Jack pine cones are largely serotinous, sealed with resin that melts in fire, so historically stand-replacing fires regenerated dense young jack pine across the sandy outwash plains of northern Michigan. Kirtland's warbler nests on the ground beneath these young trees, where the living lower branches provide cover. Once the stand grows past roughly twenty years, those lower branches die back and the birds abandon it.
Twentieth-century fire suppression closed the supply. Combined with brown-headed cowbird parasitism, the population fell to a low of around two hundred singing males in the 1970s and 1980s. Recovery required two simultaneous interventions: large-scale harvest and replanting of jack pine on a rotation that keeps a constant supply of suitable-age stands, and sustained cowbird trapping.
The count recovered past two thousand singing males and the species was delisted in the United States in 2019. The important qualifier is that this is a conservation-reliant species. The habitat is manufactured on a rotation; if the planting programme stops, the habitat ages out and the bird has nowhere to nest.