The effects of diet-shifting from invertebrates towards fruit on the condition of autumn-migrant Catharus thrushes.

Migration is an energetically challenging and risky life history stage for many animals, but could be supported by dietary choices en route, which may create opportunities to improve body and physiological condition. However, proposed benefits of diet shifts, such as between seasonally available invertebrates and fruits, have received limited investigation in free-living animals. We quantified diet composition and magnitude of autumn diet shifts over two time periods in two closely-related species of migratory songbirds on stopover in the northeastern U.S. (Swainson's thrush [Catharus ustulatus], long-distance migrant, N = 83; hermit thrush [C. guttatus], short-distance migrant, N = 79) and used piecewise structural equation models to evaluate the relationships among (1) migration timing, (2) dietary behavior, and (3) morphometric and physiological condition indices. Tissue isotope composition indicated that both species shifted towards greater fruit consumption. Larger shifts in recent weeks corresponded to higher body condition in Swainson's, but not hermit thrushes, and condition was more heavily influenced by capture date in Swainson's thrushes. Presence of "high-antioxidant" fruits in fecal samples was unrelated to condition in Swainson's thrushes and negatively related to multiple condition indices in hermit thrushes, possibly indicating the value of fruits during migration is related more to their energy and/or macronutrient content than antioxidant content. Our results suggest that increased frugivory during autumn migration can support condition, but those benefits might depend on migration strategy: a longer-distance, more capital-dependent migration strategy could require stricter regulation of body condition aided by increased fruit consumption.

Short-term mercury exposure disrupts muscular and hepatic lipid metabolism in a migrant songbird.

Methylmercury (MeHg) is a global pollutant that can cause metabolic disruptions in animals and thereby potentially compromise the energetic capacity of birds for long-distance migration, but its effects on avian lipid metabolism pathways that support endurance flight and stopover refueling have never been studied. We tested the effects of short-term (14-d), environmentally relevant (0.5 ppm) dietary MeHg exposure on lipid metabolism markers in the pectoralis and livers of yellow-rumped warblers (Setophaga coronata) that were found in a previous study to have poorer flight endurance in a wind tunnel than untreated conspecifics. Compared to controls, MeHg-exposed birds displayed lower muscle aerobic and fatty acid oxidation capacity, but similar muscle glycolytic capacity, fatty acid transporter expression, and PPAR expression. Livers of exposed birds indicated elevated energy costs, lower fatty acid uptake capacity, and lower PPAR-γ expression. The lower muscle oxidative enzyme capacity of exposed birds likely contributed to their weaker endurance in the prior study, while the metabolic changes observed in the liver have potential to inhibit lipogenesis and stopover refueling. Our findings provide concerning evidence that fatty acid catabolism, synthesis, and storage pathways in birds can be dysregulated by only brief exposure to MeHg, with potentially significant consequences for migratory performance.

Quantitative meta-analysis reveals no association between mercury contamination and body condition in birds.

Mercury contamination is a major threat to the global environment, and is still increasing in some regions despite international regulations. The methylated form of mercury is hazardous to biota, yet its sublethal effects are difficult to detect in wildlife. Body condition can vary in response to stressors, but previous studies have shown mixed effects of mercury on body condition in wildlife. Using birds as study organisms, we provide the first quantitative synthesis of the effect of mercury on body condition in animals. In addition, we explored the influence of intrinsic, extrinsic and methodological factors potentially explaining cross-study heterogeneity in results. We considered experimental and correlative studies carried out in adult birds and chicks, and mercury exposure inferred from blood and feathers. Most experimental investigations (90%) showed a significant relationship between mercury concentrations and body condition. Experimental exposure to mercury disrupted nutrient (fat) metabolism, metabolic rates, and food intake, resulting in either positive or negative associations with body condition. Correlative studies also showed either positive or negative associations, of which only 14% were statistically significant. Therefore, the overall effect of mercury concentrations on body condition was null in both experimental (estimate ± SE = 0.262 ± 0.309, 20 effect sizes, five species) and correlative studies (-0.011 ± 0.020, 315 effect sizes, 145 species). The single and interactive effects of age class and tissue type were accounted for in meta-analytic models of the correlative data set, since chicks and adults, as well as blood and feathers, are known to behave differently in terms of mercury accumulation and health effects. Of the 15 moderators tested, only wintering status explained cross-study heterogeneity in the correlative data set: free-ranging wintering birds were more likely to show a negative association between mercury and body condition. However, wintering effect sizes were limited to passerines, further studies should thus confirm this trend in other taxa. Collectively, our results suggest that (i) effects of mercury on body condition are weak and mostly detectable under controlled conditions, and (ii) body condition indices are unreliable indicators of mercury sublethal effects in the wild. Food availability, feeding rates and other sources of variation that are challenging to quantify likely confound the association between mercury and body condition in natura. Future studies could explore the metabolic effects of mercury further using designs that allow for the estimation and/or manipulation of food intake in both wild and captive birds, especially in under-represented life-history stages such as migration and overwintering.

Turning to the dark side: LED light at night alters the activity and species composition of a foraging bat assemblage in the northeastern United States.

Artificial light at night (ALAN) is a rapidly intensifying form of environmental degradation that can impact wildlife by altering light-mediated physiological processes that control a broad range of behaviors. Although nocturnal animals are most vulnerable, ALAN's effects on North American bats have been surprisingly understudied. Most of what is known is based on decades-old observations of bats around street lights with traditional lighting technologies that have been increasingly replaced by energy-efficient broad-spectrum lighting, rendering our understanding of the contemporary effects of ALAN on North American bats even less complete. We experimentally tested the effects of broad-spectrum ALAN on presence/absence, foraging activity, and species composition in a Connecticut, USA bat community by illuminating foraging habitat with light-emitting diode (LED) floodlights and comparing acoustic recordings between light and dark conditions. Lighting dramatically decreased presence and activity of little brown bats (Myotis lucifugus), which we detected on only 14% of light nights compared with 65% of dark (lights off) and 69% of control (lights removed) nights. Big brown bat (Eptesicus fuscus) activity on light nights averaged only half that of dark and control nights. Lighting did not affect presence/absence of silver-haired bats (Lasionycteris noctivagans), but decreased their activity. There were no effects on eastern red bats (Lasiurus borealis) or hoary bats (L. cinereus), which have been described previously as light-tolerant. Aversion to lighting by some species but not others caused a significant shift in community composition, thereby potentially altering competitive balances from natural conditions. Our results demonstrate that only a small degree of ALAN can represent a significant form of habitat degradation for some North American bats, including the endangered little brown bat. Research on the extent to which different lighting technologies, colors, and intensities affect these species is urgently needed and should be a priority in conservation planning for North America's bats.

Physiological indicators of habitat quality for a migratory songbird breeding in a forest invaded by non-native Japanese barberry (Berberis thunbergii).

Non-native, invasive plants can impact birds by altering food sources, nesting substrates and other critical resources. Japanese barberry (Berberis thunbergii) is one of the most invasive, non-native woody plants in in the northeastern USA, and yet almost nothing is known about its effects on birds or other wildlife. To investigate individual-level impacts of Japanese barberry on a forest-breeding bird, we compared food abundance (leaf-litter arthropod biomass) and the physiological condition of territorial male ovenbirds (Seiurus aurocapilla) between areas of a forest preserve in New York State that had high or low densities of Japanese barberry. We used haemoglobin and plasma triglyceride concentrations to indicate energetic condition, plasma uric acid and total plasma protein levels to indicate diet quality, and heterophil to lymphocyte ratios to indicate chronic stress. We found no difference in arthropod biomass between ovenbird territories that were heavily invaded by or relatively free of Japanese barberry. Perhaps largely as a result, we found no relationship between Japanese barberry density and any of our five haematological condition indices. There was also no difference in body mass, body size or age ratio between ovenbirds nesting in areas with low or high densities of Japanese barberry to suggest that relatively uninvaded forest patches were in greater demand and acquired by the most dominant individuals. Our results indicate that Japanese barberry does not reduce habitat quality for breeding male ovenbirds in a way that affects their prey abundance or physiological condition, but we caution that other species of birds and other aspects of habitat quality could be affected differently. We encourage future research on additional bird species and the effects of Japanese barberry on factors such as diet composition, pairing and nesting success and post-fledging survival to improve science-based decision-making about the extent to which conservation resources should be applied towards Japanese barberry control.

The threat of global mercury pollution to bird migration: potential mechanisms and current evidence.

Mercury is a global pollutant that has been widely shown to adversely affect reproduction and other endpoints related to fitness and health in birds, but almost nothing is known about its effects on migration relative to other life cycle processes. Here I consider the physiological and histological effects that mercury is known to have on non-migrating birds and non-avian vertebrates to identify potential mechanisms by which mercury might hinder migration performance. I posit that the broad ability of mercury to inactivate enzymes and compromise the function of other proteins is a single mechanism by which mercury has strong potential to disrupt many of the physiological processes that make long-distance migration possible. In just this way alone, there is reason to expect mercury to interfere with navigation, flight endurance, oxidative balance, and stopover refueling. Navigation and flight could be further affected by neurotoxic effects of mercury on the brain regions that process geomagnetic information from the visual system and control biomechanics, respectively. Interference with photochemical reactions in the retina and decreases in scotopic vision sensitivity caused by mercury also have the potential to disrupt visual-based magnetic navigation. Finally, migration performance and possibly survival might be limited by the immunosuppressive effects of mercury on birds at a time when exposure to novel pathogens and parasites is great. I conclude that mercury pollution is likely to be further challenging what is already often the most difficult and perilous phase of a migratory bird's annual cycle, potentially contributing to global declines in migratory bird populations.

Invasive Japanese Barberry, Berberis thunbergii (Ranunculales: Berberidaceae) Is Associated With Simplified Branch-Dwelling and Leaf-Litter Arthropod Communities in a New York Forest.

Arthropod food webs can be indirectly impacted by woody plant invasions, with cascading consequences for higher trophic levels. There are multiple bottom-up pathways by which invasive plants can alter food webs: above-ground interactions based on plant-herbivore associations and below-ground at the interface of leaf-litter and soil food webs. We compared arthropod community composition in these two food web dimensions in a New York forest that has been heavily invaded by nonnative Japanese barberry. Using two sampling protocols, we compared arthropod community composition on Japanese barberry shrubs to multiple species of native host shrubs and then compared leaf-litter arthropod assemblages between forest patches with exceptionally high Japanese barberry densities and those with relatively little to no Japanese barberry present. Fitting with trends in other woody shrub invasions, arthropod species richness was significantly lower in the leaf litter around Japanese barberry and on Japanese barberry plants themselves. Although overall arthropod abundance was also significantly lower on and in the leaf litter around Japanese barberry than on and around native shrubs, total biomass did not differ due to the taxa associated with Japanese barberry tending to be larger-bodied. We observed a dramatic reduction in predatory arthropods in response to both bottom-up pathways, particularly among ants and spiders. Our results show that Japanese barberry-invaded habitats may be experiencing trophic downgrading as result of lower numbers of generalist predators like spiders and ants, which may have rippling effects up the food web to insectivorous animals and their predators.

Environmentally relevant methylmercury exposure reduces the metabolic scope of a model songbird.

For most birds, energy efficiency and conservation are paramount to balancing the competing demands of self-maintenance, reproduction, and other demanding life history stages. Yet the ability to maximize energy output for behaviors like predator escape and migration is often also critical. Environmental perturbations that affect energy metabolism may therefore have important consequences for fitness and survival. Methylmercury (MeHg) is a global pollutant that has wide-ranging impacts on physiological systems, but its effects on the metabolism of birds and other vertebrates are poorly understood. We investigated dose-dependent effects of dietary MeHg on the body composition, basal and peak metabolic rates (BMR, PMR), and respiratory quotients (RQ) of zebra finches (Taeniopygia guttata). Dietary exposure levels (0.0, 0.1, or 0.6 ppm wet weight) were intended to reflect a range of mercury concentrations found in invertebrate prey of songbirds in areas contaminated by atmospheric deposition or point-source pollution. We found adiposity increased with MeHg exposure. BMR also increased with exposure while PMR decreased, together resulting in reduced metabolic scope in both MeHg-exposed treatments. There were differences in RQ among treatments that suggested a compromised ability of exposed birds to rapidly metabolize carbohydrates during exercise in a hop-hover wheel. The elevated BMR of exposed birds may have been due to energetic costs of depurating MeHg, whereas the reduced PMR could have been due to reduced oxygen carrying capacity and/or reduced glycolytic capacity. Our results suggest that environmentally relevant mercury exposure is capable of compromising the ability of songbirds to both budget and rapidly exert energy.

Mobilization of mercury from lean tissues during simulated migratory fasting in a model songbird.

The pollutant methylmercury accumulates within lean tissues of birds and other animals. Migrating birds catabolize substantial amounts of lean tissue during flight which may mobilize methylmercury and increase circulating levels of this neurotoxin. As a model for a migrating songbird, we fasted zebra finches (Taeniopygia guttata) that had been dosed with 0.0, 0.1, and 0.6 parts per million (ppm) dietary methylmercury and measured changes in blood total mercury concentrations (THg) in relation to reductions in lean mass. Birds lost 6-16% of their lean mass during the fast, and THg increased an average of 12% and 11% in the 0.1 and 0.6 ppm treatments, respectively. Trace amounts of THg in the 0.0 ppm control group also increased as a result of fasting, but remained extremely low. THg increased 0.4 ppm for each gram of lean mass catabolized in the higher dose birds. Our findings indicate that methylmercury is mobilized from lean tissues during protein catabolism and results in acute increases in circulating concentrations. This is a previously undocumented potential threat to wild migratory birds, which may experience greater surges in circulating methylmercury than demonstrated here as a result of their greater reductions in lean mass.

Does Exurban Housing Development Affect the Physiological Condition of Forest-Breeding Songbirds? A Case Study of Ovenbirds (Seiurus aurocapillus) in the Largest Protected Area in the Contiguous United States.

Exurban development (low-density development in rural areas) can significantly alter wildlife community composition, but it is largely unknown whether it also affects wildlife at the individual level. We investigated individual-level impacts of exurban development in New York State's Adirondack Park by comparing the physiological condition of 62 male ovenbirds (Seiurus aurocapillus) breeding in forests with low-density housing development with those in contiguous forests. We used hematocrit (HCT) volume and plasma triglyceride (TRIG) levels to compare energetic condition, plasma uric acid (UA) and total plasma protein (TPP) levels to compare diet quality, and heterophil∶lymphocyte ratios (H∶L) to compare chronic stress. HCT was the only parameter to differ, with birds near houses exhibiting lower values. The comparable TRIG, UA, and TPP that we found between treatment types suggest that ovenbird food quality and availability are unaffected by exurban development in our study area. Similar H∶L suggests that homeowner activities do not significantly change chronic stressors faced by breeding male ovenbirds. We also found no difference in body mass, body size, or age ratio to indicate that habitats in either treatment type were in higher demand or more difficult to acquire. Although our results suggest that exurban development does not reduce habitat quality for male ovenbirds in a way that affects their condition, we caution that it may still ultimately reduce fitness by attracting synanthropic predators. Further work is needed to better understand the impacts of exurban development on wildlife at all levels and provide science-based information needed to meet conservation challenges in rapidly developing exurban areas.

Mercury exposure of a wetland songbird, Agelaius phoeniceus, in the New York metropolitan area and its effect on nestling growth rate.

The potential for mercury accumulation in free-living passerine birds is now recognized to be much greater than previously assumed. However, lowest observable effect levels have yet to be well established for this taxonomic group and it is usually unknown whether levels observed in the wild are causing adverse effects. We measured total blood mercury (THg) levels and took repeated morphological measurements from nestling red-winged blackbirds (Agelaius phoeniceus; N = 39) in the New York metropolitan area to investigate whether mercury affected their growth rate. We also compared THg levels of nestlings (and parents; N = 14) between our two study sites, which included riparian habitats along a city river and surrounding ponds in a nearby suburb, to examine differences between birds within and beyond the urban core. THg levels ranged 0.009-0.284 ppm in nestlings and 0.036-0.746 ppm in adults. Adults and nestlings had significantly higher THg outside of the city than within, possibly due to the ability of rivers to flush contaminants and the higher methylation potential of ponds. Among our candidate sets, models containing THg had minimal support for explaining variation in nestling growth rate. Summed Akaike weights further showed that THg had little relative importance. Mercury pollution in our sites may be low, or feather growth may have been sufficient to protect nestlings from accumulating harmful mercury levels in living tissues.

Quantitative magnetic resonance analysis and a morphometric predictive model reveal lean body mass changes in migrating Nearctic-Neotropical passerines.

Most studies of lean mass dynamics in free-living passerine birds have focused on Old World species at geographical barriers where they are challenged to make the longest non-stop flight of their migration. We examined lean mass variation in New World passerines in an area where the distribution of stopover habitat does not require flights to exceed more than a few hours and most migrants stop flying well before fat stores near exhaustion. We used either quantitative magnetic resonance (QMR) analysis or a morphometric model to measure or estimate, respectively, the fat and lean body mass of migrants during stopovers in New York, USA. With these data, we examined (1) variance in total body mass explained by lean body mass, (2) hourly rates of fat and lean body mass change in single-capture birds, and (3) net changes in fat and lean mass in recaptured birds. Lean mass contributed to 50% of the variation in total body mass among white-throated sparrows Zonotrichia albicollis and hermit thrushes Catharus guttatus. Lean mass of refueling gray catbirds Dumetella carolinensis and white-throated sparrows, respectively, increased 1.123 and 0.320 g h(-1). Lean mass of ovenbirds Seiurus aurocapillus accounted for an estimated 33-40% of hourly gains in total body mass. On average 35% of the total mass gained among recaptured birds was lean mass. Substantial changes in passerine lean mass are not limited to times when birds are forced to make long, non-stop flights across barriers. Protein usage during migration is common across broad taxonomic groups, migration systems, and migration strategies.