Black-throated Green Warblers (Dendroica virens) typically place their nests within the dense foliage of a limb or in a branch fork against the trunk of a living conifer. I report four unusual nests from Algonquin Provincial Park, Ontario, Canada: three in feeding cavities of Pileated Woodpecker (Dryocopus pileatus) in snags (i.e., dead trees), and one in a sugar maple borer (Glycobius speciosus, Coleoptera: Cerambycidae) scar in a tree of declining health. These data are the first documentation of this species nesting within cavities and enhance our understanding of the importance of snags and trees in declining health for wildlife.