Habitat loss throughout North, Central and South America has had the greatest impact on their populations. In North America, large tracts of fields, forests and wetlands are disappearing as the result of development. Many songbirds nest and breed in the forest. Today, instead of the protection of the dense forest interior, many birds are forced to nest in smaller forest fragments and thinner edge habitat. This leaves eggs and chicks more vulnerable to predation by other birds, such as blue jays and crows, and mammals, such as raccoons and cats.
Fewer chicks survive to replace adults killed by natural causes.
Those that do must survive the often-perilous winter and migrations between North and South America. The birds' winter habitats in Central and South America are also being altered and are disappearing, in some cases, faster than breeding habitats. One way that researchers track birds is through the Breeding Bird Survey, conducted since 1966. The Breeding Bird Survey documents the number of each species noted along specific survey routes. By analyzing the data, researchers can tell whether a species is declining or not.