Changes in the composition of the urine of yellow-vented bulbuls (Pycnonotus xanthopygos) are driven by ambient temperature and not by nitrogen or water intake.

Ian G. van Tets¹, Carmi Korine¹, Lizanne Roxburgh^1,2, Berry Pinshow^1,2

¹Mitrani Department of Desert Ecology, Jacob Blaustein Institute for Desert Research and ²Department of Life Sciences, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 84990 ISRAEL

Uricotely (uric acid > 50% of urinary nitrogen) in birds was once considered ubiquitous. However, Anna’s hummingbirds (Calypte anna) have been shown to be an exception to this rule; under conditions of low ambient temperature and on a nitrogen-free diet, they increased their water intake and often became ammonotelic (ammonia > 50 % of urinary nitrogen). Our aim was to identify the effects of nitrogen intake and ambient temperature (Ta) on ammonia excretion in birds. We also wished to determine whether ammonotely was restricted to small nectarivores or whether it is a more general phenomenon. Therefore, we assayed ureteral urine composition in eight yellow-vented bulbuls (Pycnonotus xanthopygos), each randomly allocated a diet of 20 % (0.6 M) sucrose solution supplemented by either 1.03 g.l^-1 or 7.23 g.l^-1 soy protein, and held at a Ta of either 28 °C or 10 °C. Food, and therefore water, intake rates varied with nitrogen intake but not with Ta. Food intake increased significantly with decreased nitrogen intake, while concentrations of all the excretory compounds in the urine (P < 0.05) decreased, yet their proportions in the urine did not change significantly. The lower Ta had no effect on food intake or on the concentration of uric acid. However, at 10 ºC the ammonia and urea concentrations increased (P <0.05) and this led to a significant increase in the proportion of ammonia in the urine. Our results suggest that, whenever nectarivorous or frugivorous birds are exposed to low Ta, the proportion of the ammonia in their urine will increase.

Thermoregulation, and the Role of Behaviour, in Breeding Crowned Plovers

Mark Brown and Colleen T. Downs

School of Botany and Zoology, University of Natal. Private Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa.

The role of behaviour in thermoregulation in birds has been the subject of numerous studies. This study assessed the role of shading behaviour in the thermoregulation of real incubating crowned plovers (Vanellus coronatus) using radio telemetry in the field. Shading behaviour was shown to have no direct benefit for eggs, at times even causing eggs to rise close to lethal levels. Instead, shading behaviour played an important role in maintaining incubating bird temperatures at a constant level. We therefore suggest changing the term "shading" to "standing" behaviour.
In addition, a range of core bird temperature of 8.8 °C was measured during the course of the study. Both daily and seasonal differences in core bird and egg temperature occurred. Daily differences in core bird temperature are a result of daily circadian rhythms and nocturnal hypothermia while seasonal differences are explained by changes in ambient temperature. The concept of optimum incubation temperature for a species is therefore reconsidered. Differences between and within species may not be as a result of phylogenetic differences as previously reported, but may be as a result of relatively short-term changes in response to ambient temperature changes.
Finally, the validity of using taxidermic models in thermoregulatory studies was assessed by comparing data collected using both real and model birds. No direct correlations were found between real and model eggs and birds. This suggests that the patterns seen in models may not adequately reflect short term changes that occur in the real system, thus reducing the ability to use such data to make broad generalizations about thermoregulation in general. The models are, however, useful in providing insight into the heat load an animal carries under different environmental conditions, and in estimating the overall, long-term effects of metabolic heat production in a real bird and egg.

The avian metabolic continuum: zoogeographical variance and a model for the energetic determinants of heterothermy.

Andrew E. McKechnie & Barry G. Lovegrove

School of Botany and Zoology, University of Natal, Private Bag X01, Scottsville, 3209, South Africa

The majority of comparative studies of endotherm metabolic rate published in the last two decades have attempted to identify adaptive associations with various ecological and life history traits. A recent model links variance in mammalian basal metabolic rate (BMR) to climatic variability. We investigated the applicability of this model to birds, by analysing BMR variance in 219 non-migratory species. Our results indicate that similar zoogeographical variance occurs in avian BMR. Specifically, the BMR of Afrotropical, Australasian and Indomalayan species is significantly lower than that of Nearctic or Palaearctic species. In addition, the BMR of mesic and arid species are convergent in the Afrotropics and Australasia, suggesting an aridity-mimic effect. These patterns suggest that the slow-fast continuum of avian BMR is partly attributable to climatic variability.
The concept of a metabolic continuum is likely to be useful in explaining other avian energetic traits. The relative energetic benefits and ecological costs of torpor can be used to construct a model for the energetic determinants of heterothermy. We argue that heterothermy is most strongly selected for at the upper and lower extremes of the metabolic continuum. At the lower end of the continuum, selection pressures for reduced energy requirements (i.e. those that select for low BMR) probably favour the ability to reduce energy expenditure by means of heterothermy. At the upper end of the continuum, heterothermy is likely to be selected for in species with high metabolic requirements, but in which the rate of energy intake is limiting. The available data supports the predictions of this model, with heterothermy most prevalent in species at the extremes of the metabolic continuum.

Are birds preadapted to desert environments?

Berry Pinshow

Mitrani Department of Desert Ecology, Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990 Israel

Desert-dwelling mammals are well known for their physiological adaptations that facilitate survival in hot, dry environments. For example, rodents save water by highly concentrating their urine; camels and Bedouin goats can lose 25% of their body mass as water, but still maintain viable plasma volume; and both large and small mammals have labile body temperatures that allow the use of radiation, convection and conduction, rather than evaporative water loss, to dump excess heat. Conventional wisdom has it that these adaptations evolved as terrestrial mammals penetrated and occupied environments that imposed high external heat loads. In contrast with these mammals, any bird that locomotes by flapping flight is subject to an internal, metabolic heat load that might reach 10 to 15 times its resting power input. To dissipate this heat while maintaining total body water and plasma volume, birds evolved mechanisms similar to those associated with adaptation to desert life in mammals.
The traits of desert mammals are probably the result of adaptations to hot and dry environments. In contrast, the analogous traits in birds may be a by-product of physiological adaptations for flight and these may have pre-adapted birds for desert life.

Osmoregulation in avian nectarivores - an integrative approach

Todd J. McWhorter¹, Carlos Martínez del Rio² & Berry Pinshow^3,4

¹Department of Ecology & Evolutionary Biology, University of Arizona, Tucson, AZ 85721-0088 USA.
²Department of Zoology & Physiology, University of Wyoming, Laramie, WY 82071-3166 USA.
³Mitrani Department of Desert Ecology, Jacob Blaustein Institute for Desert Research and
⁴Department of Life Sciences, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 84990 ISRAEL
mcwhorte@email.arizona.edu

Nectar-feeding birds must often deal with large ingested water loads in order to meet their energetic demands. We investigated the importance of the integration of digestive and renal function for maintaining water homeostasis in these animals. We compared the fractional absorption of dietary water from the gastrointestinal tracts of Broad-tailed Hummingbirds (Selasphorus platycercus) and Palestine Sunbirds (Nectarinia osea). Hummingbirds absorbed the majority of dietary water (ca. 80%) regardless of the amount ingested, while sunbirds appeared to be able to modulate absorption of dietary water across the intestine. Fractional water absorption by sunbirds decreased (from 100% to 40%) as water intake increased, allowing them to dispose of significant amounts of dietary water at the supply side when feeding on dilute nectars. To our knowledge this is the first documentation of adaptive regulation of water flux from the gastrointestinal tract to the body. Glomerular filtration rate in Palestine Sunbirds was 42% of that predicted based on body mass (1.82 ± 0.89 mL h^-1, mean ± SD) and was not significantly correlated with water intake. The apparent ability of sunbirds to modulate the absorption of dietary water suggests that renal processing of water and recovery of filtered glucose may not limit energy assimilation because GFR (and thus glucose filtered load) remains relatively low even when birds are faced with large dietary water loads. Our results suggest that although hummingbirds and sunbirds are convergent in the ecological niche they exploit, they may deal with nectar diets in very different ways on a physiological level.

Effects of dietary protein on kidney structure and function in mammals and birds.

David L. Goldstein

Dept. of Biological Sciences, Wright State University, Dayton, OH, USA.

Many birds switch either seasonally or during ontogeny between diets differing in protein content. The kidneys are partly responsible for accommodating such changes by excreting variable amounts of nitrogenous waste. In mammals, high protein diets also have additional consequences for the kidneys, including elevated filtration rates (GFR), increased urine flow, and renal hypertrophy. These changes may follow from an enhanced sensitivity of thick ascending limbs (TAL) to peptide hormones (glucagon and anti-diuretic hormone, ADH) which, along with enhanced urinary urea, suppresses tubulo-glomerular feedback (TGF) and thereby elevates GFR. Birds lack urea excretion, and the importance of TGF remains unknown. However, published data indicate that birds do have increased urine flow on high protein diets. We have studied both growing and adult birds to explore the mechanisms responsible for the avian response to dietary protein. We do find that the avian renal medulla enlarges in birds eating protein-rich foods. However, GFR is not consistently elevated, nor does the TAL become more sensitive to stimulation by ADH or glucagon. The role for TGF in birds remains unresolved. Other mechanisms might be responsible for the rise in urine flow, including possibly water retention by the proteinaceous spherules used by birds to excrete urates.

Nocturnal energy savings in a passerine bird, the Malachite Sunbird (Nectarinia famosa).

Colleen T. Downs & Mark Brown

School of Botany and Zoology, University of Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa.

Birds, like mammals , are homeotherms and so maintain body temperature (Tb) independent of environmental temperature. However, a number of non-passerine birds show markedly decreased Tb during the rest phase. There is little evidence of this in passerines. Consequently oxygen consumption (VO₂) and Tb were measured over a range of ambient temperatures in a small passerine, the Malachite Sunbird. Surgically implanted minimitters were used to measure Tb continuously and without disturbing the birds. Minimum VO₂ during the rest phase was 1.698 (ml O₂ g^-1 h^-1) at 25 °C. As ambient temperature decreased, VO₂ minimum during the rest phase did not increase to maintain Tb . At an ambient temperature of 5 °C this resulted in a large variation in Tb of 15°C. Birds increased Tb to active phase levels with the onset of light. Consequently the Malachite Sunbirds conserved energy nocturnally by reducing metabolic rate and concomitantly Tb. This plasticity in Tb shows that daily variations in Tb of homeotherms is biologically important. Furthermore, this heterothermy, particularly nocturnal hypothermia or torpor, would be particularly important in an unpredictable environment where food resources fluctuate, to prevent an energy deficit.

Ultraviolet colour vision in birds: inferring physiology and perception from behaviour.

Emma Smith and Verity Greenwood.

Department of Biological Sciences, University of Bristol, Bristol, BS8 IUG, UK

The degree of colour vision that an animal possesses depends largely on the number of types of cone photoreceptors with differing sensitivities in its retina, and upon how the outputs of those receptors are processed within the brain. The sensation we think of as ‘colour’, or hue, is reliant on our ability to discriminate between different wavelengths of light. This discrimination is achieved by opponent coding, which compares the relative output of different cone types in response to a stimulus. The sensation of brightness, however, arises from the brain adding up the responses of all types of photoreceptor to work out how much light of all wavelengths is reflected by the stimulus. Both humans and birds have long, medium and short wavelength sensitive cones. In addition, birds generally have a violet sensitive cone that confers sensitivity to ultraviolet wavelengths (many non-passerines, for example, poultry and ducks) or a cone that is maximally sensitive t! o ultraviolet (notably passerines, for example songbirds). Birds have been shown to respond behaviourally to the presence or absence of ultraviolet, but it was unknown what type of perceptual experience ultraviolet vision would give the bird. The output of the violet/ultraviolet cone may simply be added to the output of the other cone types, which would make objects that reflect ultraviolet look brighter. If, however, the output is compared with the output of other cone types, it would enable birds to see ultraviolet as a separate hue. To investigate this, we have used psychophysical techniques based around associative learning to develop an ultraviolet ‘colour blindness’ test, which we have given to poultry (Japanese quail) and songbirds (European starlings). It appears that the output of the violet/ultraviolet cone is opponently coded, as both species appear to perceive ultraviolet as a separate hue.

Living in the cold: Energy economising in arctic breeding Kittiwakes (Rissa tridactyla)

Claus Bech¹, Ingveig Langseth¹, Børge Moe¹, Marianne Fyhn^2,3 and Geir Wing Gabrielsen²

¹Department of Zoology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway (correspondence: claus.bech@chembio.ntnu.no)
²Norwegian Polar Institute, Polar Environmental Centre, NO-9296 Tromsø, Norway
³Department of Arctic Biology, University of Tromsø, NO-9037 Tromsø, Norway

We measured rates of energy metabolism (basal metabolic rates [BMR] and field metabolic rates [FMR]), body masses and body composition in female Kittiwakes throughout the breeding season at Svalbard (79°N). At this high latitude, the time of breeding in Kittiwakes is distinguished by continuous daylight, ambient temperatures averaging only 4.5°C and occasional spells of very cold weather.
The main characteristic of the energy budget of breeding Kittiwakes, is a decrease in body mass and BMR after hatching. Also the FMR during the first part of the chick-rearing period is lower than during the pre-incubation and incubation period. The first part of the chick-rearing period is hence a period of negative energy balance, which probably is caused by time constraints, since one of the adults needs to attend the chicks at all times. After thermal independence of the chicks, at an age of about 15 days, more time is available for the adults to forage. However, the BMR is still kept at a low level. The reduction in BMR during the chick-rearing period could be an adaptation to reduce the maintenance cost of the adults, allowing the parent birds to allocate more energy into chick growth. While most internal organs are reduced in mass in direct proportion to the general decrease in body mass, the liver and the kidney masses decreases disproportionately more. Since both the liver and the kidney are known to have high intrinsic metabolic rates, the results supports the notion that the reduction in body mass is an adaptation to reduce the maintenance costs. At the end of the chick-rearing period, FMR reaches its highest level during the breeding season mainly as a result of an increased workload connected to the increased chick feeding. This occurs at a time of stable or slightly increased body mass. A large increase in metabolic intensity (FMR/BMR) during the latter part of the chick-rearing period suggests that this ratio is not a good measure of energy stress during breeding in Kittiwakes.

Thermal conditions in the burrows of the Peruvian diving-petrel (Pelecanoides garnotii)

Edda D. Kölsch1, Gregor Kölsch2, Guillermo Luna-Jorquera3, Stefan Garthe1

1 Institut für Meereskunde, Dept. Marine Zoology, Düsternbrooker Weg 20, 24105 Kiel, Germany
2 Zoological Institute, University of Kiel, Olshausenstraße 40, 24098 Kiel, Germany
3 Dep. Biología Marina, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile

The Peruvian diving-petrel (Pelecanoides garnotii) breeds on offshore islands in sheltered nests, either in rock crevices or in burrows dug by the birds in soft soil. As part of an investigation into the ecology of seabirds in the upwelling system along the Humboldt Current, these burrows were studied with regard to temperature (involving continuous registration of data for up to three days).
The burrows were about 63 cm long and dampened down the high temperatures incursed during the day (up to 47.7 °C on the ground) and the low temperatures at night (down to 13.2°C). Burrows had a relatively constant temperature (on average between 20.8 and 25.0°C, depending on habitat), which was higher than the mean temperature of the environment. The size of the burrows (length and volume) was correlated with various temperature parameters (maximum, minimum, mean and fluctuation), however, no general rule was found. The burrows had no special orientation, and the orientation of the burrows with respect to the sun or the predominant wind direction did not influence the temperature inside. The temperature at the end of burrows was, on average, higher and showed less fluctuation than at a distance of 30cm from the entrance.
Although the chicks are ectothermic during the first days after hatching, the thermal stability of the burrows soon enables the adults to leave their chicks alone and to forage at sea. A comparison among habitats (sandy plateau, beach, rocky slope) shows that the burrows in the rocky slopes had the lowest average temperatures with higher fluctuations and, therefore, seem less suitable than the burrows dug in the sediment.