Why do some birds & animals migrate?

Hint: Migration: Migration is a process through which animals move from one environment to another for breeding, searching for foods etc.
 Birds mainly migrate for breeding or in search of food due to environmental factors like weather, season etc.
Animals migrate in search for suitable shelter, food and suitable place for breeding.

Complete answer :
1. ANIMAL MIGRATION : Migration is a behavioural adaptation that helps animals to survive. Some animals travel relatively short distances to find food or more favourable living or breeding circumstances. Most animals who migrate do such because of seeking food or more livable conditions. And also there are some animals who migrate to breed.
2. BIRD MIGRATION : Birds mainly migrate for breeding or in search of food due to environmental factors like weather conditions and season. Birds that nest in the northern hemisphere tend to migrate northward in the spring season to take advantage of burgeoning insect populations, budding plants & abundance of nesting locations. As winter approaches & the availability of insects & other food sources drop, the birds move to the southern region again.

Note :
Birds and animals travel from their own habitat to a different habitat due to several reasons at a certain period of the year.
This phenomenon is called migration and the species that migrate are called migrators. They migrate to avoid suffering from thrilling heat or cold that imparts a tremendous impact on the availability of food for certain species.
Birds and animals leave such harsh conditions and look for some different habitat where they can stay comfortably and have food to survive. They also migrate to find a suitable habitat to lay eggs and raise their young due to improper and inhospitable conditions in their natural habitat.