Resolving the mystery of Cyanerpes (Thraupidae) ‘black eggs’, with notes on the breeding and exaggerated displays of Hypocnemoides antbirds (Thamnophilidae)

Between the end of the nineteenth and the middle of the twentieth centuries intermittent reports of ‘black eggs’ belonging to one or more species of Passeriformes in South America were usually attributed to small-bodied tanagers of the genera Cyanerpes and Chlorophanes. Doubts concerning these identifications solidified in the 1950s, but none of the authors concerned was able to provide a definite alternative, although it was suggested that an unknown tyrannid might be involved, especially as the nests were closed bag-shaped structures, much like those of many Tyrannidae. Following a revision of relevant material in museum collections, as well as published and our own, previously unpublished, observations of nests and eggs of the two species of Hypocnemoides antbirds, we conclude that all of the available data suggest that reports of ‘black eggs’ must have involved Black-chinned Antbird (Hypocnemoides melanopogon) and Band-tailed Antbird (Hypocnemoides maculicauda), mainly due to their distinctive nest architecture and unique egg colouration. We also provide novel information on their ‘exaggerated’ displays, reproduction and foraging behaviour of the genus.