Why do birds migrate?

Birds migrate to different areas to breed or overwinter during the change of seasons, responding to seasonal changes in the environment and resources[1]. About 19% of existing bird species migrate[2]. Migration is a way for birds to adapt to the seasons and environment. If migration is affected, it will hinder the survival and reproduction of these bird populations[3-4].

Canada geese on the move (by anjoudiscus via Birdshare)

Part 1

Why do birds migrate?

During migration, birds tend to move from areas where resources are scarce or decreasing to areas where resources are abundant or increasing. The resources that birds seek are mainly food and nesting sites, and they may also avoid monsoons and rainfall, which is conducive to hatching [5, 20]. Birds that nest in the Northern Hemisphere tend to migrate north in the spring (such as house martin) to feed on the rapidly growing insect populations there and take advantage of spring sprouts and abundant nesting sites. When autumn and winter approach, the supply of food such as insects in the north decreases, and they move south again.

As the seasons change, climate conditions and the energy supply of ecosystems also change, which may be a driving factor in bird migration [6-7]. Escaping from heat or cold may also promote bird migration, but many bird species can tolerate a certain degree of temperature variation when food supply is sufficient [5]. Some large-scale long-distance migrations are more complex and may be caused not only by different needs for resources such as food, but also by factors such as genetics, geography, and day length.

Part 2

What are the types of bird migration?

Seasonal change is one of the factors that affect bird migration. Birds can be divided into resident birds, summer migratory birds, winter migratory birds, and migratory birds according to their migration seasons.[7] For a certain place, birds that stay there all year round and do not migrate with the change of seasons are resident birds, such as sparrows; summer migratory birds, such as northern swallows; winter migratory birds, such as southern red-crowned cranes; migratory birds, such as swans, stop briefly at a place during migration but do not breed or spend the winter there and continue to migrate after stopping.

Representative birds of resident, short-distance migratory, medium-distance migratory and long-distance migratory species

(Cardinal by Kevin Bolton; Northern Bobwhite by Philip Simmons; Blue Jay by Gary Mueller; Magnolia Warbler by Gerrit Vyn)

Migration is a periodic, large-scale migration. Birds can be divided into resident birds, short-distance migrants, medium-distance migrants, and long-distance migrants according to the distance they travel.[5] Resident birds that do not migrate are often able to find sufficient food and nesting sites throughout the year; short-distance migrants travel relatively short distances, such as moving between high and low altitudes on a hillside; medium-distance migrants can cover distances of thousands of kilometers; and long-distance migrants can even cross the northern and southern hemispheres.

Part 3

Exploring the Origin of Bird Migration

It is generally believed that birds migrate to avoid intraspecific competition during the breeding season. In addition, factors such as climate change, predation, or parasitism also affect birds' choice of suitable breeding areas and promote bird migration [11,14]. As the seasons change, the key resources that birds rely on for survival also change. The seasonal changes in those important resources are another important reason for bird migration [7].

Temperate regions are rich in resources during the breeding season for migratory birds, so birds migrate to temperate regions to optimize their reproduction[10,15]; however, during the non-breeding season for wintering, temperate regions lack key resources, so birds migrate to tropical regions to optimize their survival[16]. Through migration, birds achieve staggered utilization of resources, alleviating intraspecific competition caused by seasonal tensions in key resources[17].

The evolution of migration behavior of the North American yellow warbler is consistent with the northern homeland theory

(Image source [20])

Some scholars believe that migratory behavior is gradually formed as birds search for better food or breeding opportunities. Birds benefit from small-scale annual migration, and then gradually expand the scale to evolve into migratory behavior[20]. Regarding the origin of bird migration, there are hypotheses such as the Northern Homeland Theory, the Southern Homeland Theory, and the New North-Neotropical Migration System.

Baltimore Oriole's migratory behavior fits northern home theory

(Image source [20])

The northern homeland hypothesis holds that some dramatic climate changes in history, such as glacier activity, forced birds that originally lived and bred in high-latitude areas to migrate southwards in order to survive. Under the pressure of glaciers, they had to shift their activity areas, and when the glaciers retreated northwards, they instinctively migrated back to their northern breeding grounds[8]. This migratory behavior may have increased the survival rate of birds, so birds with migratory behavior have a greater advantage in survival, and this behavior has been preserved by natural selection.

The Southern Homeland Theory hypothesis holds that bird migration evolved in tropical regions. To reduce the pressure of the breeding season, these tropical birds occasionally migrated to high latitudes. The migrating birds were able to take advantage of the abundant seasonal food, ample nesting space, and fewer natural enemies there, and thus had more offspring. Therefore, they had an advantage in natural selection and retained migratory behavior [8,18].

However, some people believe that the two theories are not mutually exclusive [9,10]. In addition, both the Northern Homeland Theory and the Southern Homeland Theory were proposed when studying bird migration in the Northern Hemisphere. As a sister system, the Southern Hemisphere is also important in studying bird migration. The Neoarctic-Neotropical migration system affirms the importance of the Southern Hemisphere and believes that bird migration in the Northern and Southern Hemispheres has the same evolutionary origin, namely Neotropical birds. This theory believes that migratory species are species in the temperate-tropical border area. They expand their range of activities, breed in temperate zones, and overwinter in tropical zones, flexibly adapt to the environment, and adjust the balance between survival and reproduction [11,12,19].

Neoarctic-Neotropical Migratory System

(The New North and New Tropical are geographical regions under the world vertebrate division system. The picture is adapted from the reference [12])

In response to different migration theories, some scholars believe that the origin of species evolution and the origin of migratory behavior evolution cannot be completely linked geographically, and the two do not have to be exactly the same [13]. In addition, the key issue of bird migration evolution is not whether birds began to migrate from low-latitude origin to high-latitude or vice versa, but whether bird migration is just to expand the breeding area to enhance adaptability or increase the number of offspring, or to better survive in the non-breeding season [10]. Moreover, these hypotheses ignore the diversity of bird migration. Birds living in different populations and different regions may have different migration formation mechanisms. Therefore, the origin of bird migration still needs further exploration and research.

In addition, we must also recognize that while migration brings some benefits, long-distance travel is also a dangerous and arduous task[5]. Migration is a test of the physical fitness of birds. Along the way, they may lack sufficient food, encounter extreme weather, and increase the risk of being preyed upon.

Part 4

What factors influence bird migration?

Bird migration is an instinct regulated by genetic and physiological factors. Birds that migrate are usually very sensitive to environmental changes.[39] There may be more than one external environmental signal that triggers bird migration. Migration may be triggered by changes in day length, lower temperatures, changes in food supply, celestial and solar signals, olfactory signals, and genetic factors.[1] Every spring and autumn, migratory caged birds experience a period of restlessness and repeatedly fly to one side of the cage.[34] German behavioral scientists named this behavior zugunruhe (migratory excitement/restlessness). Birds of different species, and even different populations of the same species, may follow different migration patterns.

Changes in day length

The rotation and revolution of the earth lead to the alternation of day and night and seasonal changes. The combined effect of these two causes periodic fluctuations in the length of day and night - that is, the days are longer and the nights are shorter in summer, and the days are shorter and the nights are longer in winter. This fluctuation in the length of day and night is a very stable factor of change, and therefore has become a key signal for regulating the biological rhythms of plants and animals. The day and night cycle affects the visual and nervous systems of birds, affecting not only the reproduction of birds, but also the migration time of migratory birds.

As the days shorten in late summer, birds’ photoreceptors trigger hormonal changes[38], stimulating many birds to grow new feathers to withstand the rigors of long-distance flight; promoting the activity of birds’ gonads in preparation for reproduction; and stimulating birds’ appetites, allowing their bodies to accumulate enough fat[40] so that they can efficiently obtain energy in the future. These hormonal changes make birds increasingly restless and in a state of excitement for migration, especially at night[21]. Until the day length shortens to a certain extent, migration is triggered.

Magnetic field information affects bird migration and positioning

(Image adapted from reference [24])

Geomagnetic field factors

Migratory birds and pigeons can use geomagnetic field information to determine migration and positioning. However, birds navigate by magnetic inclination rather than magnetic field polarity, which has been confirmed in a large number of studies on different birds [22-24]. When the local geomagnetic field strength changes extremely, birds may lose their way, but after staying in this place for a period of time, the birds can re-adapt and orient themselves with the new magnetic field strength [25]. In addition, birds' induction of magnetic fields depends on light, especially short-wavelength light. The light can be very weak but must be present. Spectral anomalies in the environment can also cause birds to lose their way [26-29]. Birds can perceive magnetic field information through their eyes or beaks [22].

Artificial training can teach birds about migration

(Image adapted from reference [36])

Socialization and acquired learning

For species that migrate in groups, experienced individuals can pass on migration information to inexperienced individuals, and young individuals can quickly gain migration-related experience and change their behavior by observing or following. Studies have shown that artificial training through vehicles, aircraft, etc. can guide birds such as wild geese, sandhill cranes, geese, and swans to fly to a certain area and form migration memories, although some birds will not cooperate or forget the route[36]. The presence of older birds in a group of whooping cranes can significantly reduce the deviation of migration paths. The older the bird, the smaller the deviation of the migration path. This is the result of its learning and memory[35].

Cryptopoietin CRY, a blue light receptor in animals and plants, plays an important role in biological rhythm regulation and migration and reproduction

(Image adapted from reference [30])

Migration-related genes

The blue light receptor gene is an important candidate gene for bird migration. The blue light receptor cryptochrome CRY is currently the only known light receptor or circadian clock element that is conserved from bacteria to animals and plants. The two genes CRY1 and CRY2 in Arabidopsis are mainly involved in blue light-inhibited stem elongation and photoperiod-induced floral organ differentiation. Cryptochrome genes in animals may play a role in the pineal gland and eyes, participating in circadian clock regulation by inhibiting circadian clock-related proteins CLOCK and BMAL1, and mediating magnetoreception through unconventional photochemical mechanisms. There are four cryptochrome protein encoding genes in bird eyes: CRY1a, CRY1b, CRY2, and CRY4. Researchers believe that cryptochrome CRY in bird eyes can produce two singlet and triplet free radical pairs with opposite or parallel electron spin directions under the induction of blue light, and sense changes in magnetic inclination through the magnetic sensitivity of free radical pairs. [30-34]

Satellites track the distance and route changes of Arctic peregrine falcons

(Image adapted from reference [37])

In addition to blue light receptor genes, genes related to memory also affect bird migration. By comparing the genomes of long-distance and short-distance migrating Arctic peregrine falcons, the study found that there were genotype differences between Arctic peregrine falcons with different migration distances. The gene ADCY8 related to memory ability was positively selected in the long-distance migratory population. Long-term memory may be one of the important factors affecting the long-distance migration of Arctic peregrine falcons [37].

References:

[1]Regulation of migration. BioScience. 2007

[2]Key conservation issues for migratory land- and waterbird species on the world's major flyways. Bird Conservation International. 2008

[3]When and where does mortality occur in migratory birds? Direct evidence from long-term satellite tracking of raptors. The Journal of animal ecology.2014

[4]Variation in survivorship of a migratory songbird throughout its annual cycle. Journal of Animal Ecology. 2002

[5]The basics of bird migration: how, why, and where. The Cornell Lab of Ornithology. 2021

[6]The role of atmospheric conditions in the seasonal dynamics of North American migration flyways. Journal of Biogeography. 2014

[7]Spring phenology of ecological productivity contributes to the use of looped migration strategies by birds. Proceedings. Biological sciences. 2014

[8]The ecology and evolution of avian migration systems. Journal of Avian Biology. 1982

[9]Perspectives on palaearctic and nearctic bird migration; comparisons and overview of life-history and ecology of migrant passerines. IBIS International journal of avian science. 1992

[10]The evolution of bird migration—a synthesis. Naturwissenschaften. 2007

[11]The evolution of avian migration systems between temperate and tropical regions of the new world. The American Naturalist. 1985

[12]Reflections across hemispheres: a system-wide approach to new world bird migration, The Auk. 2004

[13]The origin and development of bird migration: comments on Rappole and Jones, and an alternative evolutionary model. Ardea. 2005

[14]A discussion of the origin of migration. The Auk. 1904

[15]Process in the evolution of bird migration and pattern in avian ecogeography. Journal of Avian Biology. 2000

[16]Evolution of old and new world migration systems. Ardea. 2002

[17]Long-distance migration: evolution and determinants. Oikos. 2003

[18]The ecology of migrant birds—a neotropical perspective. Smithsonian Institution. 1995

[19]Evolutionary precursors of long-distance migration: resource availability and movement patterns in neotropical landbirds. The American Naturalist. 1992

[20] The evolution of bird migration—Adapted from Handbook of bird biology, The Cornell Lab of Ornithology. 2017

[21]How do birds prepare for long migrations? The Cornell Lab of Ornithology. 2009

[22]The Magnetite-based receptors in the beak of birds and their role in avian navigation. Journal of Comparative Physiology A. 2013

[23]The magnetic compass of European robins. Science. 1972

[24]Sensing magnetic directions in birds: radical pair processes involving cryptochrome. Biosensors. 2014

[25]Avian magnetic compass can be tuned to anomalously low magnetic intensities. Proceedings Biological Sciences. 2013

[26]Pigeons with a deficient sun compass use the magnetic compass. Science. 1981

[27]The effect of yellow and blue light on magnetic compass orientation in european robins, erithacus rubecula. Journal of Comparative Physiology A 1999

[28]Directional orientation of birds by the magnetic field under different light conditions. Journal of the Royal Society. 2010

[29]Red light disrupts magnetic orientation of migratory birds. Nature. 1993

[30] Light-controlled development and light signal transduction. SIPPE, SIBS, CAS

[31]Animal cryptochromes mediate magnetoreception by an unconventional photochemical mechanism. Nature. 2010

[32]Light-independent role of CRY1 and CRY2 in the mammalian circadian clock. Science. 1999

[33]Chickens' Cry2: molecular analysis of an avian cryptochrome in retinal and pineal photoreceptors. FEBS letters. 2002

[34]Resonance effects indicate a radical-pair mechanism for avian magnetic compass. Nature. 2004

[35]Social learning of migratory performance. Science. 2013

[36]Motorized Migrations: The Future or Mere Fantasy?, BioScience. 2003

[37]Climate-driven flyway changes and memory-based long-distance migration. Nature. 2021

[38]Endocrine Mechanisms of Migration. Bird Migration. 1990

[39]Arctic spring: the arrival biology of migrant birds. Acta Zoologica Sinica. 2004

[40]Strategic size changes of internal organs and muscle tissue in the Bar-tailed Godwit during fat storage on a spring stopover site. Functional Ecology. 2003

Produced by: China Science Expo x Zhihu

Author: Xinbo and his dried fish (Excellent answerer of Biology on Zhihu)

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

Please indicate the source of the public account when reprinting

Please indicate the source of the reprint. Reprinting without authorization is prohibited.

For reprint authorization, cooperation, and submission matters, please contact [email protected]