Moult-related reduction of aerobic scope in passerine birds

Abstract

It is well established that the nutrient and energy requirements of birds increase substantially during moult, but it is not known if these increased demands affect their aerobic capacity. We quantified the absolute aerobic scope of house and Spanish sparrows, Passer domesticus and P. hispaniolensis, respectively, before and during sequential stages of their moult period. The absolute aerobic scope (AAS) is the difference between maximum metabolic rate (MMR) during peak locomotor activities and minimum resting metabolic rate (RMR min), thus representing the amount of aerobic power above that committed to maintenance needs available for other activities. As expected, RMR min increased over the moult period by up to 40 and 63% in house and Spanish sparrows, respectively. Surprisingly, the maximum metabolic rates also decreased during moult in both species, declining as much as 25 and 38% compared with pre-moult values of house and Spanish sparrows, respectively. The concurrent changes in RMR min and MMR during moult resulted in significant decreases in AAS, being up to 32 and 47% lower than premoult levels of house and Spanish sparrows, respectively, during moult stages having substantial feather replacement. We argue that the combination of reduced flight efficiency due to loss of wing feathers and reduced aerobic capacity places moulting birds at greater risk of predation. Such performance constraints likely contribute to most birds temporally separating moult from annual events requiring peak physiological capacity such as breeding and migration.

Variation in erythrocyte morphology in alpine accentors (Prunella collaris Scop.) from Tian Shan, Rila and the High Tatra mountains and effects of molting

“…However, in our research, we found the largest erythrocytes occurred during the molting period, and a similar trend was observed in erythrocytes of alpine accentors in winter (Janiga et al 2017). A possible explanation is given by an interpretation of a study from Buttemer et al (2019), wherein cells remain less active despite increased metabolism of an organism such as during the molt period. Their study noted that, during the molt, the basal metabolic rate increases (BMR), but aerobic scope is substantially reduced.…”

“…In the molt phase, which is an energetically-demanding process and increases metabolic rate (Cyr et al 2008), the organism is exposed to acute stress. Molt is associated with the most energy -and nutrient -demanding periods in the annual cycle of birds (Murphy 1996), when the minimum resting metabolic rate (RMRmin) is significant increased (Buttemer et al 2019). The physiological mechanism of molting is very complex, requiring substantial stores of nutrients, an increase in the metabolism of protein (Murphy & Taruscio 1995;Murphy 1996), a loss in body weight, cessation of egg production, involution of reproductive organs (oviducts, ovaries and follicles) and thermoregulatory setback owing to loss of feathers (Harrison et al 1974;Sekimoto et al 1987).…”

The energy cost of feather replacement is not intrinsically inefficient

“…By contrast, we found that the RMR min of moulting birds was 28% higher than that of non-moulting House Sparrows, but the potential for differences in time in captivity of these cohorts to influence metabolic measurements must be considered. Measurements of RMR min in relation to moult status of a Bulgarian House Sparrow population were unaffected by time in captivity (0 to 4 days; Buttemer et al 2019), which is consistent with the lack of difference of RMR min in our non-moulting cohort held for at least 2 weeks before metabolic measurements and values for another non-moulting population in southeastern Australia measured the same day of capture (Chappell et al 1999). Furthermore, a previous study of free-living House Sparrows transferred to the samesized indoor cages as used in this study and maintained at thermoneutrality found that RMR min measured 2 weeks after captivity was statistically indistinguishable from measurements made 6 weeks later (Buttemer et al 2008).…”

“…Furthermore, a previous study of free-living House Sparrows transferred to the samesized indoor cages as used in this study and maintained at thermoneutrality found that RMR min measured 2 weeks after captivity was statistically indistinguishable from measurements made 6 weeks later (Buttemer et al 2008). Finally, the finding that RMR min of free-living Bulgarian House Sparrows measured on the first night of capture was 41% higher at a slightly later stage of moult than measurements of pre-moult birds supports our assumption that RMR min variation between our cohorts is due to differences in moult phenotype rather than time in captivity (Buttemer et al 2019).…”

“…Based on published masses for these organs during moult (Höhn 1956;Anderson 1970;Silverin et al 1999) and accounting for their water content (Corper 1912;Boyd et al 1956;Lee et al 2018), the dry mass of these two organs would be about 2% of dry feather mass of White-crowned Sparrows and would therefore place inconsequential demands on daily protein synthesis during moult. Flight muscle mass has been found to vary in some species during moult, but pectoral muscle scores of House Sparrows, although a qualitative estimate of actual muscle mass, are unaffected by moult (Buttemer et al 2019). We can therefore calculate the energy required for direct synthesis of feathers and associated structures during peak moult using the value of 75 mg of protein per day along with validated costs of protein synthesis and protein breakdown.…”