Rapid high-intensity molt of flight feathers occurs in many bird species and can have several detrimental consequences, including reductions in flight capabilities, foraging performance, parental care, and plumage quality. Many migratory New World warblers (family Parulidae) are known to have intense remigial molt, and recent work has suggested that simultaneous replacement of the rectrices may be widespread in the family as well. However, the phylogenetic distribution of simultaneous rectrix molt, and high-intensity flight feather molt more generally, has not been systematically investigated in warblers. We addressed this issue by examining flight feather molt in 13 species, representing 7 different warbler genera, at Powdermill Avian Research Center in southwestern Pennsylvania, USA. All 13 species replaced their 12 rectrices simultaneously, with the onset of rectrix molt occurring in the early-middle stages of high-intensity primary molt. As expected, single-brooded early migrants molted earlier than double-brooded species whose nesting activities extend into late summer. However, our finding that late-molting species replaced their primaries more slowly and less intensively than early molting species was unexpected, as late-molting species are widely hypothesized to be under stronger migration-related time constraints. This surprising result appears to be at least partially explained by a positive association between the pace of molt and daylength; shorter late-summer days may mandate reduced daily food intake, lower molt intensity, and a slower pace of molt. In comparison to other passerines, flight feather molt in warblers of eastern North America is extraordinarily intense; at its peak, individuals are simultaneously replacing 50–67% of their 48 flight feathers (all 12 rectrices and 6–10 remiges on each wing) for 2–3 weeks or more. Because molt of this intensity is likely to present numerous challenges for flight, avoiding predators, foraging, and parental care, the period of flight feather molt for warblers constitutes a highly demanding phase of their annual cycle.
Most birds undergo an energetically demanding annual molt in which they shed and replace all their flight feathers—the long feathers of the wing and tail that provide thrust, lift, and maneuverability.
Rapid high-intensity molt, in which birds replace multiple flight feathers simultaneously, may be necessary in migratory species that are under strong migration-related time constraints but can seriously compromise flight capabilities.
We examined the intensity of flight feather molt in 13 species of warblers that breed in eastern North America and undergo a complete late-summer molt before beginning their fall migration to the tropics and subtropics.
We found that flight feather molt in warblers is extraordinarily intense; during its peak, individuals are replacing 50–67% of their 48 flight feathers simultaneously, an intensity that is among the highest reported for any songbird.
Because molt of this intensity presents clear challenges for flight, foraging, avoiding predators, and caring for offspring, the late-summer molt constitutes a brief but highly demanding phase of the annual cycle for these colorful migratory songbirds and a phase that warrants more attention from field ornithologists.