what are the threats these species face during migration?
Since their travel involves crossing political boundaries of countries, it can put species at risk if any convservation measure is not put in place. Many spectacular migrants have already gone extinct due to this.
Bird migration
Human activities have threatened many migratory bird species. The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US. and the African-Eurasian Migratory Waterbird Agreement

The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct; during the passenger pigeon's ("Ectopistes migratorius") migration the enormous flocks were a mile (1.6 km) wide, darkening the sky and long, taking several days to pass.
when do birds start flying south for the winter It a regular seasonal movement, often north and south, undertaken in response to changes in food availability, habitat, or weather.  The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the autumn to wintering grounds in warmer regions to the south. can you name some of these migratory species? Swallows, wildfowl, the pink-footed goose and finch are some of them. how do these species navigate to a certain area? Navigation is based on a variety of senses. Many birds have been shown to use a sun compass. Some also use a combination of skills like, the ability to detect magnetic fields or use visual landmarks as well as olfactory cues. can you give me an example of how the studies were carried out for their migration? Some of the techniques include, marking the specie, radar and satellite tracking.
wiki:19526711
Bird migration [SEP] Threats and conservation
Human activities have threatened many migratory bird species. The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US. and the African-Eurasian Migratory Waterbird Agreement The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct; during the passenger pigeon's ("Ectopistes migratorius") migration the enormous flocks were a mile (1.6 km) wide, darkening the sky and long, taking several days to pass.