Unravelling the map and compass in bird navigation

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The ability to orient and navigate in space is a vital adaptation for all animals and many strategies have evolved to allow animals to return to a known goal. Studying spatial navigation has revealed much about the structure and function of the brain, how it is impacted by age, damage and disease. It has also revealed much about how sensory systems are integrated to provide information for locating position within the environment. Among the most remarkable navigators are small migratory songbirds. These animals travel thousands of kilometres between breeding areas and winter sites, and show remarkable precision, being able to return to the same breeding site, sometimes even nest, year after year. These small birds also show remarkable flexibility, being able to correct for large displacements from their normal migratory path, to places they could not have been to before, and return to their normal breeding or winter area. This appears to be learned during their first migratory journey from the breeding area they were born in, to the winter ground that they reach after their first migration. Scientists hypothesise that they navigate in this way using something akin to a map and a compass. The map step of this process is crucial, as it allows them to determine their location in relation to their desired goal and is thought to function essentially like our Cartesian coordinate system, providing latitude and longitudinal information. This is an ability that seems to be beyond humans without resorting to technology, and yet birds can do this based on cues sensed in the environment. Whilst much research effort has been expended in trying to discover how they achieve this, it remains essentially unsolved, as we do not fully understand what environmental cues are used to determine their position. Received wisdom has it that the cues and senses used in the map are separate from those used in the compass. Thus celestial cues such as the sun and stars provide compass directions, but not location. The exception to this is the Earth's magnetic field. However, It has been argued that birds require and possess two separate magnetic sensory systems, one for the compass step, located in the eye, and a second for the map step, located in the beak. However, recent evidence has called into question whether the beak based sense exists. In addition to this, new evidence indicates that one cues that is used in locating the birds' position is declination, which varies from east to west in some parts of the world, i.e. It provides a cue to longitude?. This is calculated by comparing magnetic north detected by the magnetic compass with geographic (true) north detected by a celestial compass (the sun or stars). This means that contrary to previous expectations, the sensory systems used in the map are not separate from the compass, but may be integrated into it. This discovery leaves many open questions however. How exactly to birds calculate declination? As the majority of birds are night migrants it makes sense that the star compass is the primary candidate, but some studies suggest that these birds calibrate the magnetic compass with sunset, not the stars. Do birds need two magnetic senses? If birds can calculate their longitudinal position with the magnetic sense in the eye, do they also calculate their latitudinal position with this sensory system? Do celestial cues, long relegated to the role of compass, actually play a greater role in the map, as they also could provide information on latitude. How do birds learn these cues? Birds must learn these cues on their first migratory journey, but the precise way in which they build this map is still entirely unknown. This research project will investigate these questions using a small songbird, the Eurasian reed warbler, to provide new insights into how it is able to navigate between its breeding grounds in Europe and winter grounds in Sub Saharan Africa.