Abstract
The majority of bird species studied to date have molt schedules that are not concurrent with other energy demanding life history stages, an outcome assumed to arise from energetic trade-offs. Empirical studies reveal that molt is one of the most energetically demanding and perplexingly inefficient growth processes measured. Furthermore, small birds, which have the highest mass-specific basal metabolic rates (BMRm), have the highest costs of molt per gram of feathers produced. However, many small passerines, including white-plumed honeyeaters (WPHE; Lichenostomus penicillatus), breed in response to resource availability at any time of year, and do so without interrupting their annual molt. We examined the energetic cost of molt in WPHE by quantifying weekly changes in minimum resting metabolic rate (RMRmin) during a natural-molt period in 7 wild-caught birds. We also measured the energetic cost of feather replacement in a second group of WPHEs that we forced to replace an additional 25% of their plumage at the start of their natural molt period. Energy expenditure during natural molt revealed an energy conversion efficiency of just 6.9% (±0.57) close to values reported for similar-sized birds from more predictable north-temperate environments. Maximum increases in RMRmin during the molt of WPHE, at 82% (±5.59) above individual pre-molt levels, were some of the highest yet reported. Yet RMRmin maxima during molt were not coincident with the peak period of feather replacement in naturally molting or plucked birds. Given the tight relationship between molt efficiency and mass-specific metabolic rate in all species studied to date, regardless of life-history pattern (Efficiency (%) = 35.720•10−0.494BMRm; r2 = 0.944; p = <0.0001), there appears to be concomitant physiological costs entrained in the molt period that is not directly due to feather replacement. Despite these high total expenditures, the protracted molt period of WPHE significantly reduces these added costs on a daily basis.
Highlights
Variations in life-history patterns are thought to represent a given species’ maximization of lifetime inclusive fitness within a particular environment [1,2]
The coincidence of molt/breeding overlap in individuals of some species, but not in others, provokes the question: Is molt an inherently costly and inefficient process, or are costs dependent upon other life history characteristics? We examined these questions by measuring molt costs in the White-plumed honeyeater (WPHE; Lichenostomus penicillatus), an old-endemic Australian species that includes populations inhabiting the arid zone
We found that energetic expenditure during natural molt in white-plumed honeyeaters (WPHE) was surprisingly similar to expectations based on birds from more predictable north-temperate environments, but that these increases in energy expenditure were not coincident with the peak period of feather replacement in naturally molting or plucked birds
Summary
Variations in life-history patterns are thought to represent a given species’ maximization of lifetime inclusive fitness within a particular environment [1,2]. Partitioning of molt from other life history stages is assumed to be an adaptation that minimizes physiological stress while maximizing the allocation of productive energy [16]. A bird must synthesize almost one-quarter of its total body protein in the form of feathers and other epidermal structures [17,18]. This places a high demand on energy and nutrients, especially protein [19]. Detailed knowledge of the relative energy requirements of molt is integral for gaining insight into avian life history strategies
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