High-intensity Flight Feather Molt: A Cryptic but Critical Phase of the Annual Cycle of Migratory Warblers

By Ronald L. Mumme

Linked paper: High-intensity flight feather molt and comparative molt ecology of warblers of eastern North America, by Ronald L. Mumme, Robert S. Mulvihill, David Norman, Ornithology.

The lives of migratory songbirds inhabiting the north temperate zone are built around three dramatic and energetically demanding phases of their annual cycle: spring migration, a short nesting season on the summer breeding grounds, and fall migration to the winter range. Migration and nesting have intrigued and inspired countless professional and amateur ornithologists, and have been the focus of thousands of scientific studies. 

But for most north temperate songbirds, a fourth critical phase of their annual cycle, one that is just as demanding but far less conspicuous and appreciated, is usually sandwiched between breeding and fall migration: molt and replacement of the plumage. Molt requires considerable investments of energy and protein synthesis and can have long-lasting consequences, as the feathers grown during molt will affect flight, insulation, and appearance for an entire year. How do migratory songbirds manage and resolve potential conflicts between breeding, molt, and fall migration? 

One solution, frequently employed by long-distance migrants breeding at high latitudes, is to molt as rapidly as possible, balancing the possibility of an extended breeding season (at the beginning of the molt) with a near-optimal onset of fall migration (at the end). Rapid molt in birds is usually achieved not by increasing the rate at which individual feathers grow, but by increasing molt intensity, the number of feathers being replaced simultaneously. However, rapid high-intensity molt of the flight feathers — the long feathers of the wing and tail that provide flying birds with lift, thrust, and maneuverability — can have detrimental consequences for flight performance, which in turn can affect the ability of molting birds to avoid predators, obtain food, and provide parental care to their late-season nestlings and fledglings.

We examined high-intensity molt of the flight feathers in 13 species of migratory warblers captured during summer and fall mist-netting and banding operations at Powdermill Avian Research Center (40º N latitude) in southwestern Pennsylvania. We found that molt in these species is extraordinarily intense; all 13 species replaced their 12 tail feathers simultaneously, with the onset of rectrix molt occurring in the early-middle stages of high-intensity molt of the primary flight feathers of the wings. At peak molt intensity, individuals were replacing 50–67% of their 48 flight feathers — all 12 rectrices and 6–10 remiges on each wing — for 2–3 weeks or more. Molt of this intensity is likely to present numerous challenges for flight, avoiding predators, foraging, and parental care, further complicating the tradeoffs that individual warblers must make among the competing demands of late-season reproduction, molt, and migration. 

An early-molting second-year male American Redstart captured at Powdermill Avian Research Center 13 June 2005. This bird was replacing 30 (63%) of his 48 flight feathers when captured, including all 12 rectrices and 18 remiges — 5 primaries, 1 secondary, and all 3 tertials on each wing. Photo by Robert S. Mulvihill.
Figure 1. Intensity of flight feather molt in warblers of eastern North American in comparison to 44 intensity values from 35 species reported by Vanya and Sievert Rohwer in 2013.

The exceptional intensity of flight feather molt in warblers is beautifully illustrated by comparison to data on molt intensity collected by the son-father team of Vanya and Sievert Rohwer and published in The Auk: Ornithological Applications in 2013. The intensity of flight feather molt in the 13 warbler species we investigated is greater than all, or all but a few, of the 44 measures of molt intensity recorded for 35 songbird species by the Rohwers (Figure 1).

Our findings indicate that high-intensity molt of the flight feathers is widespread in the warblers and constitutes an unusually challenging phase of their annual cycle, one that has significant implications for the timing of reproduction and migration in these colorful and much-loved songbirds. Nonetheless, we suspect that relatively few field ornithologists and birders, even those who have spent considerable time afield in eastern North America during late summer and early fall, have seen warblers during the period of peak molt intensity. Warblers at this stage of molt tend to keep quiet and are furtive and retiring, often confining themselves to areas of dense vegetation that might be far removed from their breeding territories. On the infrequent occasions when molting warblers are encountered, they do not make attractive subjects for birders and photographers and are unlikely to command the full attention of anyone seeing them. Consequently, the period of high-intensity molt, although widespread among the warblers and clearly unusually intense and demanding, is a cryptic and neglected part of the annual cycle, one that merits further attention.

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