Predictable fluctuations in environmental conditions impact the behavior and movement patterns of many animals. When northern temperate ponds freeze over in winter, bald eagles are cutoff from their prey, forced to travel in search of open water. Animals respond by travelling to such seasonal changes in carrying capacity by seeking out abundant resources in the summer and emigrating from limited, harsh resource conditions in the winter. Migration refers to these regular, seasonal journeys of animals that alternate between breeding and non-breeding locations. Many species are able to move great distances and excel at an ability to find their way. A combination of cues and mechanisms are employed to complete some of the most amazing achievements.
Animal Movements and Tinbergen's Four Aims
A series of independent questions can be addressed to explore the causation of migratory movements in animals, such as song birds.
Proximate Causation: hormonal influences on timing (restlessness), cues for navigation, ...
Ultimate Causation: For functional explanations of songbird migration, migrants must exceed local residents in the number of offspring, despite making up for additional energy costs and dangers associated with migration.
Phylogeny: Many species of birds migrate. Fossil evidence suggests that early songbirds evolved in the tropics. Migration may have arisen when individuals moved north (or southward) in spring to exploit longer daylight hours, local resource abundance in temperate zones, and release from competition in the tropics during the breeding season with its high food demands. The precise course of current major flyways have probably evolved since the last glaciation.
Ontogeny: learning of migration paths and innate preference for migration direction (European warblers, Starlings)
Homing refers to an individual's ability to return from a distant location to the only place it is certain to know something about. Animals may utilize a variety of cues and mechanisms to determine the location of its home site. Route reversal is a method used by some animals where the outgoing route is memorized/marked so the animal is able to “retrace” its steps by way of reversing the original route. Situations where the outward travel is significantly longer then the return journey, requires an ability to calculate the most direct route back via a diversity of orienting mechanisms, such as vector-based averaging or the acquisition of spatial maps.
Limpets fit their shell to one specific spot on a rock. A close fit is critical to withstand the impact of waves when submerged and prevent desiccation when exposed. Scraping algae from the rock surface, they travel from their home site, leaving behind a chemical trail. By reversing direction on this trail they are able to return to the very site they came from.
The homing pigeon has been selectively bred to be able to return to its home over extremely long distances. As a pigeon generally returns to its own nest and its own mate, human handlers have selectively bred subsets of birds that were particularly good at this task. Homing flights from as far as 1000 kilometers have been recorded by exceptional birds at average flying speeds of around 50km/h. For thousands of years humans have used homing pigeons to return messages back to home base by attaching them to the birds legs.
Migration includes all movements made in response to changes in food availability, habitat or weather.
Nomadic Migration: Individuals travel from place to place following distributions of resources. Following available forage determined by the season's weather patterns Bison roamed the American Great Plains, wildebeest travel the Serengeti
Seasonal Migration: Travel between a summer area for breeding and an area for overwintering are driving the seasonal migration of many animals, such as song birds. The energetic advantages of breeding in a site of seasonally high food resources and extended daylight hours allow diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics year-round. In autumn, as the days shorten and food sources dwindle, birds return to warmer regions where the available food supply varies little with the season. The success of migrants indicates that these advantages greatly offset the high stress, energetic costs, and other risks of the migration. Cardinals are common winter residents in northern latitudes. As members of a taxon with mostly tropical distribution, they may have reduced their winter return migration.
Navigation refers to the ability of planning and controlling one's movement from one place to another. It requires knowing one's current location on earth, the location of the desired goal, and an ability to calculate travel directions to get from here to there.
Many individuals show a distinct preference for departing in one particular direction. This applies to diurnal movements of plankton that feed at the surface during the evening and night, and sink to lower, darker regions to avoid falling prey to visual predators.
Many birds determine a particular flight direction based on environmental cues that serve as landmarks on the Earth's surface, including longitudinally magnetic lines, the lcoation and direction of the sun and other celestial objects, or prevailing wind direction and odors.
In vector navigation individuals arrive at a given destination by maintaining a compass direction (or directions) for a predetermined amount of time or distance. Dead reckoning is the process of estimating one’s present position by projecting course and speed from a known past position. Honey bees for instance communicate the location of profitable nectora sources to hive mates. Using a waggle dance they are able to transmit codes for direction and distance.
European Starlings from much of continental Europe overwinter on the channels coast of France and Britain. A group of banded individuals was moved from one part of Europe to a more western geographic location. This transfer would now require a ninety degree change in direction to reach their traditional wintering grounds. Adult starlings were admirably able to perform that course adjustment and returned at their normal wintering range. Newly-hatched juveniles, however, which had never made the trip before, continued to fly their traditional southwesterly direction and ended up on the coast of Spain.
Sahara desert ants (Cataglyphis) are scavengers. They forage for the corpses of insects and other arthropods which have succumbed to the heat stress of their desert environment. While no known land animal can live permanently at a temperature over 50°C, Sahara desert ants can sustain a body temperature well above 50°C with surface temperatures of up to 70°C. This ant ventures far from its burrow in the Sahara desert, which has almost no identifiable features. While venturing out it periodically takes measurements of its angle in respect to the Sun. By doing this the ant can venture far from its nest in search of food. Because of the blistering heat, it can only do this for about 3–5 minutes/day (the hottest time of the day, when all its predators are in hiding from the sun). In addition to obtaining information on outwards, directional angles, the ant appears to measure distances using an internal pedometer to count its steps. When the ant finds a dead insect it calculates the shortest route back to the nest through vector averaging.
Pilotage includes the use of fixed visual references to guide oneself to a destination. It refines simple dead reckoning navigation by incorporating knowledge of specific landmarks. Tinbergen explored digger wasps's (Philanthus triangulum) ability to return to the entrance tunnel of a specific nest site containing a developing larva. The wasp catches bees on hunting trips and returns this food to the offspring. While the wasp was in the burrow, Tinbergen placed a circle of pine cones around the entrance. When the wasp emerged it completed several circles around the entrance area before flying off. Tinbergen then moved the pine cones to the side of the nest and waited for the wasp to return from hunting. Upon return the wasp flew to the center of the pine cone circle even though that had been moved during her absence.
Radar images show that migrating bird flocks drift off courses in a strong crosswind, except for flocks that travel along distinct landmarks, such as rivers, coast lines, or mountain ridges. Landmarks may also arise from specific atmospheric conditions and distinct geographic features, such as Point Pelee, which jutts into Lake Erie and serves as key staging ground for migrating hawks in search of updrafts.
Visual Orientation refers to the use of geographic landmarks (rivers, coastlines, ridges, etc.). Diurnal migrants often follow such landmarks, generally of lesser importance for nocturnal migrants, although there are documented cases of nocturnal migrants following rivers or coastlines. This may be particularly important for navigation to a precise breeding or wintering locality.
Map-based navigation refers to an organism’s ability to (1) acquire a mental representation of the spatial layout of its environment, (2) to position itself within it, and (3) to determine its own position relative to a goal location. The individual is able to extrapolate this information about the surrounding even when it finds itself in an unfamiliar setting. Orienting mechanisms include mosaic and gradient maps. A cognitive map refers to an internal representation of the spatial relationships between objects in an animal’s surroundings.
Mosaic map navigation involves being able to tell what landmarks are around the goal location and how these landmarks are situated relative to each other. Successful orientation is achieved by the use of landmarks and their spatial arrangement. This generally works better on a small scale and, for many animals, the animal must be in range of its goal in order to make use of the mosaic map. An animal may use landmarks by learning how locations are spaced relative to each other. An animal at Reference Point A may recognize that its goal will be on the left. Alternatively, an animal may sum up vectors such that it learns that Point A is located 20 km distant from Reference Point B.
Gradient map navigation involves predictable variation in two geophysical gradients that run perpendicular to each (i.e., bi-coordinate map). In this fashion an animal may acquire information about its current x and y coordinates along with knowledge about the x and y values of a destination, allowing it to plot a path from the former to the latter. Examples of gradients may integrate magnetic, olfactory, or infrasound information to do this. A magnetic map utilizes the Earth’s magnetic field which acts like a large bar magnet. Consequentially, this allows for any point on Earth to be defined as a vector that is made up of intensity (the length) and direction (the horizontal and vertical components). An olfactory map is formed by certain scents that are typically only found in certain areas on Earth. This may include an area of land that is surrounded by a body of water on three of its four sides (i.e., Italy). An infrasound map may use low frequency sounds from waves crashing on the beach, mountains nearby, or similar acoustic instances. Pigeons have become disoriented when in a “zone of silence”, possibly due to their need to navigate via an infrasound map. Other animals that are known to take use of low frequency sounds include whales, elephants, and insects. An integrated map combines a combination of sensory cues in navigational tasks.
A variety of animals have been shown to use cognitive maps, including sand hoppers, ants, honey bees, lobsters, sting rays, tuna fish, salmon, newts, sea turtles, mole rats, migratory birds (European robins, garden warblers, etc.), pigeons, and sea birds (Wilson’s storm petrels, sooty terns, etc). Indigo Buntings are able to navigate by the position of celestial objects. In order to obtain precise locations they must be able to estimate altitude and azimuth relative to the horizon, in combination with precise timing information.
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