Birds are like humans. They migrate from one place to another in order to find their life needs. Birds choose particular seasons for migration. They even start migration movements in certain times in the day. Birds use some amazing methods to find their correct direction throughout their migrations journeys. Different types of segregations happen inside migration flocks. Birds face natural and unnatural types of danger during their migration journeys.
What Are the Reasons That Drive Birds to Migrate?
1. "Temperature": "Individual birds are relatively sedentary during two periods each year, at nesting time and in winter" (Lincoln 16). Most of the time, birds migrate in "waves" (Lincoln 18). There are two major migration movements, in fall and in spring. Some other birds stay. The fall's migration movement to the south takes place between the month of July and winter (Lincoln 16). The spring's migration movement to the north takes place between the beginning of spring and the following month (Lincoln 19). Birds migrate south to escape the cold weather in winter, and they return in spring for breeding (Lincoln 16; Meader 7).
2. "Food": The main reason that drives birds to migrate is food. It is true that cold weather is a difficulty for birds, but the lack of food that comes with it is more difficult (Meader 7).
What Types of Food Do Birds Look for When They Migrate?
Birds look for different types of food. Their need for migration varies from species to another. In that subject, birds are divided into 4 categories:
1. Birds That Eat Insects: In winter, they move south where there are insects, and in summer they move north because it's the season of insects' proliferation there (Meader 7).
2. "Waterfowls": Those are the birds that live next to water resources like ducks. They eat small creatures that live in the water like fish. In winter, they move south to find places where water is not frozen. The distances of their journeys "vary greatly from species to species, and from individual to individual" (Meader 7).
3. Birds That Feed on "Rodents": They move south because the numbers of rodents drop in winter in the north. Unlike other types of birds, those birds don't migrate if the food is available (Meader 7).
4. "Seed Eaters": They move in winter, and they return in spring because in summer they find a plenty of seeds that American farmers spread in their farms (Lincoln 19).
In Which Part of the Day Do Birds Migrate?
Many birds migrate during the night especially small birds (Lincoln 19). Migrating during nighttime is beneficial for birds in many ways:
1. During the dark hours of night, small birds can be safe from their enemies (Lincoln 19).
2. Birds look for food during the day only. If they start the journey at night, they will have enough energy to fly for a long time. And their bodies will store fat. Starting the journey during the hours of daytime means that birds will stop eating until next day (Lincoln 19). This will have a negative affect on their energy. And that means that the birds will reach their destination very late and they could also die during the migration journey (Lincoln 19-20). "Studies have shown that the number of days an individual lays over during a migration stop is inversely dependent upon the amount of its fat stores upon arrival" (Lincoln 20).
Flying is a process that generates heat inside a bird's body. The cool weather in the night helps the body to lose the extra heat to stay in a good temperature. Also, it saves the amounts of necessary water inside their bodies. A bird's body loses water during migration either by the increase in body temperature or by burning the stored fat. In the second case, the bird can fly "nonstop" for a longer time. Flying during nighttime helps the bird to reach that state by losing the unnecessary heat and saving the necessary water inside the body (Lincoln 20).
Some other species can only migrate during daytime like "gulls".
And that's because "their mode of flight makes them dependent on updrafts created either by thermal convection or deflection of wind by topographic features like hills and mountain ridges". In this case, birds will not use too much energy to fly. Also, some birds feed on insects that only appear during the day (Lincoln 20).
Surprisingly, there are some species of birds that can migrate and find food day or night. Most of these species live next to water resources like "Snow Geese" (Lincoln 20).
How Do Birds Know Their Directions?
1. "Sun Compass": The position of the sun in the sky helps birds to head to their destinations (Meader 9). It is true that the sun doesn't stay in one position throughout the day. That's why birds change their directions accordingly. "It seems they have a rather accurate internal clock mechanism that aids them in knowing where the Sun should be at any given time of day" (Meader 10).
2. "Magnetic Field Detection": "It is believed that birds can detect the magnetic field through small magnetite crystals that are situated near the bird’s nostrils". When birds detect the magnetic field of the Earth, they can find out which direction is the south in winter and which direction is the north in spring (Meader 10).
3. "Star Compass": Birds fly to the direction of the star, "Polaris", in spring and fly away from it in fall. The position of Polaris doesn't change. Some researchers say that birds in fact pay attention to "stars' motions", not "star patterns". Researches proved that birds watch the "continuous apparent motion of stars around Polaris" (Meader 10).
4. "Smell, Sight and Sound": Birds can use their senses to recognize many road signs. By the sense of smell, a bird can recognize many road signs such as the smell of a "bird colony". By the sense of sight, the bird can detect some road signs like "tall trees". By the sense of hearing, the bird can identify many road signs like the sounds of "highways" (Meader 11).
Segregation in Bird Migration
1. "As Individuals or Groups or Species": Many migration flocks contain many species of birds from the same family like the flocks of "wood warblers". Some other species migrate in their own groups like "Nighthawks" and "Bobolinks". The birds of those species fly fast. And that makes it hard for other species to migrate with them. "Besides flight speed, feeding habits or roosting preferences can be so species-specific as to make traveling with other species incompatible". Sometimes different species can meet each other during their migration journeys (Lincoln 32).
2. "By Age": In many bird species, the parents leave the young birds after they grow up. After that, parents prepare themselves for the migration journey. In most cases, young birds start the journey before the parents. "Mourning Doves" follow this type of age segregation in their migration movements (Lincoln 34).
In some other cases, the parents start the migration movement journey before the young birds. They start leaving right after the young birds become old enough to depend on themselves. And that happens with the "Hudsonian Godwits" (Lincoln 35).
There is another case where the mature and the young birds take off together. In this case, mature birds "undergo a wing molt that renders them flightless during the period of growth of their young so that both the adults and immatures acquire their flight capabilities at the same time". Usually, the mature birds lead the young birds in the flock. This case applies to the "Canada Geese" (Lincoln 35).
3. "By Sex": In many cases, males start the migration movement before females. Males do that to prepare themselves for the coming females for breeding. Each bird establishes his own "territory" for himself and his expected female. Also, a male creates this territory or spot to prevent other males from crossing his own private zone. After females arrive, each female chooses her favorite site and picks a male.
An example on that is the "Hermit Thrushes" that live in Chicago in the United States (Lincoln 35).
Sometimes both sexes migrate in the same companies (Lincoln 35). Males and females start breeding before the spring migration journey or even during it. After they finish the journey, they start to take care of the young birds right away. The "Osprey" birds' migration groups include both sexes (Lincoln 36).
What Types of Danger Do Birds Face During Their Migration Journeys?
1. "Storms": Strong winds are very dangerous especially when birds are flying above "broad stretches of water". Flying in a stormy weather consumes most of the birds' energy. They face the threat of drowning if there is a water resource around like rivers or lakes (Lincoln 51).
2. "Aerial Obstructions". Those obstructions include "lighthouses", "tall buildings", "monuments", and "television towers". Another problem is using the lights in these buildings at night. By nature, birds are attracted to flashing lights in the dark. And that makes birds hit the buildings and probably die (Lincoln 51).
3. Pollution: Forms of pollution include "draining wetlands, cutting down forests. Pollution of the sea, water and air also affects them". That results in the disappearance of spots where birds can take a rest during the long migration journey (Tan).
4. Hunting: Some people go out and hunt birds for unnecessary purposes including hunting as a hobby (Tan).
Birds are similar to animals and fish when it comes to migration motivations. They all migrate for breeding and finding food (Grabianowski; Larinier 1). Birds know their own needs and capabilities, and that's why they choose particular timings to start their migration movements. Birds' strong senses help them to find their directions whether it is day or night. Bird migration can be free of threats by reducing pollution and regulating hunting wild birds.