Intervening obstacles are factors that cause migrating animals challenges or prevent them from reaching their goal. Examples of intervening obstacles include mountains, forests, deserts, cities and bodies of water. Some of these barriers block the migration of some species, while they do not slow down other specie at all. For example, a large desert may prevent amphibians or insects from completing their migration, while birds fly right over it.
Birds are some of the most well known migrants, and they are especially skilled at the practice. Because most birds fly so well, they can simply go over most intervening obstacles. However, not all birds can cross large oceans, and many are limited to places which are accessible via land. Mountains can also provide significant barriers to birds that cannot fly over them. Deserts, cities and rivers are important barriers to mammalian migrations, and often mammals must travel much further to go around such obstacles.
Humans may encounter intervening obstacles during migratory travels as well. Deserts and mountains are particularly difficult for humans to traverse, while forests and cities are easy for humans to cross. Over time, humans often constructed settlements near such challenging features, as a last stop in which travelers could resupply and rest before enduring difficult travels.