Question

Describe the nature of migration in birds. Include in your answer: a. a definition of migration b. descriptions of the hypotheses for the origin of migratory patterns and the evidence in support of its northern origin c. the evidence for an endogenous, underlying genetic component for migratory behavior

Answer 1

• Nature of Migration in birds:

1) Seasonally: the timing of migration is influenced by internal “clocks” that are influenced by day length and perhaps also weather. This topic was discussed in the last lecture.

2) Over shorter time scales: the exact timing of migratory movements is greatly influenced by the weather. Birds tend to move when conditions favor flying in the direction they need to go e.g., when they have a tailwind, when air turbulence is low, when it is not raining.

3) Many birds migrate at night: There are several possible reasons for this.

(a) The atmosphere is more stable at night than during the day because there are fewer thermals “updrafts” caused by warming of the Earth’s surface.

(b) The air temperature is lower, which may make thermoregulation easier; flight generates a lot of heat that birds need to offload.

(c) Predation risk may be lower than during the day.

4) But some species do migrate by day: In particular, large soaring birds such as hawks, storks and pelicans move during the daytime. These birds use thermals to help them travel.

• Defination of Migration: it is the large-scale movement of members of a species to a different environment.it is usually a response to changes in temperature, food supply, or the amount of daylight, and is often undertaken for the purpose of breeding. Mammals, insects, fish, and birds all migrate
• There are many hypothesis for the oorigin of migratory pattern:

1.The dominance hypothesis states competition for food, According to this hypothesis, dominant individuals compete for scarce food resources during winter months; thus, they are able to stay on breeding grounds year round, while subordinates migrate to areas with reduced competition for food.

2.The arrival‐time hypothesis predicts that the sex whose fitness is more limited by intra‐sexual competition for mates will be more likely to food‐related benefits of migrating in exchange for enhanced reproductive benefits associated with remaining on the breeding grounds.

3.The body‐size hypothesis postulates that inclement weather makes year‐round residency a risky strategy, regardless of food availability. Because larger and healthier individuals can fast longer than smaller individuals due to greater fuel reserves relative to their metabolic rates, only larger individuals should risk starvation during storms, foregoing weather‐related benefits of migrating.

Thus, identifying if migratory tendency is related to age, sex, or condition is critical to determine that the migration is a fixed or conditional strategy,  hypotheses, provides insight into the nature of the trade‐offs associated with migratory decisions.

Birds migrate to move from areas of low or decreasing resources to areas of high or increasing resources. The two primary resources being sought are food and nesting locations.

Birds that nest in the Northern Hemisphere tend to migrate northward in the spring to take advantage of burgeoning insect populations, budding plants and an abundance of nesting locations. As winter approaches and the availability of insects and other food drops, the birds move south again. Escaping the cold is a motivating factor but many species, including hummingbirds, can withstand freezing temperatures as long as an adequate supply of food is available

• The evidence for an endogenous, underlying genetic component for migratory behavior:

Migrants have yielded extensive evidence that internally-generated factors exist which shows the endogenous control of migration in typical migratory birds. The most important endogenous components that have been detected in migrants.Following aspects, have been traced in part to a genetic or hereditary basis.

1. The internal annual 'clocks' or so-called 'circannual rhythms' (from 'circa' and 'annual)

2. The timing patterns involved in the detailed course of migration for particular species, or particular populations within species

3. Innate directions of migration.

4. Mechanisms controlling juvenile development and of body features (such as wing shape) associated with migration.

5. Factors involved in habitat choice during halts on migration.

6. The innate migration urge in species where only some individuals migrate.