I thought I was taking a photo of a wood thrush, turns out it was an ovenbird.
Rondeau Provincial Park.
Neighboring male Ovenbirds sing together. One male starts singing, and the second will join in immediately after. They pause, and then sing one after the other again, for up to 40 songs. The second joins in so quickly that they may sound from a distance as if only one bird is singing. Ovenbirds rarely overlap the song of their neighbors.
source - Cornell Lab of Ornithology