to the flock, its focal point. With the eye fixed in the birds head then it only takes the shape of the eye to change to keep that focus and then to reorient the body to the eye and head.
Falk Schroedi of the Anatomisches Institut in Erlanger-Nurnberg Germany points out “Intrinsic choroidal neurons (ICN) represent a peculiar feature of eyes in higher primates and birds. They account for up to 2000 in human and duck eyes but are virtually absent or rare in all other mammalian species investigated so far. It has been suggested that ICN are involved in regulation of ocular blood supply, hence influencing intraocular pressure, and changes in choroidal thickness, thus influencing accommodation”. 24 “The temporo-cranial accumulation of ICN (intrinsic choroidal neurons) in galliformes and the belt like arrangement in anseriformes may reflect special functional requirements in regions of high visual acuity” 24 .
One can envision a fairly straightforward experiment designed to first establish whether the pecune oculi is sensitive to electromagnetic waves. Put sensors at the base of the Pecten oculi and see if there is a response to stimulus made of various strengths and wavelengths of electromagnetic energy. If it is, then we would want to test whether the pecune oculi is implicated in flying flock behavior - an experiment that would be more complex. An experiment might be structured using pigeons in an established flock. First the pecking order of the flock would be established. Then a subset of the birds in the middle of the pecking order would be selected. First they would be tested to establish their visual acuity. Next the experimenters would disable their pecune oculi (what “disable” means in this context specifically would be established as part of the experiment design but here is intended to mean that it would be removed). The visual acuity of those birds would then be tested again to establish whether it was damaged by the removal of the pecune oculi. If not, the flock would be fitted with GPS and an analysis done of whether the birds’ ability to form and maintain a flock in flight had been affected.
There is such a long list of corollary questions one could address if it were shown that birds can perceive refracted/defracted light, one is tempted to deduce that they should!
VI. Conclusion
How birds form flying flocks is an authentic question, certainly. However, as a layman taking a Birders’ Certificate Program as his first biology course the writer understands he lacks standing sufficient to put forth such speculations as offered in this paper. I appreciate your indulgence. The idea was to use the discussion of how birds flock as a vehicle to organize the survey material for presentation in a conversational way. I hope that was successful. At a minimum the survey saved me from submitting a paper of pure speculation for it is likely that speculation does not course credit make. These admissions notwithstanding the speculations that precipitated this paper were only emboldened by the information gleaned from the survey. In fact there is a long list of compelling closely related questions this paper was not ambitious enough to try to address (see Appendix A) but which might be worth the reader’s consideration.
Appendix A
If it were established that some flying flock birds use diffracted light then must we prove that all birds do - all birds, some or none?
Does each species feathers have a unique periodicity - cross-sectional profile of the grooves or barbs?
If only some birds perceive polarized light, is there a correlation between those species that do and those that flock?
Are there birds of different species that flock together and if there are what can we learn about the similarity or disparity of their polarization signature, so to speak, and their sensory capability?
If all bird species perceive polarized light to some degree, then what other uses might there be for polarized light perception in non flocking birds?
Navigation is widely hypothesized.
Falconiformes, particularly those hunting fish, may benefit from polarized vision like fishermen using polarized glasses. (suggested by Dr. Dennis Crouse in personal communication)
Egg identification36
By the way, do birds in a flock beat their wings at the same rate?
If yes, do they do so in synchrony?
How would a coordinated beat affect the intensity of the polarity?
Are wings in more rapid flight more or less distorted by the increased or decreased pressure from exertion?
If yes, and if it changed the qualitative nature of the polarity, and if that nuance is proven to be perceivable, then the refined signal might provide information on force (speed) in addition to vector and direction.
Many birds molt just before migrating. Are newly grown feathers more reflective of polarized light?
How, if at all, could the perception of polarity contribute to the following flocking behaviors?
If it is established that flocking birds use polarized vision to help them, that would only partially explain flock behavior.
How are turns initiated?
Does flock structure have initiators (individual leaders, a group leader, any bird in the flock toward to the front of the flock, any group of birds sufficiently forward in the flock so that they are visible to each bird in the flock, maybe the turn ripples through the flock?) For instance, if a bird toward the back and say on the outside side of a banking flock sees a threat, how is its alarm communicated to the rest of the flock since it is behind them? Maybe it doesn’t, but just pretend until you see enough video.
How is flock lift off initiated?
For instance a flock could be startled to take off by the sound of any bird’s wing starting to flap. Rapidly complying with the four rules of flocking behavior would be particularly challenging in the this circumstance.
The concept of “recruitment” might have to be invoked to supplement Pott’s threshold theory. A couple of birds, or even one, initiates a move which gets more to follow that initiative if there is merit and others follow. Does polarized light play a role in this choreography?
Does polarized light perception play a role in an individual bird’s decision to join a flock initially? For instance could the intensity of polarized light from a flock be a determinant in the individual’s decision whether that flock is suitable?
Flocking birds tend to be darker
Do flocking birds tend to molt before migrations or other prominent flocking behaviors?
Do the Pecten Oculi of flocking birds share any characteristics collectively that differ from non flocking species?
Do the feathers emanating from the alar feather track of flocking birds differ from non flocking birds in any way?
References
1 Letter from James Gregory to John Collins, dated 13 May 1673. Reprinted in Correspondence of Scientific Men of the Seventeenth Century... (Oxford University Press, 1841).
“.... a small experiment ….. . Let in the sun’s light by a small hole into a darkened house, and at the hole place a feather (the more delicate and white the better for this purpose,) and it shall direct to a white wall or paper opposite to it a number of small circles and ovals (if I mistake them not) whereof one is somewhat white, (to wit, the middle, which is opposite to the sun,) and all the rest severally coloured.”
2 circular polarity, linear polarity and their ratio, etc, in combination with other aspects of diffracted and refracted light (whether from the electromagnetic spectrum humans can see, or not.
3Once one of these waves began, Potts found that it spread through the flock far more rapidly than could be explained by the reaction times of individual birds. A bird’s mean startle reaction time to a light flash as measured in the laboratory was 38 milliseconds, but maneuver waves spread through the flock between birds at a mean speed of less than 15 milliseconds. However, the first birds to respond to an initiator took 67 milliseconds to react. Potts proposed that birds farther away from the initiation site were able to see the wave approaching them, and could “get set” to respond before it actually reached them. He dubbed this the “chorus line hypothesis”.
www.straightdope.com/columns/read/2151/how-does-a-flock-of-birds-wheel-and-swoop-in-unison
5 the definition of light here is not restricted to the part of the electromagnetic spectrum that can be seen by humans
6 BL Partridge, The Structure and Function of Fish Schools, Scientific American, Vol.246, No.6, pp.114-123
7 Flight Plans: Airborne pecking order coordinates pigeon flocks Nature Vol 464|8 April 2010 by Mate Nagy, Zsuzsa Akos, Dora Biro and Tamas Vicsek
8 Couzin and Krause 2003
9 The Sibley Guide to Bird Life and Behavior Pg. 73
10 Potts, Wayne K. 1984. "The chorus-line hypothesis of coordination in avian flocks." Nature 24: 344-345
11 http://epod.usra.edu/blog/2006/06/black-sun-in-denmark.html
12 A phenomenological model for the collective landing of bird flocks, Istvan Daruka, The Royal Society, accepted November 3, 2008
13 1980 The coordinated aerobatics of Dunlin flocks. Anim. Behav, 668-673. doi:10.1016/s0003-3472(80)80127-8
15 “No Evidence for Polarization Sensitivity in the Pigeon Electroretinogram” byJ.J. Vos Hzn, M.A. J.M. Coemans and J.F.W. Nuboer
16 T. Vicsek, A. Czirok, E. Ben-Jacob, I. Cohen, O. Sochet, Phys. Rev. Lett., 75, 1226 (1995)
17 Hierarchical group dynamics in pigeon flocks by Mate Nagy, Zsuzsa Akos, Dora Biro & Tamas Vicsek apxiv.org/pdf/1010.5394&embedded=true
18 Collective Motion of Animals: Recent Finds From Field Study and Model Experiments by Jing Yan http://guava.physics.uiuc.edu/~nigel/courses/569/Essays_Fall2010/Files/yan.pdf
19 Kreithen, M.L. and Keeton, W.T. (1974) Detection of polarized light by the homing pigeon, Coumbia livia.J. comp. Physiol. 89 83-92
20 Polarization Vision and Its Role in Biological Signaling by Thomas W. Cronin, Ndav Shashar, Roy L. Caldwell, Justin Marshall, Alexander G. Cheroske and Tsyr-Huei Chiou, Oxford Journals Life sciences Integrative and Comparative Biology Volume 43, Issue 4 Pp 549-558
21 Information transfer in moving animal groups by David Sumpter, Jerome Buhl, Dora Biro and Iain Couzin, Theory Biosci. (2008) 127:177-186
23 Collective motion from local attraction, by Daniel Strombom, Mathematics Dept., Uppsala U.
24 Experimental Eye Research, volume 78, Issue 2, February 2004 Pages 187-196
26 How Birds Fly Together:The Dynamics of Flocking, by Kishore Dutta, Resonance, December 2010, Pg. 1097 - 1110
29 Shape and internal structure, bu C.K. Hemelrijk and H.Hildenbrandt, Interface Focus published online 22 August 2012
30 From Compromise to Leadership in Pigeon Homing, by Dora Biro, David J.T. Sumpter, Jessica Meade and Tim Guilford published in Current Biology 16, 2123-22128, November 7, 2006Elsevier Ltd.
31 Wikipedia. “In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams traveling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Such gratings can be either transmissive or reflective. Gratings can have various properties of the incident light modulated in a regular pattern including transparency (transmission amplitude gratings), reflectance, refractive index (phase gratings) and direction of optical axis (optical axis gratings).
32 Functional morphology of the pecten oculi in the nocturnal spotted eagle owl and the diurnal black kite and domestic fowl: a comparative study by S.G. Kama, J.N. Maina, J. Bhattacharjee and D. Weyrauch in the Journal of Zoology. http://journals.cambridge.org/action/displayAbstract?from Page=online&aid=81083
33 orientation of the wave's electric field at a point in space over one period of the oscillation.
34Two-dimensional transverse waves exhibit a phenomenon called polarization. A wave produced by moving your hand in a line, up and down for instance, is a linearly polarized wave, a special case. A wave produced by moving your hand in a circle is a circularly polarized wave, another special case. If your motion is not strictly in a line or a circle your hand will describe an ellipse and the wave will be elliptically polarized. Wikipedia
35 Dr. Catherine Snow quoted in the Oberlin Alumni Magazine, pg. 18 “....is more likely to be learned by students trying to answer an engaging question than by simply processing one more assignment.”
36Abernathy, V. E., Western Illinois University, USA, ve-abernathy@wiu.edu; Peer, B. D., Western Illinois University, USA, BD-Peer@wiu.edu
MECHANISMS OF EGG RECOGNITION IN BROWN-HEADED COWBIRD HOSTS: THE ROLE OF ULTRAVIOLET REFLECTANCE
The most effective adaptation against Brown-headed Cowbird Molothrus ater parasitism is rejection of the cowbird egg, yet relatively few hosts reject cowbird eggs. Studies have demonstrated that ultraviolet reflectance of eggs plays a role in egg rejection by hosts of the parasitic cuckoos, but the effects of ultraviolet light on rejection by cowbird hosts has largely been ignored. We measured the ultravio- let reflectance of the eggs of three rejecter species including the Brown Thrasher Toxostoma rufum, American Robin Turdus migratorius, and Gray Catbird Du- metella carolinensis. We also experimentally blocked the reflectance of ultraviolet light of one host egg in the clutches of these three rejecter species to determine whether they utilize ultraviolet light when rejecting eggs. We found that Brown Thrasher eggs reflected significantly more ultraviolet light than both American Robin and Gray Catbird eggs. Brown Thrashers were also significantly more likely to reject their own eggs that had the ultraviolet reflectance blocked compared to American Robins and Gray Catbirds. These findings suggest ultraviolet light is an additional factor that some hosts utilize when rejecting eggs. (6600)
37 The Mystery of Flocks, by Thomas Conuel, Pg. 7, Sanctuary Summer 2012
39 Eye-neck coupling during optokinetic responses in head-fixed pigeons (Columba livia): influence of the flying behavior. Neuroscience. 2004; 125(2): 521-31
40 Orientation saliency without visual cortex and target selection in archer fish. Proc. Natl. Acad Sci. USA 21 September 2010: 16726-16731
41 Telencephalic Input to the Pretectum of Pigeons: An Electrophysiological and Pharmacological Inactivation Study. Journal of Neurophysiology, January 1, 2004 vol.91 no. 1 274-285
42 A Dissociation of Motion and Spatial-Pattern Vision in the Avian Telencephalon: Implications for the Evolution of “Visual Streams”
Appendix B
Owls: Parliament, wisdom, study
Peafowl: Party
Pelicans: Squadron, pod, scoop
Penguins: Colony, huddle
Pheasants: Nye, bevy, bouquet
Plovers: Congregation
Ravens: Murder, congress, horde
Rooks: Clamour, parliament
Starlings: Chattering, affliction
Swallows: flight, gulp
Swans: Wedge, ballet, lamentation
Turkeys: Rafter, gobble
Woodpeckers: Descent
Wrens: Herd, chime
Birds of Prey (hawks, falcons): Cast, cauldron, kettle
Cormorants: Flight
Crows: Murder, congress, horde
Ducks: Raft, team, paddliing
Eagles: Convocation, congregation
Finches: Charm
Flamingos: Flamboyance
Game Birds (quail, grouse, ptarmigan): Covery, pack, bevy
Geese: Skein, wedge, gaggle, plump
Gulls: Colony
Herons: Siege, sedge, scattering
Hummingbirds: Charm
Jays: Band, Party, scold
Larks: Bevy, exaltation, ascension
