Mind The Gaps: What's Missing in Our Understanding of Feather Molt

1E-mail: ebridge@ou.edu Manuscript received 15 November 2010; accepted 5 January 2011. The life histories of all birds feature two major events, reproduction and molt, and many species of the temperate and polar zones add migration as a third major energetic and scheduling demand to their annual cycle. Although we are far from a complete understand of any of these phenomena, it seems that for a given species basic molt data are most often lacking. Open almost any field guide and you will find information on the timing and location of breeding and migration for almost all species included. For information on molt one must resort to more detailed species descriptions such as the Birds of North America series or the works of Pyle (1997, 2008), and even these species accounts may lack adequate data. If your species of interest is not included in a series that has descriptions of molt, you will probably find that there are no data at all. A few years ago I reviewed molt strategies in seabirds and found no information on molt for 25% of the 314 species I wanted to include in the review (Bridge 2007). The situation for terrestrial species is similar (Ryder and Wolfe 2009). Birds demonstrate a bewildering array of molting strategies. The diversity of avian molts is such that an active debate over molt terminology continues more than 100 years after the first systematic studies of feather replacement (Dwight 1900, Howell et al. 2003, Thompson 2004, Willoughby 2004, Pyle 2005). For almost every rule we try to impose on the patterns or timing of molt, there are invariably several exceptions. However, one rule that does appear to apply to all species is that there is a physiological limit on the rate at which feathers can grow (Rohwer et al. 2009). All of the raw materials used to synthesize a feather must be conducted through a collar of cells that surrounds the base of the feather follicle (Lucas and Stettenheim 1972). Presumably, this bottleneck limits the rate of individual feathers’ growth regardless of their size when fully grown. If we look across species ranging in mass from a few grams to several kilograms, there is little difference in the rate at which feathers are grown (Rohwer et al. 2009). For instance, a 27-g House Sparrow (Passer domesticus) grows its feathers at a rate of 2.7 mm per day, whereas a 9000-g Whooper Swan (Cygnus cygnus) grows 9 mm of feather per day (Prevost 1983). In other words, we see a 300-fold increase in body mass but only a 3-fold increase in feather-growth rate. This constraint on feather growth is evident when birds are pressured to accelerate molt. Across several taxa, speeding up molt results in feathers of low quality according to several metrics, including mass (Dawson 2004), integrity (Dawson et al. 2000, Serra 2001), and coloration (Serra et al. 2007, Griggio et al. 2009). The effect of rapidly grown plumage has been linked to decreased survival (Nilsson and Svensson 1996, Morales et al. 2007). The limit on feather growth gives rise to some interesting life-history tradeoffs as birds balance the various costs of molt and other activities (e.g., breeding and migration). We often see the most complex molt strategies in large birds, in which the time constraints associated with growing large feathers are most pronounced. For large birds like albatrosses that must not only molt long feathers but maintain their ability to fly over vast distances, molt imposes a heavy toll, which is likely to affect other aspects of their annual cycle, such as breeding. More specifically, one might presume that large, aerial foragers should be forced to reduce their molting effort to breed successfully and vice versa. However, it has been difficult to conclusively identify a clear link between variation in molt and breeding success in large birds. The first study to do so appears in this issue of the Condor. Rohwer et al. (2011) found that Black-footed Albatrosses (Phoebastria nigripes) with more worn feathers are more likely to be unsuccessful in their attempts to breed. The problem for the Black-footed Albatross is that there is insufficient time in its annual cycle for it to both breed and molt enough feathers to offset accumulated feather wear, and as a result some individuals accrue a molt debt that eventually erodes their ability to breed successfully. Hence, periodically birds skip breeding altogether to devote more time to growing feathers and paying off their molt debt. In the Black-footed Albatross, the molt debt is usually borne out in primaries 6 and 7 (P6 and P7). These feathers may be molted just prior to the outermost three primaries, or they may remain unmolted for several years, presumably as a means of reducing the time devoted to molt (Langston and Rohwer 1996; Fig. 1). A key finding in Rohwer et al. (2011) is that if P6 and P7 accumulate 2 or more years of wear, the birds are less capable of rearing offspring. Although Rohwer et al. (2011) have made an important contribution to the ornithological literature, their work also makes evident some of the gaps in our knowledge of feather molt. Why should something as obvious as a molt/breeding tradeoff only now find empirical support? One reason is that, for many species, we lack comprehensive information about feather molt. Without documentation of basic molt data such as timing, location, sequence, intensity, completeness (for species with incomplete molts), and degree of individual variation, comparative studies aimed at describing the origin and maintenance of molt strategies The Condor 113(1):1–4 The Cooper Ornithological Society 2011 COMMENTARY