Aggressive nest intrusions by male Humboldt Penguins

During observations at a breeding colony of Humboldt Penguins (Spheniscus h umboldti), we noted five male penguins enter nests occupied by breeding pai rs and engage in aggressive interactions with the resident adult. These nes t intrusions resulted in egg loss or infanticide, and accounted for 11.1% o f breeding failure in the colony. Two intruder males ultimately mated with respective resident females. This behavior had not been observed previously in Humboldt Penguins and we suggest that it may occur when males are unabl e to obtain mates through advertising at the nest. Intrusion behavior may s low down population recovery in this endangered species.