The decline of the Kirtland’s warbler population was due to the loss of its breeding habitat, caused by reduced wildfires (which decreased the growth of young jack pines) and habitat modification. Kirtland’s warblers are closely associated with young jack pine trees: They build their nests under the lowest branches, which conceal the birds from predators. Jack pine reproduction relies on periodic wildfires, which was inhibited by widespread fire suppression during the 20th century.