Intervening obstacle is an environmental or cultural feature that hinders migration Intervening opportunities Intervening opportunities are the nature and number of possible alternative migration destinations which may exist between place A (migration origin) and place B (migration destination).
Intervening obstacles are factors that cause migrants 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.
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.
Sources:
Rashid's Blog: Portal for Inquisitive Learners 