High-intensity flight feather molt and comparative molt ecology of warblers of eastern North America

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 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.