Questions 1-2 are about the following passage.

    Bird vocalization is the term used to describe the sounds that birds make. Them are many different types of bird vocalization, but they are mainly divided into two general categories: songs and calls. They are distinguished from one another by length, inflection, and context.

   Birds make these sounds for a variety of reasons. Songs which are relatively long and more continuous usually indicate mating, while short and simple calls are often used for alerting the flock of danger and keeping the flock together. Commonly, the male bird is the one that delivers songs in order to attract the female. It is very likely that the quality of songs is connected to a bird's genetic health. Calls are used as alarms to warn of predators or bad weather. These alarms can be understood not only by birds of the same species, but also other birds and animals. Sometimes calls are used for finding individual birds, evident from adult birds that use calls to find their chicks.

   There are many other bird sounds that have unclear purposes. For example, "duetting" is the term for when two birds call at the same time using the same sounds. Some species of birds can sing so in sync that it is impossible to distinguish the two birds. Scientists present numerous theories, but the reason why certain birds do duetting remains unclear.