Lucy's is the only warbler besides prothonotary to nest in cavities. It uses natural cavities in cactus or trees or holes excavated by woodpeckers or verdin in prior years. Unlike the prothonotary, Lucy's warbler has been known to utilize man-made nest boxes, even using a hollowed-out gourd. If using a woodpecker hole, the warbler may fill the cavity nearly to the top with debris and put the nest on top so the small birds can see outside of it. This species nests in some of the densest aggregations of any warbler, with as many as 12 pairs per ha/5 pairs per acre. The birds migrate to western Mexico in winter.