Sept. 19 (UPI) -- North America's bird population has declined by nearly 3 billion in the last five decades, according to a study published Thursday.
"Species extinctions have defined the global biodiversity crisis, but extinction begins with loss in abundance of individuals that can result in compositional and functional changes of ecosystems," the study states.
Researchers analyzed range-wide population estimates of 529 species of birds and included biomass estimates from weather radar to track birds during their biannual migrations in order to calculate how populations have changed.
The decline was most severe in birds found in grasslands such as sparrows, warblers, blackbirds and finches as their populations fell 53 percent in the past half-century.
Nearly three-quarters of all grassland species are experiencing a decline in population as these areas are prone to habitat loss and exposure to toxic pesticides in part due to the presence of farmlands.
"You only need to fly across the country to see that we've drastically changed the face of the earth," said Peter Marra, senior other of the study and director of the Georgetown Environment Initiative at Georgetown University.
Use of pesticides has harmed birds by contaminating the seeds they eat, which causes them to lose weight and makes it more difficult to migrate. It has also led to a decline in the insect population, removing birds' primary source of prey.
Window collisions also result in about 600 million bird deaths each year, while cats hunt down as many as 4 billion birds annually.