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Theories of Olfactory Navigation in Rock Pigeons and Their Counterparts, Homing Pigeons (Columbia livia)
Olfactory navigation in Rock pigeons (Columbia livia), and their urban counterparts, homing pigeons, has been the subject of debate since the early 1970s, when it was first investigated by behavioural scientists. Based on a review of the literature, what’s clear is that pigeons don’t directly use chemicals in the atmosphere to orient themselves in the direction of their home point, but will use chemicals to develop a process for navigation. How pigeons use olfactory mechanisms is still under debate by scientists, however, from a review of the major literature, most of the dominant theories involve using, what most researchers have termed, a “cognitive map” derived from sensing odours. There is an exception to this cognitive map theory, however, which will also be discussed, and is logical given the diverse nature of the current debate.
This paper will attempt to compare the various dominant theories surrounding olfactory navigation, weighing their strengths and weaknesses, and arrive at a conclusion regarding which of these theories seems the most legitimate. A collection of examples of these prominent theories are reviewed below. Interestingly, much of the literature on olfactory navigation arose from experiments conducted in the 1980s and early 1990s, suggesting a concentration and collaboration of research on this subject during this time.
A brief collection of researchers and their findings is highlighted below, giving a sampler of a variety of arguments. Jerry Waldvogel and H.G. Wallraff are among the majority of scientists, leaning towards a cognitive map idea. Waldvogel tests both existing variations of the cognitive map: the “mosaic” model, developed from Floriano Papi’s research, and the “gradient” model. The mosaic model postulates that pigeons will recognise a collage of odour cues through direct experience with odours at their home position and during training flights, with more distant odours that are transported by wind being integrated into the pigeon’s mosaic olfactory map. They then determine distance from home by recognising these odours at their release position. The “gradient” model claims that odours can extend for long distances and exist as spatial gradients. Pigeons learn the strength of these odours at their home point during ontogeny, such that, when in an unfamiliar environment, they can compare the value of those odours to the values at home, determining how far and in what direction they would need to fly. Waldvogel concludes that meteorological conditions and the range of atmospheric chemical transport can affect how effectively pigeons use odours, and the type of olfactory navigation they might employ (Waldvogel, 1987).
Wallraff has run numerous experiments on homing pigeons, comparing unaltered individuals to treatment, smell-affected individuals, concluding that while pigeons are able to deduce location information from atmospheric odours, their success also largely depends on the environmental conditions, including differences in the density of environments (e.g. between plains and forests) and atmospheric activity, such as winds (Wallraff, 1992 and 2003). Walraff’s conclusions regarding atmospheric odours lending positional information is an indicator of cognitive map theory, however, Wallraff rejected the term “cognitive map”, citing that pigeons need only olfactory sensory information to fly in a perceived direction towards home, and that spatial cognition serves no special purpose other than giving the pigeon cues as to where they are initially (2003).
Floriano Papi, one of the pioneer researchers on this subject, believed that pigeons learn the traits of various odours reaching their home loft from surrounding areas, and associate them in their source directions (Ioalé and Benvenuti, 1983). Ioalé and Benvenuti decided to test Papi’s conclusion, reaching the summary that pigeons would need sensory information from at least two wind directions, because they sense not only the local odour at the release site, but also need to perceive the difference in change of odours present at their home loft and ones at the release site (1983). Even before the academic recognition of the two olfactory navigation models presented by Waldvogel, the beginnings of the mosaic model of olfactory navigation can be seem in these findings, suggesting that Ioalé, Benvenuti and Papi may have contributed to this model. Benvenuti and Ioalé later reported findings which supported the mosaic model, stating that pigeons only needed odour cues received during the initial journey to the release site to find their way home (1994).
N.E. Baldaccini from Parma, Italy, discussed Kramer’s “map and compass” concept, which has also been referred to by other animal behaviourists. The 1953 theory determines that there are two steps that pigeons take when establishing orientation: the “map step,” in which the animal calculates where it is, in respect to its destination, and the “compass step,” where it chooses a bearing to return home (Baldaccini, 1983). There are arguments showing favour for this in an olfactionary context and criticising this theory altogether; for example, one German researcher works her theory along these lines (Wiltschko, 1983).
But apart from reviewing a small collection of theories, such as the ones above, in order to truly understand the nature of these navigation models and evaluate their effectiveness in explaining pigeon behaviour, it is also necessary to investigate the anatomy of pigeon olfactory navigation—what physical mechanisms contribute to their navigational behaviour, and what effects have sectioning off or deactivating these areas had on pigeons’ navigation? The latter question will be explored through an investigation of experiments performed on this topic.
This comparison of theories is relevant because there is a large amount of debate regarding this topic and most of the literature shows a skew towards favouring certain theories. A thorough investigation may shed some light onto which direction the literature may be heading and provide new ground for more research.
Significance of Olfactory Navigation
How significant olfaction is in pigeon navigation is also a subject of widespread debate. Papi rebuts research suggesting that even anosmic pigeons can be homeward oriented. He guides attention to research implying that olfaction is a largely significant aspect of navigation, examining how clock-shifted pigeons can still be homeward oriented when they have become familiar with the odours at the release site (1990). In a study discussed in the UK newspaper The Observer, Anna Gagliardo, a University of Pisa researcher, sectioned off trigeminal nerves associated with magnetic navigation in 24 pigeons, and in another sample of 24, sectioned off olfactory nerves. When she released both test samples 30 miles from their home loft, virtually all magnetism-affected birds returned to the loft, while olfaction-affected birds “fluttered all over the skies of northern Italy” (McKie, 2006), also suggesting a significant role played by olfaction in navigation. Gagliardo is also an adherent to the mosaic model of navigation (McKie, 2006).
However, there are also others claiming that, while olfaction may be used by pigeons in their homing strategies, it doesn’t play a crucial part in these behaviours. A study in 1979 at the Tübingen University in West Germany concluded that, from spraying chemicals into the noses of control and treatment pigeons in deflector cages, there was no significant difference in deviation from the home site route attributed to the affected olfaction in pigeons, and rather, other cues affected navigation more significantly than the olfaction cue (Kiepenheuer, 1979). Roswitha Wiltschko found a difference in how birds were raised between Upstate New York, Frankfurt, and Italy; she also explored the varieties of birds used by Italian researchers and Wallraff. When she ran an experiment comparing Italian-style raised birds and German-style raised birds, she found the German-style pigeons, with numbed olfactory senses, orienting similarly to the Italian controls, casting doubt on olfaction’s significance (1989).
In the end, however, it seems that arguments for olfaction’s significance trumps the arguments casting doubt on its significance. Firstly, much of the literature casting doubt on its significance comes from the late 1970s, a time in which olfaction in homing pigeons was just beginning to be explored—besides, the observed effect from the deflector cages may have been due to other factors, such as uncontrolled conditions interacting with the cages. Secondly, olfaction tested in a tropical, wet climate such as south Eastern Brazil was deemed the same level of importance to the birds as olfaction in northern Italy, a predominantly drier climate (Benvenuti and Ranvaud, 2004), while Frankfurt receives similar amounts of precipitation to Sao Paulo, Brazil. The differences observed in Wiltschko’s study, therefore, may have been due to other factors, or an underlying difference in her methods.
Physical Mechanisms of Navigation
Apart from the nostrils, the primary organ used to sense odours, the majority of odour processing occurs in the brain, particularly around the hippocampus and olfactory bulbs (Mehlhorn et al., 2010). From a review of the available literature, no thorough overview of the olfactory system and its function in pigeon brains was found, although the above two segments of the brain appeared in literature covering olfactory navigation, and were seen as significant parts of the brain to study. In other words, while a holistic approach to investigating neural processing in olfaction hasn’t been widely adopted, concentration in specific areas allows researchers to draw conclusions and expand on research towards new goals.
The pigeon’s olfaction processing area sits on top of the brain. The hippocampus, situated around the same region, is responsible for long-term memory and acquiring spatial navigation, and can be larger in birds actively using it and in homing pigeons with navigational experience (Mehlhorn et al., 2010; Adelson, 2002). Indeed, in a study conducted by a large team of researchers, led by Verner Bingman, pigeons with lesioned hippocampi were compared to those with ordinary hippocampi. The research team found that learning a navigational map is impaired in lesioned pigeons while hippocampi-intact birds succeeded in learning a map under the same conditions (Bingman et al., 2005). The hippocampus may play an important role in allowing odours from the olfactory bulbs to be remembered and indeed, has been attributed to odour remembrance in mammals and birds (Eichenbaum, 1991).
Experiments conducted on homing pigeons show that they can become disoriented when physical organs related to their olfactory abilities are altered. Some of these have involved blocking olfactory abilities by applying zinc to their nostrils, which would have an anosmic effect, and affect their ability to navigate by smell (Benvenuti et al., 1998). As mentioned earlier, in an experiment conducted by Gagliardo, the pigeons that had their olfactory nerves cut navigated much worse compared to those who had severed trigeminal nerves (McKie, 2006). Experiments performed by Wallraff in the 1980s and Papi in 1976 show that pigeons have a difficult experience homing from unfamiliar locations when made anosmic (seen in Baldaccini, 1983). The two above experiments seem to show how crucial the olfactory nerves are, and also reinforce olfaction’s importance in navigation. When scientists from the eastern US created lesions in the hippocampi of homing pigeons, they observed that these pigeons had difficulty learning and memorising cues around their environment, and were confused when homing in a familiar area (Bingman and Mench, 1990). As deduced before, the disconnection these pigeons experienced between the hippocampus and the other parts of the brain, including the olfactory processing area, contributed to difficulties in forming a homing route.
Although olfaction is one of the dominant navigational mechanisms pigeons use, other mechanisms have also been accredited in relationships with the olfactory system, suggesting an interrelation between olfactory mechanisms and other navigational cues possessed by birds. Wiltschko determined that birds are born with an imprinted compass, and this gives them the ability to distinguish navigational directions early in life. As they begin gaining flight experience, they begin recognising orientation factors around their home loft and start establishing a “map,” which eventually replaces initial journey information they gather on their way to the release site (1983). These factors can include odour cues, which may then be aligned with a navigational compass, according to Wiltschko’s findings. Similar to the Bingman study mentioned above, a UC Berkeley researcher proposed that the formation of the hippocampus is related to a cognitive map useful for long-distance navigation, and detailed how an integrated map, consisting of both a bearing map showing compass landmarks, and a sketch map based on relative positions of landmarks, is used by the organism to find a route toward home point (Jacobs 2003). This cognitive map-hippocampus relation gives further support to proponents of the olfactory map school of thought, and implies that maps are the result of environmental navigation cues such as odour.
Upon initial examination, the jumble and variety of theories present to for this phenomenon can seem confusing and branching off into different directions, ideas and thoughts. Upon closer examination however, much of the argument is polarised towards certain issues, and the majority of arguments presented by authors derives from experimental data gained beforehand and work done by predecessors in the field. The data covers a wide time period, from literature in the 1950s to research and new findings occurring now. Though there is a wide coverage of different viewpoints and analyses, much of the research argues over the same issues: theories of navigation, why one theory should or shouldn’t apply etc., only using fundamental and initial research and not extrapolating further. While some research has been conducted into divergent, yet important topics, such as homing behaviour in microenvironments like forests and open space, these have opened up further research areas that are not being explored to its full potential. This may be due to the controversial nature of this topic, and no-one wants to be seen as “radical” or contributing even more to the controversy. Furthermore, it seems that a portion of the research on olfaction, while aging, can’t be deemed questionable or outdated because some of the contentions they make are still debated among a collection of researchers; thus reinforcing the idea that scientists must perform more studies to discover accurate explanations and solutions to these debates.
A detail that warrants attention is the earlier assertion by H.G. Wallraff that terming the pigeons’ navigation system as a navigational or olfactory map is incorrect because pigeons don’t actively use spatial representations in their navigation (2003). However, findings by other researchers, as seen above, suggest otherwise, as pigeons seem to recognise landmarks in accordance with their navigation, possibly using other mechanisms. For example, Jacobs’ research on the medial palladium—a segment of the avian forebrain just below the hippocampus—in homing pigeons suggests they can interpret spatial representations, even though more research is needed to draw evidence on the matter (2003). Moreover, Swiss researchers experimenting with neurologgers found that pigeons’ brain wave “frequencies spiked as the birds passed over features such as roads” (Leybold-Johnson, 2009), indicating a likely recognition of that landmark (Leybold-Johnson, 2009), and a clear refuting of, and dominance over, Wallraff’s original belief.
One of the biggest areas of debate is between the two aforementioned main models of cognitive mapping: the “gradient” model, which suggests that chemicals are distributed widely in the atmosphere, and when pigeons come into contact with these they can compare the strength of these odours at their current position with home position data to determine in which direction and for how long they would need to fly; and the “mosaic” model—a term coined by Wallraff (quoted in Papi, 1990)—which posits that odour information is perceived as a patchwork of different types by the homing birds at home, and at release, pigeons recognise these odours and formulate a route based on the information present in them. The literature seems divided on one or the other, seemingly in part due to author differences and preferences. H.G. Wallraff’s seems to be leading the arguments of the pro-gradient group, based on experiments on Columbia livia. Upon a closer investigation of his methods, the vast majority of his experiments follow a repeated outline: two groups of birds, one group being able to smell outside air at the release site, while the other group receives altered air, most commonly in a filtered form, but variations of this do exist. Together with Silviano Benvenuti, they simulated experiments through allowing them to smell natural air at a dummy release site, then moving them to the actual release area. Their controls, which were transported directly to the release site, smelt natural air for 3 hours at the site. During their journey, the pigeons received filtered air, an unchanged condition for a third pigeon group, who smelt the same air at the release site. Under anaesthesia, controls oriented the best, while experimentals flew in the direction of the false site to home, missing the target. Filters were disoriented altogether. They concluded that false olfactory data can misguide pigeons (Benvenuti and Wallraff, 1985), as gradient distribution can vary due to factors such as wind, and other experiments concluded with results in favour of the gradient model (Wallraff, 2003; Wallraff and Andreae, 1999).
There is also a cohort of scientists who see otherwise to this model, and barrack for the mosaic theory. Many of the leading scientists seem to have collaborated with each other to further data on pigeon homing, especially seen between Floriano Papi’s work and the developments and analyses made by other researchers. Anna Gagliardo, for example, reported to The Observer that she sees olfactory navigation centred around “a patchwork of odours” (McKie, 2006). Implicit biases aside, credit must be given to Waldvogel once again, for doing a fantastic and thorough job on analysing the implications of this model. Through the Cross Appalachian Tracer Experiments in 1983, the American recognises that, while this model should theoretically function for moderate distances (up to and including 200 km away from the home loft), practically, its range is limited due to the prerequisite that pigeons must have direct experience with winds blowing toward their loft for the distance to be fully realised, otherwise, they will just become acclimatised to odours specific to their home loft, limiting the distance away from home they can navigate. Complicating the winds’ journeys to the loft is the turbulence and mixing of air in the lower troposphere, where pigeons conduct their training and exercise flights—typically, in the early morning, the air is usually still and unaffected by mixing actions, however, it can be a different story in the afternoon. Even when conditions are favourable, factors such as topography and atmospheric turbulence impact these odours’ concentrations when reaching the home loft, such that, rather than an ideal 200 km range, the pigeons will have half that range. Therefore, it would make sense if other mechanisms were used alongside it (Waldvogel, 1987), such as the ontogenic compass and hippocampal spatial representations discussed earlier; indeed, Papi believes that odour direction determination is connected to use of a magnetic compass (Papi, 1990, and Benvenuti and Ranvaud, 2004). Interestingly, Wallraff reached a similar conclusion in the 1990s, posing that pigeons use other navigational cues alongside olfaction in familiar environments (1993). Therefore, it can be concluded that the particular experiment Gagliardo ran would have been affected by how far away the release point for the pigeons was from the home loft.
He also adduces that the gradient model seems to avoid this shortcoming in distance limitation, because while also being affected by wind and some turbulence, its distribution, according to the gradient model itself, is much greater, extending for a range up to 1000 km from the loft (Waldvogel, 1987). Based on these revelations, it is tempting to, just get rid of the mosaic model altogether and focus on developing the gradient model. However, Waldvogel points out that in areas like the Northeastern US, the gradient model is ineffective due to odour plume sizes—if they cover the release and home sites, the pigeons are disoriented. Furthermore, even if odour plumes are well defined, the line of symmetry drawn down their middles would have equal odour concentration on both sides, resulting in disorientation of tens of kilometres (Waldvogel, 1987). Papi himself comments on the gradient model, claiming that the mosaic model serves as a better olfactory map explanation over short distances than does the gradient model. Interestingly, some scientists working on the mosaic model, such as Papi and Waldvogel, have developed a unique conclusion: rather than emphasising their findings and arguing for the results’ legitimacy, they conclude that the literature and available research on pigeon olfactory homing is too small to declare any one theory dominant over the other. In 1990, Papi called for an investigation into atmospheric gaseous substance distribution to support either of the two models, and even then, both models are “working proposals...to the need to offer a model for the physical substrate” (Papi, 1990)—he doesn’t doubt, however, that there is a long-distance mechanism explanation of olfaction (1990). At this point, it is reasonable to follow Papi and Waldvogel’s conclusion, as the debate seems tied, with plausible evidence on both sides.
Another concept worthy of assessment is Kramer’s “map and compass” concept. To review, Kramer believed in a two-step process to navigation: in the map phase, the pigeon locates itself in respect to its home direction, and the compass phase involves deciding on a direction towards home. While the theory has been an accepted cornerstone of avian biology since its inception, Wallraff sees some innate issues built into it. He cites clockshifting issues, among others, into how a pigeon would determine its orientation and navigation direction, as a clockshifted pigeon interprets the sun’s position differently to an unaffected, control individual. Instead, he proposes a unique solution to the map-and-compass dilemma, from his point of view. Throughout his discussion, he reiterates the wind’s role in distributing odours and affecting atmospheric turbulence, similar to details in the mosaic model. In his re-examination of Kramer’s theory, he inserts the wind in the “and” segment, explaining that as his experiments have shown, a lack of wind has a similar effect to clockshifting a pigeon, as they rely on the wind to calibrate their internal olfactory maps and compasses. A pigeon shielded from winds in an aviary would not develop these complex spatial associations between odours and direction, and would have difficulty navigating home from a release point, as clock-shifts impact interpreting winds leading back to the loft and the pigeon would orient itself differently in respect to home loft. He calls for an abandonment of the traditional map-and-compass concept and its replacement by an integrated model, similar to the one he had described (Wallraff, 2003).
Wiltschko, on the other hand, suggests that the current map-and-compass model is legitimate because pigeons seem to use the compass as a primary portion of their navigational toolset (1983); however, this contention is severely limited because it doesn’t account for olfactory inputs and odours, as these may not have been established as significant primary navigational tools at the time. Baldacinni’s analysis also comes under this particular clout, leading to a definite lack in the literature on these subjects around this time. However, this was changed in light of Papi’s research, as his integration between olfactory inputs and the map-and-compass concept was one of the first forays into critiquing Kramer’s theory. In experiments later mirrored successfully by Wallraff, of all others (see above for details on Benvenuti and Wallraff’s experiment), Papi found that anosmic pigeons oriented homeward at a much lesser rate when released than the controls, unless they received sufficient time to acclimate to odours at the site and passed on by the wind (Papi, 1990). Ironically, Wallraff’s arguments for the gradient model can be used by a mosaic modeler, to support arguments relating to a distinctly different theoretical approach to pigeon navigation. Papi contends, however, that odours used for navigation must be the same odours sensed at home, and from experiments using shielded lofts, a favourite of Wallraff’s, that at least two wind-borne odour directions must be accounted for (Papi, 1990), similar to the conclusions two Italian researchers reached seven years earlier (Ioalé and Benvenuti, 1983). Not surprisingly, Papi’s findings are supported by some critics—Wiltschko performed one of the more prominent studies going against this research, concluding that, from the data she received in comparing pigeons living on the roof of a small structure and those living at ground level, anosmiating pigeons didn’t reveal any significant results that could be attributed to changes in odour conditions (Wiltschko, 1989). However, Italian scientists attempted to replicate this study twice, receiving different results in both trials. Apart from Wallraff’s replications, other researchers from Europe and North America also replicated Papi’s experiments, producing similar results such that a sufficient amount of literature exists on olfaction’s impact on the map-and-compass theory and significance in navigating towards the home loft. Based on the data received and arguments made, it only seems fitting giving credit to both Papi and Wallraff for their findings on wind affected odours and impact of these on the pigeon’s decision to approach a compass direction heading home. Not only have these been replicated successfully, but they attempt to improve on the original model, and leave open the room for a cohesive model taking into account these factors and arguments.
From examining literature on olfactory navigation in Columbia livia, this paper presented a plethora of arguments, both discrediting its significance and hailing it as one of the most important mechanisms pigeons use to travel home. The topic is still as controversial as it was 40 years ago, even though academics have performed more experiments and attempted to evaluate each others’ theories. However, there is some consensus arising between some dominant researchers, such as the experiments concerning Kramer’s hypothesis and its development into an integrated model, and this may help shape the direction of future research. A concerning area, for lack of concentrated research, is the fact that no thorough study has been conducted on the olfactory organs of pigeons, and no holistic picture has been developed as of late. Even though it has been established in most scientific circles that olfaction is a significant part of navigation, it’s surprising that no review has been performed, even though physiological reviews have been performed for magnetic compasses and navigational maps. Nonetheless, it is highly possible that a review will take place in the near future. In addition, biological data is clear and available insofar as the technology used to gather that information is accurate and the latest available models are used. It would aid to have better technology available in the future to test these theories brought to light by various researchers, and finally lessen some controversy on the issue.
A key direction for research to head now, especially given the contemporary impact of anthropogenic actions, is how human actions influence what odours are used to travel home—more specifically, how would pigeons living in densely populated urban areas use olfaction to navigate? Especially since the world’s largest cities (e.g. New York, Los Angeles, London, Sydney) are also the largest polluters, would pigeons use human pollutants as odour cues, and if so, how? For pigeons who are exposed to both urban and rural environments, would they tag odours differently compared to just rural or urban populations? Would the mosaic model be predominantly used? This is also an interesting topic that appeals to wider audiences, as pigeons are a common species. Wallraff’s mention of anthropogenically sourced odours in his 2003 paper lays the track for further examination, and he also reasons that certain anthropogenic chemicals emitted into the atmosphere are used by pigeons when navigating (Wallraff, 2003). Further experiments investigating the odour composition in some areas would be useful in both understanding any health-related issues the pigeons may encounter from using these odours for a prolonged period of time and shed some light onto the call for a new model instigated by Papi, by possibly investigating how migrating pigeons may change their olfactory habits when encountering differences between urban and rural areas, possibly finding a unifying theme between the gradient and mosaic models. Varied research such as this will also seem more enticing to promising scholars, as it’s different and unusual from much of the literature.
While pigeons and their navigational habits will remain a mystery for much of the near future, it is possible to draw comparisons between different theorists and clarify theoretical issues, lessening the controversy around this subject. In the past 40 years, researchers have deduced many things on olfactory navigation practices. In the next 40 or 50 years, there maybe completely new, more accurate models to aid scientists in navigating the mysteries of the pigeon navigation.
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