9: Well big groups of birds are often found competing for food, causing fights and even death due to the fight. If a flock of birds from population A goes to population B, there would be more competition due to birds wanting food and nesting places that are good.
10: Many species evolve to suit environmental changes, but let's say this change in the environment was bad. Pollution, species of aggressive animals, which would eat the food the animals need and cause a downgrade in food leaving the area bare, and hunting, which would bring humans to the area and cause them to camp out, eat the animals and wild vegetables, such, can cause negative population changes. Due to these things birds could get sick and die off, causing population to go down. If trash and sewage are not cleaned up it could affect the birds food source and cause it to be bad for a long time, the aggressive animals would kill and eat the birds causing a continuous spiral of death, and hunting could cause them to die.
