Serial Descendant Primary Molt or Staffelmauser in Black-Crowned Night-Herons

Abstract

Primary molt in Black-crowned Night-Herons (Nycticorax nycticorax) has been described as irregular. However, through an examination of museum specimens we determined that the molt of juvenal primaries progresses as a single descendant wave as primaries molt in sequence from primary 1 to primary 7 or 8. Molt then skipped to primary 10, the terminal primary, in most birds before the skipped primary or primaries were molted. The molt of juvenal primaries was arrested in 56% of specimens and a median of eight primaries was replaced (range 2-10). In contrast to the molt of juvenal primaries, 88% of adults had at least two descendant waves (range 1-4) of molt simultaneously active in primaries. Adults replaced from 2-10 primaries during a period of molt. The number of primaries replaced was bimodal with modes at 3.5 and nine. We were unable to track the transition from the single wave of molt evident in juvenal primary molt to the serial descendant waves in the adult primary molts. However, our findings for the juvenal and adult molts indicate that primary molt of Black-crowned Night-Herons is similar to serial descendant molt (Staffelmauser) in other species. Typically, serial waves in the adults arise through the arrest and continuation of descendant waves generated from a proximally located molt center. The variation in the number of primaries replaced by night-herons is evidence that the waves that typify serial descendant molt are simply a consequence of molt arresting due to time constraints on the length of the molt period. On the basis of available data for night-herons, serial descendant molt does not minimize the aerodynamic impact of primary molt or equalize the frequency of primary replacement.