Cues indicating location in pigeon navigation

Cumulative route improvements spontaneously emerge in artificial navigators even in the absence of sophisticated communication or thought

Homing pigeons (Columba livia) navigate by solar and magnetic compass, and fly home in idiosyncratic but stable routes when repeatedly released from the same location. However, when experienced pigeons fly alongside naive counterparts, their path is altered. Over several generations of turnover (pairs in which the most experienced individual is replaced with a naive one), pigeons show cumulative improvements in efficiency. Here, I show that such cumulative route improvements can occur in a much simpler system by using agent-based simulation. Artificial agents are in silico entities that navigate with a minimal cognitive architecture of goal-direction (they know roughly where the goal is), social proximity (they seek proximity to others and align headings), route memory (they recall landmarks with increasing precision), and continuity (they avoid erratic turns). Agents' behaviour qualitatively matched that of pigeons, and quantitatively fitted to pigeon data. My results indicate that naive agents benefitted from being paired with experienced agents by following their previously established route. Importantly, experienced agents also benefitted from being paired with naive agents due to regression to the goal: naive agents were more likely to err towards the goal from the perspective of experienced agents' memorised paths. This subtly biased pairs in the goal direction, resulting in intergenerational improvements of route efficiency. No cumulative improvements were evident in control studies in which agents' goal-direction, social proximity, or memory were lesioned. These 3 factors are thus necessary and sufficient for cumulative route improvements to emerge, even in the absence of sophisticated communication or thought.

Avian navigation: the geomagnetic field provides compass cues but not a bicoordinate "map" plus a brief discussion of the alternative infrasound direction-finding hypothesis

The geomagnetic field (GMF) is a worldwide source of compass cues used by animals and humans alike. The inclination of GMF flux lines also provides information on geomagnetic latitude. A long-disputed question, however, is whether horizontal gradients in GMF intensity, in combination with changes in inclination, provide bicoordinate "map" information. Multiple sources contribute to the total GMF, the largest of which is the core field. The ubiquitous crustal field is much less intense, but in both land and marine settings is strong enough at low altitudes (< 700 m; sea level) to mask the core field's weak N-S intensity gradient (~ 3-5 nT/km) over 10 s to 100 s of km. Non-orthogonal geomagnetic gradients, the lack of consistent E-W gradients, and the local masking of core-field intensity gradients by the crustal field, therefore, are grounds for rejection of the bicoordinate geomagnetic "map" hypothesis. In addition, the alternative infrasound direction-finding hypothesis is briefly reviewed. The GMF's diurnal variation has long been suggested as a possible Zeitgeber (timekeeper) for circadian rhythms and could explain the GMF's non-compass role in the avian navigational system. Requirements for detection of this weaker diurnal signal (~ 20-50 nT) might explain the magnetic alignment of resting and grazing animals.

Enhanced Hippocampus-Nidopallium Caudolaterale Interaction in Visual-Spatial Associative Learning of Pigeons

Learning the spatial location associated with visual cues in the environment is crucial for survival. This ability is supported by a distributed interactive network. However, it is not fully understood how the most important task-related brain areas in birds, the hippocampus (Hp) and the nidopallium caudolaterale (NCL), interact in visual-spatial associative learning. To investigate the mechanisms of such coordination, synchrony and causal analysis were applied to the local field potentials of the Hp and NCL of pigeons while performing a visual-spatial associative learning task. The results showed that, over the course of learning, theta-band (4-12 Hz) oscillations in the Hp and NCL became strongly synchronized before the pigeons entered the critical choice platform for turning, with the information flowing preferentially from the Hp to the NCL. The learning process was primarily associated with the increased Hp-NCL interaction of theta rhythm. Meanwhile, the enhanced theta-band Hp-NCL interaction predicted the correct choice, supporting the pigeons' use of visual cues to guide navigation. These findings provide insight into the dynamics of Hp-NCL interaction during visual-spatial associative learning, serving to reveal the mechanisms of Hp and NCL coordination during the encoding and retrieval of visual-spatial associative memory.

Comprehensive Transcriptomic and Metabolomic Analysis Revealed the Functional Differences in Pigeon Lactation between Male and Female during the Reproductive Cycle

Lactation is a unique reproductive behavior in pigeons, with the crop serving as the organ responsible for secreting pigeon milk. Both male and female pigeons can produce crop milk and rear their offspring through a division of labor. Since the time of the secretion of pigeon crop milk is different in the process of feeding the young, whether the metabolism and formation of pigeon milk use the same mechanism is a very interesting scientific question. However, the metabolic dynamics and underlying genetic mechanisms involved in the formation of pigeon crop milk remain unclear, particularly during the incubation-feeding reproductive cycle. In this study, we integrated lactation-associated metabolism and transcriptome data from the crop tissues of both male and female pigeons during the brooding and feeding stages. We mapped the changes in metabolites related to milk formation in the crop tissues during these stages. Through metabolome profiling, we identified 1413 metabolites among 18 crop tissues. During the breeding cycles, the concentrations of estrone, L-ergothioneine, and L-histidine exhibited the most dynamic changes in females. In contrast, estrone, L-anserine, 1-methylhistidine, homovanillate, oxidized glutathione, and reducing glutathione showed the most dynamic changes in males. Gender-specific differences were observed in the metabolome, with several metabolites significantly differing between males and females, many of which were correlated with cytokine binding, immunity, and cytochrome P450 activity. Using this dataset, we constructed complex regulatory networks, enabling us to identify important metabolites and key genes involved in regulating the formation of pigeon milk in male and female pigeons, respectively. Additionally, we investigated gender-associated differences in the crop metabolites of pigeons. Our study revealed differences in the modulation of pigeon crop milk metabolism between males and females and shed light on the potential functions of male and female pigeon milk in the growth, development, and immunity of young pigeons, an area that has not been previously explored. In conclusion, our results provide new insights into the metabolic regulation of pigeon crop milk formation during the brooding and breeding stages. Furthermore, our findings lay the foundation for the accurate development of artificial pigeon milk.

Magnetic maps in animal navigation

In addition to providing animals with a source of directional or ‘compass’ information, Earth’s magnetic field also provides a potential source of positional or ‘map’ information that animals might exploit to assess location. In less than a generation, the idea that animals use Earth’s magnetic field as a kind of map has gone from a contentious hypothesis to a well-established tenet of animal navigation. Diverse animals ranging from lobsters to birds are now known to use magnetic positional information for a variety of purposes, including staying on track along migratory pathways, adjusting food intake at appropriate points in a migration, remaining within a suitable oceanic region, and navigating toward specific goals. Recent findings also indicate that sea turtles, salmon, and at least some birds imprint on the magnetic field of their natal area when young and use this information to facilitate return as adults, a process that may underlie long-distance natal homing (a.k.a. natal philopatry) in many species. Despite recent progress, much remains to be learned about the organization of magnetic maps, how they develop, and how animals use them in navigation.

The discovery of the use of magnetic navigational information

The magnetic field of the Earth provides animals with various kinds of information. Its use as a compass was discovered in the mid-1960s in birds, when it was first met with considerable skepticism, because it initially proved difficult to obtain evidence for magnetic sensitivity by conditioning experiments. Meanwhile, a magnetic compass was found to be widespread. It has now been demonstrated in members of all vertebrate classes, in mollusks and several arthropod species, in crustaceans as well as in insects. The use of the geomagnetic field as a 'map' for determining position, although already considered in the nineteenth century, was demonstrated by magnetically simulating displacements only after 2000, namely when animals, tested in the magnetic field of a distant site, responded as if they were physically displaced to that site and compensated for the displacement. Another use of the magnetic field is that as a 'sign post' or trigger: specific magnetic conditions elicit spontaneous responses that are helpful when animals reach the regions where these magnetic characteristics occur. Altogether, the geomagnetic field is a widely used valuable source of navigational information for mobile animals.

Waterfall low-frequency vibrations and infrasound: implications for avian migration and hazard detection

Many researchers have suggested that birds may use natural infrasound sources for navigation and hazard avoidance. However, there is a need to define the sound levels and frequencies to characterize potential infrasound sources. This paper summarizes new measurements from Niagara Falls which define a stable, powerful infrasound source that could be detected by birds on a regional scale of over 400 km. Measurements made in the vicinity of Niagara Falls show that exceptional infrasonic pressure levels can occur in the regions of large waterfalls (> 100 Pa at a range of about 500 m). This paper reviews investigator assessments of avian use of infrasound. A review of the results of Cornell researchers on pigeon hearing provides a basis for estimating avian detection ranges of waterfalls. It is possible that migrating birds use sounds from waterfalls as beacons- a component of their "navigation toolbox" as well as infrasound for hazard avoidance.

Einstein, von Frisch and the honeybee: a historical letter comes to light

The work of the Nobel Laureate Karl von Frisch, the founder of this journal, was seminal in many ways. He established the honeybee as a key animal model for experimental behavioural studies on sensory perception, learning and memory, and first correctly interpreted its famous dance communication. Here, we report on a previously unknown letter by the Physicist and Nobel Laureate Albert Einstein that was written in October 1949. It briefly addresses the work of von Frisch and also queries how understanding animal perception and navigation may lead to innovations in physics. We discuss records proving that Einstein and von Frisch met in April 1949 when von Frisch visited the USA to present a lecture on bees at Princeton University. In the historical context of Einstein’s theories and thought experiments, we discuss some more recent discoveries of animal sensory capabilities alien to us humans and potentially valuable for bio-inspired design improvements. We also address the orientation of animals like migratory birds mentioned by Einstein 70 years ago, which pushes the boundaries of our understanding nature, both its biology and physics.

Behavioural traits of individual homing pigeons, Columba livia f. domestica, in their homing flights

Homing tracks of two groups of pigeons, Columba livia f. domestica, were analyzed in view of difference between individual birds and correlations between characteristic variables, looking at the initial phase while the pigeons were still at the release site, and the homing phase separately. Individual birds differed significantly in their flying speed during the initial phase, and one pigeon tended to stay longer at the release site than the others. There were no significant differences in steadiness and efficiency, indicating that all pigeons homed equally well. Differences in correlation dimension, a variable reflecting the complexity of the navigational process, reflect differences in the use of navigational information, with one bird apparently using less complex information than others. The flying speed during the initial phase was positively correlated with the flying speed during the homing phase. During the homing phase, the steadiness of flight and the efficiency of homing were closely correlated, and both tended to be positively correlated with the correlation dimension, suggesting that birds that use more complex navigational information home more efficiently.

Considerations on the role of olfactory input in avian navigation

A large amount of data documents an important role of olfactory input in pigeon navigation, but the nature of this role is not entirely clear. The olfactory navigation hypothesis assumes that odors are carrying essential navigational information, yet some recent experiments support an activating role of odors. This led to an ongoing controversy. An important, often-neglected aspect of the findings on olfaction is that olfactory deprivation affects avian navigation only at unfamiliar sites. The orientation of anosmic birds at familiar sites remains an enigma; earlier assumptions that they would rely on familiar landmarks have been disproven by the home-oriented behavior of anosmic pigeons additionally deprived of object vision, which clearly indicated the use by the birds of non-visual, non-olfactory cues. However, if odors activate the establishing and enlarging of the navigational 'map' and promote the integration of local values of navigational factors into this map, it seems possible that such a process needs to occur only once at a given site, when the birds are visiting this site for the first time. If that were the case, the birds could interpret the local factors correctly at any later visit and orient by them. This hypothesis could explain the oriented behavior of birds at familiar sites, and it could also help to reconcile some of the seemingly controversial findings reported in the literature, where the effect of olfactory deprivation was reported to differ considerably between the various pigeon lofts, possibly because of different training procedures.

Marine biorhythms: bridging chronobiology and ecology

Marine organisms adapt to complex temporal environments that include daily, tidal, semi-lunar, lunar and seasonal cycles. However, our understanding of marine biological rhythms and their underlying molecular basis is mainly confined to a few model organisms in rather simplistic laboratory settings. Here, we use new empirical data and recent examples of marine biorhythms to highlight how field ecologists and laboratory chronobiologists can complement each other's efforts. First, with continuous tracking of intertidal shorebirds in the field, we reveal individual differences in tidal and circadian foraging rhythms. Second, we demonstrate that shorebird species that spend 8–10 months in tidal environments rarely maintain such tidal or circadian rhythms during breeding, likely because of other, more pertinent, temporally structured, local ecological pressures such as predation or social environment. Finally, we use examples of initial findings from invertebrates (arthropods and polychaete worms) that are being developed as model species to study the molecular bases of lunar-related rhythms. These examples indicate that canonical circadian clock genes (i.e. the homologous clock genes identified in many higher organisms) may not be involved in lunar/tidal phenotypes. Together, our results and the examples we describe emphasize that linking field and laboratory studies is likely to generate a better ecological appreciation of lunar-related rhythms in the wild.

This article is part of the themed issue ‘Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals’.

Pigeonetics takes flight: Evolution, development, and genetics of intraspecific variation

Intensive artificial selection over thousands of years has produced hundreds of varieties of domestic pigeon. As Charles Darwin observed, the morphological differences among breeds can rise to the magnitude of variation typically observed among different species. Nevertheless, different pigeon varieties are interfertile, thereby enabling forward genetic and genomic approaches to identify genes that underlie derived traits. Building on classical genetic studies of pigeon variation, recent molecular investigations find a spectrum of coding and regulatory alleles controlling derived traits, including plumage color, feather growth polarity, and limb identity. Developmental and genetic analyses of pigeons are revealing the molecular basis of variation in a classic example of extreme intraspecific diversity, and have the potential to nominate genes that control variation among other birds and vertebrates in general.

Mathematical analysis of the homing flights of pigeons based on GPS tracks

To analyse the effect of magnetic and olfactory deprivation on the homing flight of pigeons, we released birds from a familiar site with either their upper beak or their nostrils anaesthetized. The tracks were analysed by time lag embedding to calculate the short-term correlation dimension, a variable that reflects the degrees of freedom and thus the number of factors involved in a system. We found that higher natural fluctuations in the earth's magnetic field characterized by A _P-indices of 8 and above caused a reduction of the correlation dimension of the control birds. We thus separated the data into two groups according to whether they were recorded on magnetically quiet days or on days with higher magnetic fluctuations. Anaesthetizing the upper beak had no significant effect. Making pigeons anosmic reduced the correlation dimension on magnetically quiet days, but did not cause any reduction on days with higher fluctuations. Altogether, our data suggest an involvement of magnetic cues and olfactory factors during the homing flight and point to a robust, multi-factorial map.

Collective Behaviour: Leadership and Learning in Flocks

A new study has decoded which birds become leaders in homing pigeon flocks, finding an unexpected benefit of leadership: faster birds emerge as leaders, and these leaders learn more about their environment than their followers.

Age-Dependent Neurogenesis and Neuron Numbers within the Olfactory Bulb and Hippocampus of Homing Pigeons

Many birds are supreme long-distance navigators that develop their navigational ability in the first months after fledgling but update the memorized environmental information needed for navigation also later in life. We studied the extent of juvenile and adult neurogenesis that could provide such age-related plasticity in brain regions known to mediate different mechanisms of pigeon homing: the olfactory bulb (OB), and the triangular area of the hippocampal formation (HP tr). Newly generated neurons (visualized by doublecortin, DCX) and mature neurons were counted stereologically in 35 pigeon brains ranging from 1 to 168 months of age. At the age of 1 month, both areas showed maximal proportions of DCX positive neurons, which rapidly declined during the first year of life. In the OB, the number of DCX-positive periglomerular neurons declined further over time, but the number of mature periglomerular cells appeared unchanged. In the hippocampus, the proportion of DCX-positive neurons showed a similar decline yet to a lesser extent. Remarkably, in the triangular area of the hippocampus, the oldest birds showed nearly twice the number of neurons as compared to young adult pigeons, suggesting that adult born neurons in these regions expanded the local circuitry even in aged birds. This increase might reflect navigational experience and, possibly, expanded spatial memory. On the other hand, the decrease of juvenile neurons in the aging OB without adding new circuitry might be related to the improved attachment to the loft characterizing adult and old pigeons.

Collaboration in the competitive world of science: lessons to be learned from William T. Keeton

The tremendous increase in the number of authors and institutional affiliations on papers published in the natural sciences over the last few decades is commonly interpreted as an indicator of an increase in the collaborative spirit. However, a closer analysis suggests that this development reflects an increase in cooperation (defined as a strategy to divide labor among participants), rather than a rise in collaboration (defined as a mutual engagement of participants in a coordinated effort to jointly solve a problem). An exception to this development was William T. Keeton (1933-1980), who, as a faculty member at Cornell University, pioneered research into pigeon homing. A direct result of his willingness to openly share ideas and collaborate with other investigators is the article by Hagstrum and Manley (J Comp Physiol A, 2015) in this issue of the Journal of Comparative Physiology A. Their study is based on data from experiments Keeton and his collaborators conducted some 40 years ago. Despite the age of these data, their analysis and the interpretation of the results are likely to stimulate fruitful discussion in the field of avian orientation.