Phylogeny: Migrate or not to migrate, is that the question?

An instance of the phylogeny of Arctic Terns can be found here. This was published in 1990 by Sibley and Ahlquist, using DNA hybridization data. It provides a rough idea of the relationship between 32 bird species.

Most birds migrate due to the change in food availability. Most migratory birds tend to be insectivorous, the reason being that availability of their major food source tends to fluctuate dramatically between the summer and the winter seasons.

It should be noted that the populations of insects and other bird foods are heavily dependent on the seasonal changes, this is particularly evident at the extreme ends of the temperate regions, the  Arctic and Antarctica.

Fish eating birds such as the Arctic Tern are affected mainly when the water bodies from which they feed are frozen during the winter months.

One factor that may have greatly affected bird migration, particularly temperate birds. is the last major ice age which ended about 15,000 years ago (Ref**). It has been theorized that a lot of bird species that are being studied today are still evolving their migratory routes in response to that natural event.  Given the situation of the ice age, which involved polar ice sheets expanding south and covering a great deal of the Northern hemisphere continents, such as Europe and North America, it is possible to see how Arctic Terns evolved migration to move so far down south. These birds moved so far down south in order to avoid the arctic winters as much as possible.

Migration may have evolved in temperate birds in general, partially in response to the pressures of predators. During the warm seasons, in addition to the abundance of food sources such as fruits, insects and fish, there is the presence of many predators, that prey on the young or eggs of these birds and at times the birds themselves. This is evident from the case where certain birds of prey such as large hawks (Ref**) migrate in order to follow their food. Migration may have developed to get away from a higher population of traditional predators

It is believed that evolution may have also developed as a quick response mechanism to drastic changes in the local weather conditions such as the development of the last major ice age. Research conducted by Peter Berthold on migration of a certain species of Blackcaps found that the behavior  can develop between 4 to 6 generations (Ref**). This shows that migration is a trait that allows birds to move or remain sedentary, depending on which is more beneficial. Whichever phenotype in the environment has the greatest fitness will become predominant in the population. However, the population has the ability to change from sedentary to migratory if the climate dictates it.

The main evolutionary basis for Arctic Tern migration and bird migration in general lies in the fact that it has to be genetic. The genetic link is most evident in the way birds choose their routes and also site fidelity. Year after year, the same bird is able to return to within a few meters of its breeding or wintering grounds. The theory is that since birds are able to do this repeatedly and in some cases are able to achieve this as newly fledged adults without the help of their parents, shows that theres is some map of the route and/ or location passed on genetically from one bird to another.