The population could be reduced where the migrants are moving from.
unsuitable food , environment ,poaching,etc .....
Gazelles are grazing animals, so they acquire food by migrating around their habitat and eating whatever green material they can find.
I was taught in college that some animals, such as birds, geese, are guided by magnetic forces, not forcing them to go this way, but guiding them
some animals need to migrate because of weather conditions or food problems
Animals can adapt to a food shortage in various ways, such as reducing their activity levels to conserve energy, changing their diet to consume different types of food, migrating to areas with more abundant food sources, or entering a period of dormancy. Some animals may also engage in behaviors like hoarding, scavenging, or hunting in groups to improve their chances of finding food.
Wolves.
Nomad's traveled in order to get food. As they were hunter-gatherers, they had to follow the migrating animals that they hunted, and the plants that only grow in season. If they had stayed in place, they would have starved from lack of game animals.
Temporary structures and a migrating food source.
Reverse migrants are animals that can go from salt to fresh water while migrating. One reverse migrant is a salmon.
Most likely bears and animals like that.
non uniform motion