Point of decision: when do pigeons decide to head home?

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.

Grade-control outdoor turning flight of robo-pigeon with quantitative stimulus parameters

Introduction

The robo-pigeon using homing pigeons as a motion carrier has great potential in search and rescue operations due to its superior weight-bearing capacity and sustained flight capabilities. However, before deploying such robo-pigeons, it is necessary to establish a safe, stable, and long-term effective neuro-electrical stimulation interface and quantify the motion responses to various stimuli.

Methods

In this study, we investigated the effects of stimulation variables such as stimulation frequency (SF), stimulation duration (SD), and inter-stimulus interval (ISI) on the turning flight control of robo-pigeons outdoors, and evaluated the efficiency and accuracy of turning flight behavior accordingly.

Results

The results showed that the turning angle can be significantly controlled by appropriately increasing SF and SD. Increasing ISI can significantly control the turning radius of robotic pigeons. The success rate of turning flight control decreases significantly when the stimulation parameters exceed SF > 100 Hz or SD > 5 s. Thus, the robo-pigeon's turning angle from 15 to 55° and turning radius from 25 to 135 m could be controlled in a graded manner by selecting varying stimulus variables.

Discussion

These findings can be used to optimize the stimulation strategy of robo-pigeons to achieve precise control of their turning flight behavior outdoors. The results also suggest that robo-pigeons have potential for use in search and rescue operations where precise control of flight behavior is required.

GPS-profiling of retrograde navigational impairments associated with hippocampal lesion in homing pigeons

The avian hippocampal formation (HF) is homologous to the mammalian hippocampus and plays a central role in the control of spatial cognition. In homing pigeons, HF supports navigation by familiar landmarks and landscape features. However, what has remained relatively unexplored is the importance of HF for the retention of previously acquired spatial information. For example, to date, no systematic GPS-tracking studies on the retention of HF-dependent navigational memory in homing pigeons have been performed. Therefore, the current study was designed to compare the pre- and post-surgical navigational performance of sham-lesioned control and HF-lesioned pigeons tracked from three different sites located in different directions with respect to home. The pre- and post-surgical comparison of the pigeons' flight paths near the release sites and before reaching the area surrounding the home loft (4 km radius from the loft) revealed that the control and HF-lesioned pigeons displayed similarly successful retention. By contrast, the HF-lesioned pigeons displayed dramatically and consistently impaired retention in navigating to their home loft during the terminal phase of the homing flight near home, i.e., where navigation is supported by memory for landmark and landscape features. The data demonstrate that HF lesions lead to a dramatic loss of pre-surgically acquired landmark and landscape navigational information while sparing those mechanisms associated with navigation from locations distant from home.

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.

Head-mounted sensors reveal visual attention of free-flying homing pigeons

Gaze behavior offers valuable insights into attention and cognition. However, technological limitations have prevented the examination of animals' gaze behavior in natural, information-rich contexts; for example, during navigation through complex environments. Therefore, we developed a lightweight custom-made logger equipped with an inertial measurement unit (IMU) and GPS to simultaneously track the head movements and flight trajectories of free-flying homing pigeons. Pigeons have a limited range of eye movement, and their eye moves in coordination with their head in a saccadic manner (similar to primate eye saccades). This allows head movement to act as a proxy for visual scanning behavior. Our IMU sensor recorded the 3D movement of the birds' heads in high resolution, allowing us to reliably detect distinct saccade signals. The birds moved their head far more than necessary for maneuvering flight, suggesting that they actively scanned the environment. This movement was predominantly horizontal (yaw) and sideways (roll), allowing them to scan the environment with their lateral visual field. They decreased their head movement when they flew solo over prominent landmarks (major roads and a railway line) and also when they flew in pairs (especially when flying side by side, with the partner maintained in their lateral visual field). Thus, a decrease in head movement indicates a change in birds' focus of attention. We conclude that pigeons use their head gaze in a task-related manner and that tracking flying birds' head movement is a promising method for examining their visual attention during natural tasks.

The Influence of Social Parameters on the Homing Behavior of Pigeons

Homing pigeons develop preferred routes when released alone several times from the same site, but they sometimes diverge from their preferred route when subsequently released with another pigeon. Additionally, group flights show a better homing performance than solo flights. But this knowledge is based on studies involving both sexes and lacks analyses of social parameters such as mating or breeding status, even though it is known that such parameters have an influence on behavior and on motivation for specific behavioral patterns. GPS trackers were used to track 24 homing pigeons (9 breeding pairs and 6 unmated females) as they performed a familiar 10km route in various pair and group combinations. Comparisons of efficiency indices (quotient between straight-line distance and pigeon’s track) reveal that unmated females show the best efficiency in single flights. Generally, group flights show the best efficiency followed by pair flights with a social partner of the opposite sex. Pair flights with the mated partner exhibit the poorest performance. Additionally, just before squabs hatching, females show a higher efficiency index when released at 8 am, compared to releases at 2 pm. Our results indicate that homing flight efficiency can provide insight into individual motivation and that social parameters have an influence on homing performance on a familiar route.

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.

Novel Insights into the Map Stage of True Navigation in Nonmigratory Wild Birds (Stone Curlews, Burhinus oedicnemus)

In the map-and-compass model of true navigation, animals at unfamiliar sites determine their position relative to a destination site (the map stage) before progressing toward it (the compass stage). A major challenge in animal navigation research is to understand the still cryptic map stage in general and the map stage for free-ranging wild animals in particular. To address this challenge, we experimentally translocated wild, nonmigratory birds (stone curlews [Burhinus oedicnemus]) far from their nests and GPS-tracked their subsequent movements at high resolution and for long durations. Homing success was high and cannot be explained by random chance or landmark navigation, implying true navigation. Although highly motivated to return home, the homing trajectories of translocated birds exhibited a distinct, two-phase pattern resembling the map and compass stages: a long, tortuous wandering phase without consistent approach home, followed by a short and direct return phase. Birds retranslocated to the same site initially repeated the original wandering path but switched to the return phase earlier and after covering a smaller area; they returned home via a different path but with similar movement properties. We thus propose the map learning hypothesis, asserting that birds resolve the map by acquiring, potentially through learning, the relevant navigation cues during the wandering phase.

Collective Decision-Making in Homing Pigeons: Larger Flocks Take Longer to Decide but Do Not Make Better Decisions

Social animals routinely are challenged to make consensus decisions about movement directions and routes. However, the underlying mechanisms facilitating such decision-making processes are still poorly known. A prominent question is how group members participate in group decisions. We addressed this question by examining how flocks of homing pigeons (Columba livia) decide their homing direction. We released newly formed flocks varying in size and determined the time taken to choose a homing direction (decision-making period) and the accuracy of that choice. We found that the decision-making period increases exponentially with flock size, which is consistent with a participatory decision-making process. We additionally found that there is no effect of flock size on the accuracy of the decisions made, which does not match with current theory for democratic choices of flight directions. Our combined results are better explained by a participatory choice of leaders that subsequently undertake the flock directional decisions. However, this decision-making model would only entirely fit with our results if leaders were chosen based on traits other than their navigational experience. Our study provides rare empirical evidence elucidating decision-making processes in freely moving groups of animals.

Following the sun: a mathematical analysis of the tracks of clock-shifted homing pigeons

We analysed the tracks of clock-shifted pigeons from six releases to determine how they cope with the conflict between their sun compass and other navigational cues. Time-lag embedding was used to calculate the short-term correlation dimension, a parameter that reflects the complexity of the navigational system, and with it, the number of factors involved. Initially, while pigeons were still at the release site, the short-term correlation dimension was low; it increased as the birds left the site, indicating that the birds were now actively navigating. Clock-shifted pigeons showed more scatter than the control birds, and their short-term correlation dimension became significantly smaller than that of the controls, remaining lower until the experimental birds reached their loft. This difference was small, but consistent, and suggests a different rating and ranking of the navigational cues. Clock-shifted pigeons do not seem to simply ignore the information from their manipulated sun compass altogether, but appear to merely downgrade it in favour of other cues, like their magnetic compass. This is supported by the observation that the final part of the tracks still showed a small deviation in the expected direction, indicating an effect of clock-shifting until the end of the homing flight.

Altered orientation and flight paths of pigeons reared on gravity anomalies: a GPS tracking study

The mechanisms of pigeon homing are still not understood, in particular how they determine their position at unfamiliar locations. The “gravity vector” theory holds that pigeons memorize the gravity vector at their home loft and deduct home direction and distance from the angular difference between memorized and actual gravity vector. However, the gravity vector is tilted by different densities in the earth crust leading to gravity anomalies. We predicted that pigeons reared on different gravity anomalies would show different initial orientation and also show changes in their flight path when crossing a gravity anomaly. We reared one group of pigeons in a strong gravity anomaly with a north-to-south gravity gradient, and the other group of pigeons in a normal area but on a spot with a strong local anomaly with a west-to-east gravity gradient. After training over shorter distances, pigeons were released from a gravitationally and geomagnetically normal site 50 km north in the same direction for both home lofts. As expected by the theory, the two groups of pigeons showed divergent initial orientation. In addition, some of the GPS-tracked pigeons also showed changes in their flight paths when crossing gravity anomalies. We conclude that even small local gravity anomalies at the birth place of pigeons may have the potential to bias the map sense of pigeons, while reactivity to gravity gradients during flight was variable and appeared to depend on individual navigational strategies and frequency of position updates.

Development of the navigational system in homing pigeons: increase in complexity of the navigational map

In the present study we analysed GPS-recorded tracks from pigeons of different ages from 11 sites between 3.6 and 22.1 km from the home loft, which revealed changes in the navigational system as the birds grew older and became more experienced. The efficiency of juveniles in their first year of life, at only 0.27, was rather low, indicating that the young birds covered more than three times the direct distance home. In the second year, after a standard training programme, the efficiency of the same birds increased to 0.80 and was no longer different from that of older pigeons. The short-term correlation dimension, a variable that reflects the number of factors involved in the navigational process, also increased with age. In juveniles, it was markedly lower than in the other two groups, but even in yearlings it was still significantly lower than that of old pigeons, indicating that the navigational map of yearlings is still developing. Our results indicate that the map system, although functional in the first year of life, continues to become more complex - older pigeons seem to either consider more navigational factors than younger ones or at least weigh the same factors differently.

Tracking pigeons in a magnetic anomaly and in magnetically "quiet" terrain

Pigeons were released at two sites of equal distance from the loft, one within a magnetic anomaly, the other in magnetically quiet terrain, and their tracks were recorded with the help of GPS receivers. A comparison of the beginning of the tracks revealed striking differences: within the anomaly, the initial phase lasted longer, and the distance flown was longer, with the pigeons' headings considerably farther from the home direction. During the following departure phase, the birds were well homeward oriented at the magnetically quiet site, whereas they continued to be disoriented within the anomaly. Comparing the tracks in the anomaly with the underlying magnetic contours shows considerable differences between individuals, without a common pattern emerging. The differences in magnetic intensity along the pigeons' path do not differ from a random distribution of intensity differences around the release site, indicating that the magnetic contours do not directly affect the pigeons' routes. Within the anomaly, pigeons take longer until their flights are oriented, but 5 km from the release point, the birds, still within the anomaly, are also significantly oriented in the home direction. These findings support the assumption that magnetically anomalous conditions initially interfere with the pigeons' navigational processes, with birds showing rather individual responses in their attempts to overcome these problems.

Waiting time before release increases the motivation to home in homing pigeons (Columba livia)

When performing homing experiments with individual releases, pigeons have to wait in a transport box for a certain amount of time before being released and hence perceive the departure of companions. Quite often, the last pigeons disappear straightforward from the release site. The question is whether this reflects improved orientation because of prolonged exposure to the release place or whether it reflects increased homing motivation. By releasing pigeons from a familiar site, we investigated the effects of the time spent at the release site on homing performance, recording pigeons' flights with GPS loggers. Our results show that, despite individual peculiarities of flight patterns, the waiting time at release site had a positive effect on homing speed and time, and reduced the time spent circling around the release point. However, the overall path efficiency as derived from GPS tracking was not influenced. These results suggest that a longer waiting time before release improves homing performance and this is related not only to increased navigational abilities but also to increased homing motivation.

A strong magnetic anomaly affects pigeon navigation

Pigeons were released in a strong magnetic anomaly with fast changes in intensity and gradients directions, about 60 km from their loft, and, for comparison, at the border of the anomaly and at a control site. The vanishing bearings were found to be closely related to the home direction, but unrelated to the local gradient directions. The vector lengths and the vanishing intervals, however, were significantly correlated with the maximum difference in intensity within a 2.5 km radius around the release site. This correlation was negative for the vector lengths and positive for the vanishing intervals,indicating that steep local gradients increase scatter between pigeons and delay their departure. These findings suggest that an irregular, fast changing magnetic field as found in the anomaly leads to confusion during the navigational processes. This, in turn, implies that pigeons can sense the respective changes in magnetic intensity. Magnetic cues seem to be included in the normal navigational processes that determine the departure direction.