Abstract
The functional life span of feathers is always much less than the potential life span of birds, so feathers must be renewed regularly. But feather renewal entails important energetic, time and performance costs that must be integrated into the annual cycle. Across species the time required to replace flight feather increases disproportionately with body size, resulting in complex, multiple waves of feather replacement in the primaries of many large birds. We describe the rules of flight feather replacement for Hemiprocne mystacea, a small, 60g tree swift from the New Guinea region. This species breeds and molts in all months of the year, and flight feather molt occurs during breeding in some individuals. H. mystacea is one to be the smallest species for which stepwise replacement of the primaries and secondaries has been documented; yet, primary replacement is extremely slow in this aerial forager, requiring more than 300 days if molt is not interrupted. We used growth bands to show that primaries grow at an average rate of 2.86 mm/d. The 10 primaries are a single molt series, while the 11 secondaries and five rectrices are each broken into two molt series. In large birds stepwise replacement of the primaries serves to increase the rate of primary replacement while minimizing gaps in the wing. But stepwise replacement of the wing quills in H. mystacea proceeds so slowly that it may be a consequence of the ontogeny of stepwise molting, rather than an adaptation, because the average number of growing primaries is probably lower than 1.14 feathers per wing.
Highlights
The feathers of birds have a functional life span that is always much shorter than the potential life span of adult birds
the innermost primary (P1) is nodal; this suggests that the primaries constitute a single molt series
The first is that the biology of flight feather replacement in H. mystacea has many interesting life history implications
Summary
The feathers of birds have a functional life span that is always much shorter than the potential life span of adult birds. For birds that were not growing primaries or secondaries when collected but that had two generations of feathers in their wings, each feather was scored as a 1 or 2 to indicate its replacement in the last (1) or next-to-last (2) episode of molting These data constituted the raw data table illustrated in Rohwer [23] and are not presented in this paper. We could not do this for H. mystacea because the replacement of flight feathers progresses so slowly that directionality scores required comparing a growing feather with its neighbors and scoring directionality on the basis of whether the neighbor was new or old With this annotated molt summary table we used various versions of the COUNTIF function in Excel to create the molt summary table. These interruptions generate the mechanism by which multiple waves of feather replacement that characterize stepwise molting are generated because, in all species where the ontogeny of multiple waves has been established, molt reinitiates both where it was arrested and again at primary one [16],[17]
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