Physical aerobic activity is oxygen demanding, butparticularly for birdsthere is still little understanding of how blood contributes to oxygen supply under various activity levels. In a two-factorial experimental design, we investigated the long-term effect of daily flight training and the immediate effect of a short exercise bout on a set of haematological variables: haemoglobin (Hb) content, haematocrit (Hct), and red blood cell number (RBC count) and size (RBC area) in zebra finches (Taeniopygia guttata). For a period of 6 weeks, birds were either trained daily for 3 h in a flight arena or remained untrained. Subsequently, half of each group was blood sampled either in the resting condition or after a 5 min exercise bout in a flight-hover wheel. We found significantly lower Hb content, Hct and RBC count compared with that in untrained controls in response to training, while RBC area did not differ between treatments. Response to an exercise bout revealed the opposite pattern, with significantly higher Hb content and Hct compared with that in non-exercised birds. Additionally, RBC area was significantly smaller immediately after exercise compared with that in non-exercised birds, and such short-term flexibility represents a novel finding for birds. This contrasting response in erythrocyte characteristics with respect to long-term training and short exercise bouts appears as a clear pattern, presumably underlain by changes in water balance. We infer alterations of blood flow to be involved in adequate oxygen supply. During an exercise bout, RBC area flexibility may not only enhance oxygen delivery through improved erythrocyte surface area to volume ratio but also improve blood flow through a compensatory effect on blood viscosity.
Thermoregulation constitutes an important share of the energy budget of endotherms. Elevated thermoregulatory requirements must be met by oxygen supply through the blood, as heat is produced mainly via aerobic processes. In contrast to mammal studies, it remains unclear whether elevated thermoregulatory needs are followed by changes in haematological variables in birds. We investigated haematocrit (HCT), haemoglobin content per volume of blood (HGB), number of red blood cells (RBCcount), and size of the erythrocytes (RBCarea) in zebra finches Taeniopygia guttata acclimated to either cold or thermoneutral ambient temperatures under laboratory conditions. Seventy‐nine females were maintained for six weeks either in cold (T = +12°C) or thermoneutral (T = +32°C) ambient temperature prior to blood collection. On average, HGB, HCT and RBCcount were significantly lower by about 10% in cold acclimated compared to thermoneutral acclimated birds. Only RBCarea was not different between the two acclimation temperatures. Mean HCT, one of the most commonly measured haematological variable for example was 53 ± 0.9% (LSM ± SEM) in thermoneutral and 49 ± 0.8 % (LSM ± SEM) in cold acclimated zebra finches. On first sight, the observed lower values for three out of the four determined haematological variables in response to acclimation to cold question oxygen supply to be indeed a limiting factor for heat production. However, higher demands of oxygen supply due to increased thermoregulation in birds may instead require specific optimisation of blood viscosity and modulation by other cardiovascular properties. Nucleated red blood cells in birds may pose different strain on blood viscosity compared to non‐nucleated mammalian erythrocytes and explain the contrasting response in haematological variables to temperature acclimation between birds and mammals.
Activation of an immune response (IR) upon exposure to pathogens is crucial to ensure adequate organismal performance and is directly linked to survival. Fitness benefits of the response may be associated with costs in terms of increased energy expenditure and may compete for resources and compromise such fitness benefits. Trade‐offs between immune function and other traits relevant for fitness are well documented, however, it remains unknown if such trade‐offs are energetically mediated. We manipulated the flight activity of 70 zebra finches Taeniopygia guttata to investigate the energy reallocation to the immune system in rested and exercised birds. Four experimental groups exhibiting different flight intensity were used: trained, untrained and birds that either stopped or started flight training after the immune challenge. If costs associated with the IR and flight activity compete for energy, we predicted the extent of inhibition of IR would be dependent on energy allocated to physical activity. Daytime resting metabolic rate was measured before and after the immune challenge, induced using sheep red blood cells (SRBC). Strength of the response was measured as the concentration of anti‐SRBC antibodies six days post‐challenge. We found no evidence for the predicted inhibition of the immune function between trained and untrained birds, as there was no difference in resting metabolic rate between experimental groups. However, resting metabolic rate following the challenge was negatively correlated with the IR. Surprisingly, individuals with relatively low resting metabolic rates following immune challenge were able to up‐regulate their IR, indicating a trade‐off in the use of the energy resource independent of flight activity levels. Our results suggest that energy allocation to mount the IR may represent a constraint that is possibly linked to the circadian pattern of the energy budget but appears to be independent of energetic challenges brought on by different levels of flight activity.
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