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All animals route assimilated nutrients to their tissues where they are used to support growth or are oxidized for energy. These nutrients are probably not allocated homogeneously among the various tissue and are more likely to be preferentially routed toward some tissues and away from others. Here we introduce an approach that allows researchers to identify and compare nutrient routing among different organs and tissues. We tested this approach by examining nutrient routing in birds. House sparrows Passer domesticus were fed a meal supplemented with one of seven 13C-labeled metabolic tracers representing three major classes of macronutrients, namely, carbohydrates, amino acids, and fatty acids. While these birds became postabsorptive (2 h after feeding), we quantified the isotopic enrichment of the lean and lipid fractions of several organs and tissues. We then compared the actual 13C enrichment of various tissue fractions with the predictions of our model to identify instances where nutrients were differentially routed and found that different classes of macronutrients are uniquely routed throughout the body. Recently ingested amino acids were preferentially routed to the lean fraction of the liver, whereas exogenous carbohydrates were routed to the brain and the lipid fraction of the liver. Fatty acids were definitively routed to the heart and the liver, although high levels of palmitic acid were also recovered in the adipose tissue. Tracers belonging to the same class of molecules were not always routed identically, illustrating how this technique is also suited to examine differences in nonoxidative fates of closely related molecules. Overall, this general approach allows researchers to test heretofore unexamined predictions about how animals allocate the nutrients they ingest. [ABSTRACT FROM AUTHOR]

Size and hydrolytic activity of the gastrointestinal tracts of altricial birds undergo large and rapid changes during ontogeny. However, nothing is known about the development of the capacity of absorption of products of digestion, a factor that can limit total digestive performance. Using pharmacokinetic methods applied to wild-collected and laboratory-raised altricial nestlings of house sparrows (Passer domesticus), we addressed several questions of general significance about absorption in young birds. We found that both rate and efficiency of absorption of radiolabeled 3-O-methyl-d-glucose (3-OMD-glucose; absorbed by both transporter-mediated and nonmediated mechanisms) increased significantly between days 3 and 12 posthatch. We hypothesize that these changes can explain improvements in whole-diet digestion rate and efficiency observed in the young of house sparrows and of many other avian species, even after intestinal growth has ceased. We also tested the hypothesis that a high level of nonmediated, paracellular glucose absorption, as is typical in adult house sparrows, would already be observed in nestlings, and that their glucose absorption efficiency would not depend on glucose load because absorption rate is nonsaturable and is matched to substrate concentration. Using l-glucose (which is absorbed by nonmediated mechanism[s]), we found that, as predicted, paracellular absorption accounted for the majority of total absorption in nestlings of all ages, and starch content (0% vs. 25%) in the diet of laboratory-raised nestlings had no effect on efficiency of absorption of 3-OMD-glucose. Presumably, reliance on nonmediated absorption in young sparrows can save energy for growth. Also, during the transition from an almost starch-free, insect-based diet during the first days posthatch to the starch-rich, seed-based diet that is typical of adults, reliance on passive absorption is advantageous because the rate of absorption can easily match the current carbohydrate level in the intestines and the activity of hydrolytic enzymes. [ABSTRACT FROM AUTHOR]

Antioxidants protect against free-radical damage, and free radicals, in turn, are thought to underlie aging. Thus, measuring antioxidants may aid field ecologists in understanding the physiological mechanisms that underlie life-history trade-offs. Antioxidant levels are known to vary markedly in response to the stress of capture in many birds. These changes in antioxidants could result from regulation (e.g., by stress-related hormones) or consumption (e.g., by an increase in free radicals due to increased metabolic rate). Here we experimentally test the effect of increased metabolic rate on circulating antioxidant and corticosterone concentrations in two wild passerine bird species, house sparrows (Passer domesticus) and gray catbirds (Dumetella carolinensis). We increased metabolic rate via exposure to low ambient temperatures overnight in captivity and measured circulating antioxidant capacity, uric acid, corticosterone, and oxygen consumption in cold-exposed and control individuals. Both species showed increases rather than decreases in all antioxidant parameters overnight, contradicting a consumption-by-energy-expenditure hypothesis. Both positive and negative correlations between antioxidant response and corticosterone response were occasionally but not consistently present, refuting a generalized regulation-by-corticosterone hypothesis. High baseline uric acid predicted diminished response of corticosterone and all antioxidants. Thus, high uric acid may reflect recent stress, poor condition, or a compensatory response. Relationships among metabolic rate, antioxidants, and corticosterone differed qualitatively between the species. [ABSTRACT FROM AUTHOR]

Skin is an important avenue of water loss in terrestrial birds, so environmental conditions that necessitate water conservation should favor physiological mechanisms that reduce cutaneous water loss (CWL). Skin resistance to CWL is conferred by a barrier of lipid molecules located in the stratum corneum (SC), the outer layer of the epidermis. In mammals, SC barrier function depends on the conversion of cerebrosides to ceramides by the enzyme ß-glucocerebrosidase (ß-GlcCer'ase). Avian SC contains both cerebrosides and ceramides, suggesting that observed plasticity in CWL may be mediated by changes in ß-GlcCer'ase activity and resultant SC lipid composition. We tested the hypothesis that changes in ambient humidity would alter ß-GlcCer'ase activity by acclimating house sparrows (Passer domesticus) to either dry (6.5 g H2O m-3 absolute humidity) or humid (31 g H2O m-3) conditions for 5 and 21 d at 30°C and then measuring ß-GlcCer'ase activity from SC homogenates. Our results provide the first characterization of ß-GlcCer'ase activity in any nonmammalian vertebrate. Relative to nonacclimated controls, both dry- and humid-acclimated sparrows had significantly elevated ß-GlcCer'ase activity at 21 d postacclimation. Across individuals, we observed negative correlations between ß-GlcCer'ase activity and both CWL and SC ceramide content. Although dry- and humid-acclimated sparrows did not differ in ß-GlcCer'ase activity, these results are consistent with our findings that both humidity treatments caused a reduction in CWL and similar changes in SC lipid composition. Our results demonstrate physiological plasticity in CWL and provide tentative support for a role of ß-GlcCer'ase in mediating this response. [ABSTRACT FROM AUTHOR]

Resistance to water-vapor diffusion through the skin is thought to be conferred by lipids in the stratum corneum (SC), the outer layer of the epidermis. We tested the effect of ambient humidity on cutaneous water loss (CWL) and lipid composition of the SC by acclimating house sparrows (Passer domesticus) to either a dry (6.5 g/m3 absolute humidity) or a humid (31 g/m3) environment for 3 wk at a thermoneutral temperature (30°C). Sparrows in the dry-acclimated group reduced CWL by 36% compared with those in the humid environment. Relative to initial values, both groups of sparrows decreased CWL, 45% in the dry-acclimated group and 23% in the humid group, suggesting that temperature is also an important stimulus for CWL apart from humidity. Both groups of acclimated sparrows decreased quantities of cholesterol, free fatty acids, and cerebrosides and increased the proportion of ceramides in their SC. Lipid amounts or proportions in the SC did not differ between dry- and humid-acclimated sparrows, but the free fatty acid ceramide ratio was significantly lower in dry-acclimated birds. Also, lipid composition was only correlated with CWL in dry-acclimated sparrows, suggesting that structural changes to SC lipids are more tightly linked to CWL regulation in response to low humidity. Our results demonstrate phenotypic flexibility in CWL and lipid composition of the SC and provide support for a functional relationship between these traits. [ABSTRACT FROM AUTHOR]

Many vertebrates show seasonality in immune defenses, perhaps because of trade-offs with other physiological processes. Trade-offs between reproduction and immune function have been well studied, but how other life cycle events such as molt affect immune function remains unclear. Here, we hypothesize that one possible explanation is that accumulative dissociated processes (e.g., resource deficits generated over the long term by physiological processes) can have delayed effects on immune activity. To test this hypothesis, we compared cutaneous immune responses in groups of captive female house sparrows (Passer domesticus) photoperiodically induced into six different life cycle stages. We predicted that if delayed trade-offs occur, immune activity would be reduced after a mature life state was reached (e.g., postmolt) and not just compromised when other tissues were actively growing (instantaneous trade-off). We found evidence for both types of trade-offs: immune responses were weakest in sparrows that had just completed postnuptial molt, but they were also weak in birds growing reproductive tissues or feathers. Birds in mature reproductive states or light molt had strong immune responses comparable with birds in a nonbreeding/nonmolting state. Altogether, our results indicate that immune activity in female house sparrows can be influenced by both instantaneous and delayed trade-offs. [ABSTRACT FROM AUTHOR]

Passerines that overwinter in temperate climates undergo seasonal acclimatization that is characterized by metabolic adjustments that may include increased basal metabolic rate (BMR) and cold-induced summit metabolism (Msum) in winter relative to summer. Metabolic changes must be supported by equivalent changes in oxygen transport. While much is known about the morphology of the avian respiratory system, little is known about respiratory function under extreme cold stress. We examined seasonal variation in BMR, Msum' and ventilation in seasonally acclimatized house sparrows from Wisconsin. BMR and Msum increased significantly in winter compared with summer. In winter, BMR increased 64%, and Msum increased 29% over summer values. The 64% increase in winter BMR is the highest recorded for birds. Metabolic expansibility (Msum/ BMR) was 9.0 in summer and 6.9 in winter birds. The metabolic expansibility of 9.0 in summer is the highest yet recorded for birds. Ventilatory accommodation under he lox cold stress was due to changes in breathing frequency (f), tidal volume, and oxygen extraction efficiency in both seasons. However, the only significant difference between summer and winter ventilation measures in helox cold stress was f Mean f in helox cold stress for winter birds was 1.23 times summer values. [ABSTRACT FROM AUTHOR]

The immune system requires energetic and nutritional resources to optimally defend organisms against pathogens and parasites. Because resources are typically limited, immune function may require a trade-off with other physiologically demanding activities. Here, we examined whether photoperiodically induced seasonal states (breeding, molting, or nonbreeding) affected the cutaneous immune response of captive male house sparrows (Passer domesticus). To assess immune function in these birds, we injected the mitogen phytohemagglutinin (PHA) into the patagium and measured the resulting wing web swelling. Molting and nonbreeding birds had similar immune responses to PHA injection. However, males in a breeding state showed lower immune responses than both molting and nonbreeding birds even though they did not actually breed. We tested whether this decrease in the PHA swelling response in birds in a breeding state was due to elevated plasma concentrations of testosterone (7) by administering T to birds in a nonbreeding state. Contrary to some evidence in the literature, T did not suppress the response to PHA in house sparrows. Our data show that passerine birds show seasonal modulation in immune function, even in benign environmental conditions. However, even though T is often cited as a strong immunosuppressant, it is not fully responsible for this seasonal modulation. [ABSTRACT FROM AUTHOR]

Six decades of studies have speculated that digestive capacity might limit avian growth rate or that developmental changes in the gut might determine developmental changes in digestive efficiency. However, there are no studies on digestive enzymes during avian development, except for studies on mainly domestic birds that exhibit the precocial mode of development. We studied alimentary organ masses, intestinal enzyme activities (sucrase, maltase, isomaltase, aminopeptidase-N), and pancreatic enzyme activities (amylase, trypsin, chymotrypsin) during development of a wild passerine bird exhibiting the altricial mode of development. Wild nestling house sparrows were studied immediately after removal from the nest (days 0, 3, 6 of age; day 0 = hatch), whereas captives were raised in the laboratory beginning day 3 on a formulated casein/starch-based diet until fledging age (after day 12). Digestive biochemistry was dynamic. Tissue-specific activities of some digestive enzymes continued to increase through fledging, by > 10 times in some cases (e.g., sucrase and maltase in midintestine). Total pancreatic amylase activity increased 100 times between hatch and day 12 through a combination of increases in tissue-specific activity and pancreas mass. House sparrows differ from poultry, in whom after about 2 wk of age the specific activity of intestinal and pancreatic digestive enzymes is generally constant or declines during development. The data on intestinal and pancreatic enzymes help explain why digestive efficiency of nestling house sparrows improves with age, and the data seem consistent with the idea that digestive capacity might limit feeding rate and hence growth rate. [ABSTRACT FROM AUTHOR]

Presents a study which hypothesized that switching from a low-protein to a high-protein diet would induce responses in birds similar to those found in mammals. Description of the renal morphology in house sparrows, Passer domesticus; Details on the adenylyl cyclase activity in sparrows; Avian renal response to dietary protein.