Manx shearwater compass calibration
Creators: name: Lewin, Patrick
Publisher: name: OSF Registries
Year: 2023
Subjects: subject: Behavior and Ethology
subject: Life Sciences
subject: Ecology and Evolutionary Biology
subject: Zoology
subject: FOS: Biological sciences
subject: Animal Sciences

description: The planned experiment aims to test whether shearwaters rely on information from the magnetic compass to account for changes in the sun's arc resulting from seasonal progression or latitudinal movement. Birds, including shearwaters, are known to use the sun's azimuth as a compass cue. However, due to the earth's rotation, the sun appears to move across the sky from east to west. As a result, birds using a sun compass at varying times of day must keep track of the time of day and adjust their directional response relative to the sun's azimuth accordingly. This is called time compensation. Evidence suggests that birds and other animals use an internal clock entrained on the timing of dawn and dusk to time-compensate their use of the sun compass. However, the sun's arc is not constant through time or space (it varies with latitude and season, due to the tilt of the earth's axis) so the azimuth represented by the sun at a particular time relative to dawn or dusk changes. As the sun's arc becomes longer or shorter by symmetrical changes at both ends of the day, a clockwise change in the sun's dawn azimuth predicts an equal anticlockwise change in the sun's dusk azimuth. As such, birds must be able to adjust their time-compensation program in order to account for these spatial and temporal changes and use the sun compass accurately. If animals are able to keep track of the season, their own movements and the interaction between them, they could account for these changes endogenously. However, there is no evidence for such an ability and it would require complex encoding. Instead, animals may rely on environmental cues such as day length or other compass cues to account for changes in the sun's arc. If birds use another compass to measure the sun's azimuth at dawn or dusk and thus predict the sun's arc, this would be an example of compass calibration. There is evidence that birds may calibrate their magnetic compass using the sun or star compass, or that they may calibrate these celestial compasses with their magnetic compass. This experiment will involve the deployment of neodymium magnets with co-deployed GPS on adult Manx shearwaters over a number of weeks during their incubation period in May and early June. The target sample size is 80 birds, split equally between treatments (with magnets) and controls (with brass beads of equal dimensions and mass). Due to the potential for failure of GPS devices and logistical constraints of fieldwork, the final sample size may be lower. During this period, the sun's arc is longer at higher latitudes due to the boreal summer and gets longer as the season approaches the summer solstice. If shearwaters rely on the magnetic compass to calibrate their sun compass, shearwaters which are denied magnetic compass information and cannot measure the sun's azimuth at dawn or dusk should become disoriented, relative to controls. The magnitude of this disorientation should increase with latitudinal displacement from the site of their last calibration and with seasonal change in the sun's arc. The direction of the shift will depend on whether the experienced sun arc is longer or shorter than during the last calibration before treatment. It will also depend on the time of day: shearwaters which expect the sun to rise clockwise of where it is truly rising should expect it to set anticlockwise of where it will truly set. As such, the deflection should be greatest at dawn and dusk, smallest around midday and should switch from anticlockwise to clockwise over the course of the day. Thus, southerly movements from the colony after treatment will predict clockwise deflections at dawn and anticlockwise deflections at dusk. Northerly movements predict the opposite. Deflections should also become more anticlockwise at dawn and more clockwise at dusk as day length gets longer towards the summer solstice. There are also other potential uses of the geomagnetic field for navigation in birds. Shearwaters may simply rely on the use of a magnetic compass, or could use parameters of the earth's magnetic field, such as inclination angle, as map cues. There is evidence suggesting that shearwaters may not use geomagnetic cues in these ways, as past experiments in other shearwater species have found no effect of jostling magnets. Nonetheless, we will be able to test these hypotheses, which would predict constant disorientation of shearwaters under the magnetic treatment, which would not be affected by season, time of day or latitude.