earlier you had mentioned some migratory species, in which country/region is the first species commonly found?
Europe, Africa and Northern America region.
Swallow
The swallows, martins, and saw-wings, or Hirundinidae, are a family of passerine birds found around the world on all continents, including occasionally in Antarctica. Highly adapted to aerial feeding, they have a distinctive appearance. The term "swallow" is used colloquially in Europe as a synonym for the barn swallow. Around 90 species of Hirundinidae are known, divided into 19 genera, with the greatest diversity found in Africa, which is also thought to be where they evolved as hole-nesters. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are nonmigratory.
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. 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. can you name a few birds who migrate in the night? UNANSWERABLE
wiki:19571650
Swallow [SEP] Introduction
The swallows, martins, and saw-wings, or Hirundinidae, are a family of passerine birds found around the world on all continents, including occasionally in Antarctica. Highly adapted to aerial feeding, they have a distinctive appearance. The term "swallow" is used colloquially in Europe as a synonym for the barn swallow. Around 90 species of Hirundinidae are known, divided into 19 genera, with the greatest diversity found in Africa, which is also thought to be where they evolved as hole-nesters. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are nonmigratory.