Corticosterone treatment has no effect on reproductive hormones or aggressive behavior in free-living male tree sparrows, Spizella arborea

We examined the effect of corticosterone on plasma levels of reproductive hormones (testosterone, dihydrotestosterone, and luteinizing hormone) and territorial defense behavior in male tree sparrows, Spizella arborea. Birds receiving Silastic implants filled with corticosterone (B) had significantly higher plasma levels of B than control birds, which received empty implants, and exhibited pectoral muscle wastage and a decrease in body mass. We evaluated the hormonal and agonistic responses of the two implanted groups of birds using a simulated territorial intrusion (STI) 2 to 4 days after they were implanted. Corticosterone-treated and control birds did not differ in their circulating levels of reproductive hormones or in their behavioral responses to STI (latency to respond to intrusion, number of songs, and closest approach to a decoy and tape recording). Unlike previous studies of north temperate passerines, high physiological levels of exogenous B had no effect either on circulating levels of reproductive hormones or on territorial behaviors associated with breeding. Nonetheless, untreated tree sparrows do mount a robust adrenocortical response, having a two- to fourfold increase in plasma B levels during a 1-h period of capture. Thus, adrenocortical responsiveness is maintained in these birds, but elevated levels of glucocorticoids do not suppress reproductive hormones or associated behaviors. We believe that this hormonal and behavioral refractoriness to glucocorticoids-or uncoupling of the stress response from the reproductive axis-may be advantageous for species having extreme temporal constraints on their breeding schedules.     

Inexplicable Inefficiency of Avian Molt? Insights from an Opportunistically Breeding Arid-Zone Species,Lichenostomus penicillatus

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 (BMR_m), 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 (RMR_min) 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 RMR_min during the molt of WPHE, at 82% (±5.59) above individual pre-molt levels, were some of the highest yet reported. Yet RMR_min 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.     

Why fly the extra mile? Using stress biomarkers to assess wintering habitat quality in migratory shorebirds

Migratory birds make decisions about how far to travel based on cost-benefit trade-offs. However, in many cases the net effect of these trade-offs is unclear. We sought to address this question by measuring feather corticosterone (CORT_f), leucocyte profile, avian malaria parasite prevalence and estimating fueling rates in three spatially segregated wintering populations of the migratory shorebird ruddy turnstone Arenaria interpres during their stay in the winter habitat. These birds fly from the high-Arctic breeding ground to Australia, but differ in that some decide to end their migration early (Broome, Western Australia), whereas others travel further to either South Australia or Tasmania. We hypothesized that the extra costs in birds migrating greater distances and overwintering in colder climates would be offset by benefits when reaching their destination. This would be evidenced by lower stress biomarkers in populations that travel further, owing to the expected benefits of greater resources and improved vitality. We show that avian malaria prevalence and physiological stress levels were lower in birds flying to South Australia and Tasmania than those overwintering in Broome. Furthermore, our modeling predicts that birds in the southernmost locations enjoy higher fueling rates. Our data are consistent with the interpretation that birds occupying more costly wintering locations in terms of higher migratory flight and thermoregulatory costs are compensated by better feeding conditions and lower blood parasite infections, which facilitates timely and speedy migration back to the breeding ground. These data contribute to our understanding of cost-benefit trade-offs in the decision making underlying migratory behaviour.     

The effect of corticosterone on standard metabolic rates of small passerine birds

Summary Resting metabolic rates of Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) and pine siskins (Carduelis pinus) were evaluated at thermoneutral temperatures before and after administration of corticosterone (B) at physiological doses. There was no effect of B on basal metabolic rate of either species, but nocturnal metabolic rate varied significantly less over the 3-h period of measurement in B-treated sparrows and siskins than in control birds. These results, coupled with observations of caged birds, suggest that corticosterone has no direct effect on avian resting metabolism but does reduce the responsiveness of birds to external stimuli and thus promotes nocturnal restfulness.Abbreviations B corticosterone - VO ₂ rate of oxygen consumption - T ₃ tri-iodothyromine - T ₄ tetra-iodothyronine, thyroxine     

Testosterone does not affect basal metabolic rate or blood parasite load in captive male White-plumed Honeyeaters Lichenostomus penicillatus

We examined physiological and morphological responses of reproductively active male White-plumed Honeyeaters Lichenostomus penicillatus to exogenous testosterone (T). Birds used in the experiment were held in captivity for two weeks and then were given subcutaneous implants which were either empty (controls) or T-filled silastic capsules (T-treated). Basal metabolic rates (BMR) measured just prior to implantation and again 4-weeks later were not affected by treatment and were indistinguishable from BMR measured in free-living White-plumed Honeyeaters, averaging 3.2 ml O2/g/h. Plasma T levels measured at the time of the final BMR evaluation were 1.2, 1.8, and 5.5 ng/ml in free-living, control, and T-treated birds, respectively. Body mass was unaffected by T treatment. All captive birds used in the experiment displayed Haemoproteus sp. infection, but blood parasite loads did not differ temporally between T-treated and control birds during their 6-week captivity. Corticosterone secretion during a 60-min capture-handling protocol did not differ between control and T-treated birds, but corticosterone levels at each sampling period in captive birds were significantly lower than those of free-living males. Three morphological variables of White-plumed Honeyeaters were affected by T treatment: T-treated birds had significantly higher haematocrits and larger testes than control birds and also showed no change in bill colour. Control birds, by contrast, showed substantial lightening of the proximal half of their bills. We conclude that BMR, resistance to Haemoproteus infection, and stress responsiveness are not directly affected by T levels in male White-plumed Honeyeaters.     

Metabolic rate and membrane fatty acid composition in birds: a comparison between long-living parrots and short-living fowl

Both basal metabolic rate (BMR) and maximum lifespan potential (MLSP) vary with body size in mammals and birds and it has been suggested that these are mediated through size-related variation in membrane fatty acid composition. Whereas the physical properties of membrane fatty acids affect the activity of membrane proteins and, indirectly, an animal’s BMR, it is the susceptibility of those fatty acids to peroxidation which influence MLSP. Although there is a correlation between body size and MLSP, there is considerable MLSP variation independent of body size. For example, among bird families, Galliformes (fowl) are relatively short-living and Psittaciformes (parrots) are unusually long-living, with some parrot species reaching maximum lifespans of more than 100 years. We determined BMR and tissue phospholipid fatty acid composition in seven tissues from three species of parrots with an average MLSP of 27 years and from two species of quails with an average MLSP of 5.5 years. We also characterised mitochondrial phospholipids in two of these tissues. Neither BMR nor membrane susceptibility to peroxidation corresponded with differences in MLSP among the birds we measured. We did find that (1) all birds had lower n-3 polyunsaturated fatty acid content in mitochondrial membranes compared to those of the corresponding tissue, and that (2) irrespective of reliance on flight for locomotion, both pectoral and leg muscle had an almost identical membrane fatty acid composition in all birds.     

Passive flow through an unstalked intertidal ascidian: orientation and morphology enhance suspension feeding in Pyura stolonifera

Passive flow is believed to increase the gains and reduce the costs of active suspension feeding. We used a mixture of field and laboratory experiments to evaluate whether the unstalked intertidal ascidian Pyura stolonifera exploits passive flow. We predicted that its orientation to prevailing currents and the arrangement of its siphons would induce passive flow due to dynamic pressure at the inhalant siphon, as well as by the Bernoulli effect or viscous entrainment associated with different fluid velocities at each siphon, or by both mechanisms. The orientation of P. stolonifera at several locations along the Sydney-Illawarra coast (Australia) covering a wide range of wave exposures was nonrandom and revealed that the ascidians were consistently oriented with their inhalant siphons directed into the waves or backwash. Flume experiments using wax models demonstrated that the arrangement of the siphons could induce passive flow and that passive flow was greatest when the inhalant siphon was oriented into the flow. Field experiments using transplanted animals confirmed that such an orientation resulted in ascidians gaining food at greater rates, as measured by fecal production, than when oriented perpendicular to the wave direction. We conclude that P. stolonifera enhances suspension feeding by inducing passive flow and is, therefore, a facultatively active suspension feeder. Furthermore, we argue that it is likely that many other active suspension feeders utilize passive flow and, therefore, measurements of their clearance rates should be made under appropriate conditions of flow to gain ecologically relevant results.     

Constrained mate choice in social monogamy and the stress of having an unattractive partner

In socially monogamous animals, mate choice is constrained by the availability of unpaired individuals in the local population. Here, we experimentally investigate the physiological stress endured by a female (the choosy sex) when pairing with a non-preferred social partner. In two experimental contexts, female Gouldian finches (Erythrura gouldiae) socially paired with poor-quality mates had levels of circulating corticosterone that were three to four times higher than those observed in females that were paired with preferred mates. The elevated level of this stress hormone in response to partner quality was observed within 12 h of the experimental introduction and maintained over a period of several weeks. Our findings demonstrate the extent of intra-individual conflict that occurs when individuals are forced to make mate-choice decisions that are not perfectly aligned with mate-choice preferences. The elevated level of corticosterone also suggests a mechanistic route through which females might adaptively manage their responses to intersexual conflict over reproductive investment.     

Energy relations of winter roost-site utilization by American goldfinches (Carduelis tristis)

Summary American goldfinches (Carduelis tristis) were observed roosting in Colorado blue spruce (Picea pungens), which comprised part of a mixed stand of conifers. Their winter roost-sites were distally situated among the most densely-needled branches on the leeward sides of these trees. Heated and unheated taxidermic goldfinch mounts were placed within these sites and at the same height in an adjacent clearing. The radiative and convective characteristics of these locations were monitored simultaneously and compared to predicted power requirements of live goldfinches (based on laboratory calibration of heated mounts) and operative temperatures (T _e ; based on body temperatures of unheated mounts). The winter roost-sites significantly reduced radiative and convective heat exchanges between goldfinches and the environment. Based on body composition data for winter goldfinches, all but two birds sampled could endure a 15-h roost period at average overnight T _e 's as low as-40°C. In contrast, if these birds were prevented from feeding the following day, only 30% could survive the imposition of a 39-h fast at average T _e 's of-2°C. Winter roost-site selection may be more constrained by thermoregulatory considerations in small birds than in larger species.