When the ice age comes, under what circumstances will animals begin to be forced to migrate?

Modern biologists believe that the fundamental reason for animal migration is natural selection. Migratory species can leave more offspring than non-migratory species. So, what makes the migrating animals reproduce successfully? First of all, migration can enable animals to use resources that do not exist at any time in various habitats. There is a long sunshine time in summer in the Arctic and Antarctic, and invertebrates, both insects on land and krill in the sea, exist in large numbers in a short time, which is extremely beneficial to the reproduction of many birds and whales that feed on them, but in winter, the food they depend on is reduced; These animals can't live in the polar regions, so they must migrate. Blue whales, right whales and humpback whales spend the summer in Antarctic waters from 65438+February to March of the following year, and then move northward to tropical and subtropical waters for the winter. Another example is the swallow; According to statistics, 3 1 species of swallows are resident birds on the African continent. Every year, two species of swallows migrate from the Mediterranean to Africa and three species of swallows migrate from Western Europe. Because flying insects disappear in the air in the north temperate zone in winter, it can be safely speculated that the ancestors of these three migratory swallows had previously bred in the African continent. After a long evolution, they moved to the north to breed, where they met fewer food competitors, which could increase the time of catching insects during the day; Fewer predators attack eggs and chicks. These factors enable migrating swallows to breed successfully. It is estimated that the number of swallows migrating to Africa every year may be more than all the birds in Africa. Migration makes it more possible for animals to make use of those changeable and short-lived food resources and win in natural selection. In addition, there are some special examples: iguanas living in Central America have to wade to an island to breed; Because it is safer to breed there. The migration of Australian lizards is more interesting. They live in the granite exposed area in the desert, and the larger granite exposed area is their most suitable breeding ground. When spring comes, new lizards will move into these areas to breed, while young lizards will be pushed out to live on smaller granite bare land. Wait until they are mature, and then return to the most suitable breeding ground. Therefore, in the most suitable breeding sites, regular immigration and emigration are ongoing. Once animals are in conditions suitable for migration, they will start to migrate as long as they have some environmental factors that induce migration. Black fish and locusts wait for good weather conditions to start, and fish wait for the right tide; The migration of aphids must wait until the temperature reaches a certain value.