Woodstars are a clade of small hummingbirds with poorly known life history. One reason why they are insufficiently characterized is they tend to be little represented in museum collections. The Cornell Lab of Ornithology’s Macaulay Library, an online database of user-uploaded photographs, is a complementary source of phenological information. Here, stimulated by our own field observations, we analyzed up to 200 photos per species from the Macaulay Library to investigate whether adult male Slender-tailed Woodstar (Chaetocercus [Microstilbon] burmeisteri), Amethyst Woodstar (Calliphlox amethystina), Chilean Woodstar (Eulidia yarrellii), Peruvian Sheartail (Thaumastura cora), and Purple-collared Woodstar (Thaumastura [Myrtis] fanny) molt into a drab-throated, nonbreeding plumage distinct from the better known iridescent-throated breeding plumage. We investigated these 5 species because adult males have elongated, dimorphic tails, meaning that we could distinguish adult males from immature males by their tail morphology alone. The photos show that, post-breeding, entire populations of Slender-tailed Woodstar and Peruvian Sheartail males replace their iridescent throat feathers with non-iridescent feathers, then molt back into their breeding plumage a few months later. The holotype of Microstilbon insperatusTodd 1913, type species of the genus Microstilbon, is an adult male of Slender-tailed Woodstar in the nonbreeding plumage that we describe here. Our data also suggest that male Amethyst, Chilean, and Purple-collared woodstars have a second distinct nonbreeding plumage. In these 3 species, photos showing the apparent drab-throated male plumage were rare, suggesting these species might have complex breeding phenology with some individuals molting into the nonbreeding plumage at the same time that others were breeding in iridescent-throated plumage. The rapidly growing number of photographs available in the Macaulay Library makes this resource valuable for documenting phenology, including for rare species such as woodstars.

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