Migration Notes

Introduction to Movement and Migration

Movement and migration are vital aspects of animal life cycles. Bird migrations involve seasonal round trips between locations. Other migrations, like those of locusts, don't involve individual round trips, and some, like monarch butterfly migrations, take multiple generations to complete a round trip.

Migration Records

Migration distances can be substantial:

  • Leatherback Turtle: 20,558 km

  • Humpback Whale: Over 9,800 km each way

  • Monarch Butterfly: Up to 4,750 km

  • Arctic Tern: 70,900 km round trip

  • Bar-headed Goose: Altitudes of 9,000 m

Bar-Tailed Godwit Example

Satellite telemetry tracked a bar-tailed godwit (E7) flying 11,680 km non-stop from Alaska to New Zealand in nine days, averaging 56 km/h at 3000–4000 m altitude, losing over half its body mass. Migrations increase survival and reproductive success.

Definition of Migration

A broad definition of migration is movement from one habitat to another, triggered by various reasons like crowding or depleted resources.

Energetic Costs of Migration

Migration is energetically costly, needing preparation. Locomotion costs vary by medium (land, water, air). For example, a 100g mammal running 1 km uses 2.4 kJ of energy, while a similar-sized bird flying 1 km uses 0.68 kJ, and a fish swimming 1 km uses 0.24 kJ. Birds may store up to 50% of their body mass as lipid reserves before departure.

Bird Migration

Bird migration behaviours are valuable for studying migration processes. Not all bird species migrate, and some populations are residents year-round, while others show partial migration. Migration is common in seasonal environments due to food supply changes. Up to half of bird species (around five billion individuals) migrate annually between breeding and non-breeding areas, mainly on a north-south axis.

Migration Paths and Flyways

Migrants are categorized as short-distance (overland within continents) or long-distance (crossing continents, often with sea crossings). Long-distance migrants use flyways, which are migration corridors. There are three main global flyways: the Americas, the African-Eurasian, and the East Asian-Australasian.

Introduction to Selection

Natural selection requires variation within a population. Adaptation is central to evolution. Three types of natural selection:

  • Directional Selection: Favors one extreme phenotype.

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Disruptive Selection: Favors both extreme phenotypes.

Natural Selection Example: Finches

Peter and Rosemary Grant studied medium ground finches on Daphne Major. During a 1977 drought, small seeds decreased, and only finches with large beaks could crack larger seeds. This led to a rapid increase in average beak size in the finch population due to natural selection.

Evolution of Migratory Behavior in Birds

Migration evolved independently in different bird groups, indicating selective advantages. Migration should occur where it increases survival and breeding success. Birds migrate from high latitudes to warmer, lower latitudes in autumn due to food scarcity and shorter days. Selective advantages must outweigh the potential costs, such as mortality during the journey.

Variation in Migratory Behavior

Migration types include obligate (genetically controlled, consistent timing) and facultative (flexible response to conditions). Partial migration is common, with some individuals migrating while others don't. A continuous range exists between rigid and flexible migratory behaviours.

Genetic Control of Migratory Traits

Evidence suggests that birds have an innate capacity for migration. Studies on blackcaps (Sylvia atricapilla) show that migratory behaviour, including inclination, timing, distance, and direction, are under genetic control.

Changes in Migratory Behavior

Migratory behaviour is changing, including later departure, earlier arrival, and shorter distances. Temperature changes from global warming are implicated. Blackcaps have established a new wintering area in the British Isles, shortening migration distance. This is influenced by factors like human feeding and warmer winters.

Introduction to Homeostasis

Homeostasis is the ability of an organism to regulate its internal environment. Endotherms rely on internal metabolic processes, while ectotherms rely on external heat sources. Negative feedback mechanisms maintain a preset state. Metabolic rate is the energy used per unit of time, and the basal metabolic rate (BMR) is the minimum rate for basic functions. BMR is maintained within the thermoneutral zone.

Thermoregulation in Migratory Birds

Migratory birds have high metabolic rates and body temperatures. They need to assimilate and conserve energy. Before migration, birds increase food intake and store lipids. They avoid heat and cold stress. Some species can lower their body temperature to conserve energy. For example, blackcaps can lower their body temperature by up to 10 °C at night during stopovers.