Spruce grouse decline in maturing lowland boreal forests of New York

•Spruce grouse (Falcipennis canadensis) are endangered in NY.•Numbers of occupied patches declined 25% (71% in extent) over the last 30years.•Older patches of conifer tended to be devoid of spruce grouse.•Larger patches closer to colonizing sources tended to be occupied.•Maintaining areas with mid-successional conifer near occupied areas is critical.