Anomalous Double-Clutch Laying in the Black Vulture (Coragyps atratus): Replacement Eggs or Biochemical Alterations of the Breeding Cycle?

We describe the first documented case of anomalous double-clutching behavior in Coragyps atratus (Black Vulture) in northeastern North America. The female laid 2 eggs, which hatched successfully, and laid an additional clutch of 2 eggs, probably post-hatching, which it did not incubate, but were eventually consumed by the adults. Although we cannot determine the reason for this rare nesting behavior, the possibility of a double-brooding attempt was unlikely due to the timing of egg laying and the eggs not being incubated, yet we are not able to discard the possibility of an event of intraspecific nest parasitism. Since egg-laying is expensive in terms of energy, and may compromise adults' future reproduction and survival, this redundant expenditure suggests that (1) this pair of Black Vultures may have a resource-rich environment, and/or (2) the food availability derived from anthropogenic byproducts might be altering the biochemical cycles of reproduction timing. Further research is needed to ascertain whether the recent range expansion of Black Vultures in North America due to human activity may hide a threat that could compromise its populations in the future.