Continent-wide variation in feather colour of a migratory songbird in relation to body condition and moulting locality

Understanding the causes of variation in feather colour in free-living migratory birds has been challenging owing to our inability to track individuals during the moulting period when colours are acquired. Using stable-hydrogen isotopes to estimate moulting locality, we show that the carotenoid-based yellow-orange colour of American redstart (Setophaga ruticilla) tail feathers sampled on the wintering grounds in Central America and the Caribbean is related to the location where feathers were grown the previous season across North America. Males that moulted at southerly latitudes were more likely to grow yellowish feathers compared with males that moulted more orange-red feathers further north. Independent samples obtained on both the breeding and the wintering grounds showed that red chroma-an index of carotenoid content-was not related to the mean daily feather growth rate, suggesting that condition during moult did not influence feather colour. Thus, our results support the hypothesis that feather colour is influenced by ecological conditions at the locations where the birds moulted. We suggest that these colour signals may be influenced by geographical variation in diet related to the availability of carotenoids.     

Rainfall at African wintering grounds predicts age‐specific probability of haemosporidian infection in a migratory passerine bird

In migratory species breeding in temperate zones and wintering in tropical areas, the prevalence of blood parasites may be affected by migratory strategies and winter habitat choice. We explored whether African winter habitat was linked to the probability of haemosporidian infection in the House Martin Delichon urbicum breeding in Spain, and tested for potential differences between age‐classes. As a proxy for winter habitat features, we analysed stable isotope (δ²H, δ¹³C and δ¹⁵N) values of winter‐grown feathers moulted in tropical Africa. Rainfall at the African winter grounds was related to the probability of being infected with haemosporidians and this effect differed among age‐classes. We found that haemosporidian prevalence was similar for young and experienced birds wintering in habitats of higher rainfall (²H‐depleted), whereas there were great differences in winter habitats of lower rainfall (²H‐enriched), with young having a much higher prevalence compared with experienced birds. Likewise, experienced birds wintering in habitats of higher rainfall had a higher probability of haemosporidian infection compared with experienced birds wintering in habitats of lower rainfall. By contrast, young birds wintering in habitats of lower rainfall had a higher probability of haemosporidian infection compared with young birds wintering in habitats of higher rainfall. These outcomes highlight the interaction of age with haemosporidian infection in the migratory ecology of the House Martin, which may drive carry‐over effects in this long‐distance aerial insectivore.     

Hatching date influences winter habitat occupancy: Examining seasonal interactions across the full annual cycle in a migratory songbird

Birds experience a sequence of critical events during their life cycle, and past events can subsequently determine future performance via carry‐over effects. Events during the non‐breeding season may influence breeding season phenology or productivity. Less is understood about how events during the breeding season affect individuals subsequently in their life cycle. Using stable carbon isotopes, we examined carry‐over effects throughout the annual cycle of prairie warblers (Setophaga discolor), a declining Nearctic–Neotropical migratory passerine bird. In drier winters, juvenile males that hatched earlier at our study site in Massachusetts, USA, occupied wetter, better‐quality winter habitat in the Caribbean, as indicated by depleted carbon isotope signatures. For juveniles that were sampled again as adults, repeatability in isotope signatures indicated similar winter habitat occupancy across years. Thus, hatching date of juvenile males appears to influence lifetime winter habitat occupancy. For adult males, reproductive success did not carry over to influence winter habitat occupancy. We did not find temporally consecutive “domino” effects across the annual cycle (breeding to wintering to breeding) or interseasonal, intergenerational effects. Our finding that a male''s hatching date can have a lasting effect on winter habitat occupancy represents an important contribution to our understanding of seasonal interactions in migratory birds.     

Global change and the distributional dynamics of migratory bird populations wintering in Central America

Understanding the susceptibility of highly mobile taxa such as migratory birds to global change requires information on geographic patterns of occurrence across the annual cycle. Neotropical migrants that breed in North America and winter in Central America occur in high concentrations on their non‐breeding grounds where they spend the majority of the year and where habitat loss has been associated with population declines. Here, we use eBird data to model weekly patterns of abundance and occurrence for 21 forest passerine species that winter in Central America. We estimate species’ distributional dynamics across the annual cycle, which we use to determine how species are currently associated with public protected areas and projected changes in climate and land‐use. The effects of global change on the non‐breeding grounds is characterized by decreasing precipitation, especially during the summer, and the conversion of forest to cropland, grassland, or peri‐urban. The effects of global change on the breeding grounds are characterized by increasing winter precipitation, higher temperatures, and the conversion of forest to peri‐urban. During spring and autumn migration, species are projected to encounter higher temperatures, forests that have been converted to peri‐urban, and increased precipitation during spring migration. Based on current distributional dynamics, susceptibility to global change is characterized by the loss of forested habitats on the non‐breeding grounds, warming temperatures during migration and on the breeding grounds, and declining summer rainfall on the non‐breeding grounds. Public protected areas with low and medium protection status are more prevalent on the non‐breeding grounds, suggesting that management opportunities currently exist to mitigate near‐term non‐breeding habitat losses. These efforts would affect more individuals of more species during a longer period of the annual cycle, which may create additional opportunities for species to respond to changes in habitat or phenology that are likely to develop under climate change.     

Warming and depth interact to affect carbon dioxide concentration in aquatic mesocosms

1. Climate change may significantly influence lake carbon dynamics and consequently the exchange of CO₂ with the atmosphere. Warming will accelerate multiple processes that either absorb or release CO₂, making predicting the net effect of warming on CO₂ exchange with the atmosphere difficult. Here we experimentally test how the CO₂ flux of deep and shallow systems responds to warming. To do this, we conducted a greenhouse experiment using mesocosms of two depths, experiencing either ambient or warmed temperatures. 2. Deeper mesocosms were found to have a lower average CO₂ concentration than shallower mesocosms under ambient temperature conditions. In addition, warming interacts with mesocosm depth to affect the average CO₂ concentration; there was no effect of warming on the average CO₂ concentration of deep mesocosms, but shallow mesocosms had significantly lower average CO₂ concentrations when warmed. 3. The difference in CO₂ concentration resulting from the depth manipulation was due to varying loss rates of particulate carbon to the sediments. There was a strong negative correlation between CO₂ and sedimentation rates in the deep mesocosms suggesting that high particulate carbon loss to the sediments lowered the CO₂ concentration in the water column. There was no correlation between CO₂ and sedimentation rates observed for shallow mesocosms suggesting enhanced carbon regeneration from the sediments was maintaining higher CO₂ concentrations in the water column. 4. Relationships between CO₂ and algal concentrations indicate that the reduction in CO₂ concentrations resulting from warming is due to increased per capita algal turnover rates depleting CO₂ in the water column. Our results suggest that the carbon dynamics and CO₂ flux of shallow systems will be affected more by climate warming than deep systems and the net effect of warming is to increase CO₂ uptake.     

Individual condition,but not fledging phenology,carries over to affect post-fledging survival in a Neotropical migratory songbird

Migratory animals face severe time and energy constraints during their annual cycle. These constraints may be exacerbated in young animals by conditions experienced during development that can affect both phenotype and phenology. For young migratory songbirds, the period between fledging and autumn migration, the post-fledging period, is believed to represent a time of intense selective pressure. However, there has yet to be a study that has assessed post-fledging survival for the entirety of the post-fledging period, probably due to the challenge of following juveniles as they move broadly across the landscape (tens to hundreds of kilometres). To overcome this challenge, we used an automated radiotelemetry array spanning 60 000 km² in southern Ontario, Canada, and miniature digital radiotelemetry tags to track 216 juvenile Barn Swallows Hirundo rustica continuously from fledging to migration. We hypothesized that young that fledged in better condition and earlier in the breeding season would have higher survival relative to birds fledging in poorer condition, because they have more energy to deal with resource constraints, and that early-fledging birds would depart on migration earlier than late-fledging birds because there is probably a fixed period of time required post-fledging to prepare for migration. We found that average cumulative apparent survival was 42% and that condition in the nest was a strong positive predictor of post-fledging apparent survival. We also found that birds that fledged earlier in the season departed on migration earlier in the autumn relative to late-fledging birds. Contrary to our prediction, average apparent survival was equal for early- and late-fledging birds. Our results suggest that factors during development that promote better nestling condition are critical for predicting future apparent survival prior to migration. Differences in annual apparent survival between early- and late-fledging songbirds, as commonly observed, may be driven by events occurring at later stages of the annual cycle.     

Altering Rainfall Timing and Quantity in a Mesic Grassland Ecosystem: Design and Performance of Rainfall Manipulation Shelters

Global climate change is predicted to alter growing season rainfall patterns, potentially reducing total amounts of growing season precipitation and redistributing rainfall into fewer but larger individual events. Such changes may affect numerous soil, plant, and ecosystem properties in grasslands and ultimately impact their productivity and biological diversity. Rainout shelters are useful tools for experimental manipulations of rainfall patterns, and permanent fixed-location shelters were established in 1997 to conduct the Rainfall Manipulation Plot study in a mesic tallgrass prairie ecosystem in northeastern Kansas. Twelve 9 x 14–m fixed-location rainfall manipulation shelters were constructed to impose factorial combinations of 30% reduced rainfall quantity and 50% greater interrainfall dry periods on 6 x 6–m plots, to examine how altered rainfall regimes may affect plant species composition, nutrient cycling, and above- and belowground plant growth dynamics. The shelters provided complete control of growing season rainfall patterns, whereas effects on photosynthetic photon flux density, nighttime net radiation, and soil temperature generally were comparable to other similar shelter designs. Soil and plant responses to the first growing season of rainfall manipulations (1998) suggested that the interval between rainfall events may be a primary driver in grassland ecosystem responses to altered rainfall patterns. Aboveground net primary productivity, soil CO₂ flux, and flowering duration were reduced by the increased interrainfall intervals and were mostly unaffected by reduced rainfall quantity. The timing of rainfall events and resulting temporal patterns of soil moisture relative to critical times for microbial activity, biomass accumulation, plant life histories, and other ecological properties may regulate longer-term responses to altered rainfall patterns.     

Breeding experience and population density affect the ability of a songbird to respond to future climate variation

Predicting how populations respond to climate change requires an understanding of whether individuals or cohorts within populations vary in their response to climate variation. We used mixed-effects models on a song sparrow (Melospiza melodia) population in British Columbia, Canada, to examine differences among females and cohorts in their average breeding date and breeding date plasticity in response to the El Niño Southern Oscillation. Climatic variables, age and population density were strong predictors of timing of breeding, but we also found considerable variation among individual females and cohorts. Within cohorts, females differed markedly in their breeding date and cohorts also differed in their average breeding date and breeding date plasticity. The plasticity of a cohort appeared to be due primarily to an interaction between the environmental conditions (climate and density) experienced at different ages rather than innate inter-cohort differences. Cohorts that expressed higher plasticity in breeding date experienced warmer El Niño springs in their second or third breeding season, suggesting that prior experience affects how well individuals responded to abnormal climatic conditions. Cohorts born into lower density populations also expressed higher plasticity in breeding date. Interactions between age, experience and environmental conditions have been reported previously for long-lived taxa. Our current results indicate that similar effects operate in a short-lived, temperate songbird.     

Adcyap1 polymorphism covaries with breeding latitude in a Nearctic migratory songbird,the Wilson's warbler (Cardellina pusilla)

Understanding the genetic background of complex behavioral traits, showing multigenic control and extensive environmental effects, is a challenging task. Among such traits, migration is known to show a large additive genetic component. Yet, the identification of specific genes or gene regions explaining phenotypic variance in migratory behavior has received less attention. Migration ultimately depends on seasonal cycles, and polymorphism at phenological candidate genes may underlie variation in timing of migration or other aspects of migratory behavior. In this study of a Nearctic–Neotropical migratory songbird, the Wilson's warbler (Cardellina pusilla), we investigated the association between polymorphism at two phenological candidate genes, Clock and Adcyap1, and two aspects of the migratory phenotype, timing of spring migration through a stopover site and inferred latitude of the breeding destination. The breeding destination of migrating individuals was identified using feather deuterium ratio (δ²H), which reliably reflects breeding latitude throughout the species' western breeding range. Ninety‐eight percent of the individuals were homozygous at Clock, and the rare heterozygotes did not deviate from homozygous migration phenology. Adcyap1 was highly polymorphic, and allele size was not significantly associated with migration date. However, Adcyap1 allele size significantly positively predicted the inferred breeding latitude of males but not of females. Moreover, we found a strong positive association between inferred breeding latitude and Adcyap1 allele size in long‐distance migrating birds from the northern sector of the breeding range (western Canada), while this was not the case in short‐distance migrating birds from the southern sector of the breeding range (coastal California). Our findings support previous evidence for a role of Adcyap1 in shaping the avian migratory phenotype, while highlighting that patterns of phenological candidate gene–phenotype associations may be complex, significantly varying between geographically distinct populations and even between the sexes.     

Ecosystem fragmentation drives increased diet variation in an endemic livebearing fish of the Bahamas

One consequence of human‐driven habitat degradation in general, and habitat fragmentation in particular, is loss of biodiversity. An often‐underappreciated aspect of habitat fragmentation relates to changes in the ecology of species that persist in altered habitats. In Bahamian wetlands, ecosystem fragmentation causes disruption of hydrological connectivity between inland fragmented wetlands and adjacent marine areas, with the consequent loss of marine piscivores from fragmented sections. We took advantage of this environmental gradient to investigate effects of ecosystem fragmentation on patterns of resource use in the livebearing fish Gambusia hubbsi (Family Poeciliidae), using both population‐ and individual‐level perspectives. We show that fragmentation‐induced release from predation led to increased G. hubbsi population densities, which consequently led to lower mean growth rates, likely as a result of higher intraspecific competition for food. This was accompanied by a broadening of dietary niches via increased interindividual diet variation, suggesting a negative effect of predation and a positive effect of intraspecific competition on the degree of diet variation in natural populations. Our results therefore indicate that habitat fragmentation can greatly impact the ecology of resilient populations, with potentially important ecological and evolutionary implications.     

Precipitation amount and event size interact to reduce ecosystem functioning during dry years in a mesic grassland

Ongoing intensification of the hydrological cycle is altering rainfall regimes by increasing the frequency of extreme wet and dry years and the size of individual rainfall events. Despite long‐standing recognition of the importance of precipitation amount and variability for most terrestrial ecosystem processes, we lack understanding of their interactive effects on ecosystem functioning. We quantified this interaction in native grassland by experimentally eliminating temporal variability in growing season rainfall over a wide range of precipitation amounts, from extreme wet to dry conditions. We contrasted the rain use efficiency (RUE) of above‐ground net primary productivity (ANPP) under conditions of experimentally reduced versus naturally high rainfall variability using a 32‐year precipitation–ANPP dataset from the same site as our experiment. We found that increased growing season rainfall variability can reduce RUE and thus ecosystem functioning by as much as 42% during dry years, but that such impacts weaken as years become wetter. During low precipitation years, RUE is lowest when rainfall event sizes are relatively large, and when a larger proportion of total rainfall is derived from large events. Thus, a shift towards precipitation regimes dominated by fewer but larger rainfall events, already documented over much of the globe, can be expected to reduce the functioning of mesic ecosystems primarily during drought, when ecosystem processes are already compromised by low water availability.     

Winter body condition in relation to age,sex and plumage ornamentation in a migratory songbird

Winter body condition may play important roles in the life history of migratory birds, but it is difficult to estimate. We used the growth rate of winter‐grown tail feathers of Collared Flycatchers Ficedula albicollis as an indicator of winter body condition, comparing this trait between age classes and sexes and relating it to plumage ornamentation (forehead and wing patch sizes). Adults and males were in better nutritional condition during winter, as indicated by their faster tail feather growth rates, than were yearlings and females, respectively, which could indicate differences in individual quality and foraging ability with age, or age‐ and sex‐related winter habitat segregation. However, feather growth rate was related neither to the size of the winter‐grown forehead patch nor to the size of the summer‐grown wing patch, suggesting weak condition‐dependence for the winter‐grown ornament and complex life‐history consequences for the summer‐grown ornament.     

Connecting the dots: Stopover strategies of an intercontinental migratory songbird in the context of the annual cycle

The phases of the annual cycle for migratory species are inextricably linked. Yet, less than five percent of ecological studies examine seasonal interactions. In this study, we utilized stable hydrogen isotopes to geographically link individual black‐and‐white warblers (Mniotilta varia) captured during spring migration with breeding destinations to understand a migrant's stopover strategy in the context of other phases of the annual cycle. We found that stopover strategy is not only a function of a bird's current energetic state, but also the distance remaining to breeding destination and a bird's time‐schedule, which has previously been linked to habitat conditions experienced in the preceding phase of the annual cycle. Birds in close proximity to their breeding destination accumulate additional energy reserves prior to arrival on the breeding grounds, as reflected by higher migratory condition upon arrival, higher refueling rates measured via blood plasma metabolites, and longer stopover durations compared to birds migrating to breeding destinations farther from the stopover site. However, late birds near their breeding destination were more likely to depart on the day of arrival (i.e., transients), and among birds that stopped over at the site, the average duration of stopover was almost half the time of early conspecifics, suggesting late birds are trying to catch‐up with the overall time‐schedule of migration for optimal arrival time on the breeding grounds. In contrast, birds with long distances remaining to breeding destinations were more likely to depart on the day of arrival and primarily used stopover to rest before quickly resuming migration, adopting similar strategies regardless of a bird's time‐schedule. Our study demonstrates that migrants adjust their en route strategies in relation to their time‐schedule and distance remaining to their breeding destination, highlighting that strategies of migration should be examined in the context of other phases of the annual cycle.     

The effects of an experimental drought on the ecophysiology and fruiting phenology of a tropical rainforest palm

据预测,人为气候变化将季节性地增加平均温度和降雨。热带雨林物种将如何应对这种气候变化仍不确定。本研究分析了对澳大利 亚昆士兰北部丹特里雨林的一种澳大利亚特有棕榈(Normambya normanbyi)进行4年降雨实验的影响,目的是了解模拟降雨减少对物种生理 过程和果实物候的影响。我们考察了这种本地丰富的棕榈的果实物候和生理生态学特性,以确定该物种对干旱的生态响应。2015年5月, 通过排涝实验,降低了约30%的土壤水分有效性。我们总共监测了8年(2009–2018年)的月度果实活性,包括排涝实验开始之前的4年。在最 近几年的研究中,我们测量了干、湿两季幼嫩和成熟叶片的光合速率、气孔导度和碳稳定同位素等生理参数。研究结果表明,所有棕榈树 的月度果实活性主要受光周期、平均太阳辐射和平均温度的驱动。然而,暴露于较低土壤水分的植株,其果实活性、光合速率和气孔导度 均显著下降。我们还发现这些生理表现受到排涝实验、季节以及两者的相互作用的影响。2018年观察到排涝实验的棕榈的果实活力有所恢复,土壤浅层水分也有所增加(与前几年相比)。我们的研究结果表明,像N. normanbyi 这样的棕榈树对未来的气候变化非常敏感,建议对其 进行长期监测,以确定其对种群规模的影响。     

The influence of climate and habitat on stable isotope signatures and the isotopic niche of nestling White‐headed Woodpeckers (Dryobates albolarvatus)

The majority of landbird species feed their nestlings arthropods and variation in arthropod populations can impact reproductive outcomes in these species. Arthropod populations in turn are influenced by climate because temperature affects survival and reproduction, and larval development. Thus, climate factors have the potential to influence many bird species during their reproductive phases. In this study, we assessed climate factors that impact the diet of nestling White‐headed Woodpecker (Dryobates albolarvatus), an at‐risk keystone species in much of its range in western North America. To do this, we measured stable isotope signatures (δ¹³C and δ¹⁵N) in 152 nestlings across six years and linked variation in isotopic values to winter (December–February) and spring (June) precipitation and temperature using mixed effects models. We also explored habitat factors that may impact δ¹³C and δ¹⁵N and the relationship between δ¹⁵N and nest productivity. Last, we estimated isotopic niche width for nestlings in different watersheds and years using Bayesian standard ellipses, which allowed us to compare dietary niche width and overlap. We found that colder winter temperatures were associated with an increase in δ¹⁵N and δ¹⁵N levels had a weak positive relationship with nest productivity. We also found that sites with a more diverse tree community were associated with a broader isotopic niche width in nestlings. Our findings suggest that nestling diet is affected by climate, and under future warming climate scenarios, White‐headed Woodpecker nestling diet may shift in favor of lower trophic level prey (prey with lower δ¹⁵N levels). The impact of such changes on woodpecker populations merits further study.     

Population trends of breeding birds in British woodlands over a 32-year period: relationships with food, habitat use and migratory behaviour

This paper outlines population trends (with confidence intervals) for 49 species in woodland habitats in Britain as monitored by the British Trust for Ornithology's (BTO) Common Bird Census (CBC) between 1967 and 1999. Additionally, the possible causes of these population trends are investigated by relating the ecological characteristics of species to the degree of population change they have undergone over different time periods. Over the whole period, 17 species showed significant decreases in abundance and 12 species showed significant increases. Whilst population trajectories were diverse, long-distance migrants showed more negative trends than other species and the timing of the changes in their populations was related to their wintering latitude, suggesting that these species may be suffering from environmental changes in the non-breeding season. There was also support for habitat specializations being related to population changes, with species classified as scrub and understorey specialists declining on average, but this was only evident across the entire study period. Additionally, species eating seeds in summer declined and those eating vegetation and making use of the agricultural landscape matrix increased. Therefore wide-scale factors such as landscape-scale processes or processes operating outside of Britain appear to be important in addition to local habitat change, especially for long-distance migrants.     

The impact of climate change on the structure of Pleistocene food webs across the mammoth steppe

Species interactions form food webs, impacting community structure and, potentially, ecological dynamics. It is likely that global climatic perturbations that occur over long periods of time have a significant influence on species interaction patterns. Here, we integrate stable isotope analysis and network theory to reconstruct patterns of trophic interactions for six independent mammalian communities that inhabited mammoth steppe environments spanning western Europe to eastern Alaska (Beringia) during the Late Pleistocene. We use a Bayesian mixing model to quantify the contribution of prey to the diets of local predators, and assess how the structure of trophic interactions changed across space and the Last Glacial Maximum (LGM), a global climatic event that severely impacted mammoth steppe communities. We find that large felids had diets that were more constrained than those of co-occurring predators, and largely influenced by an increase in Rangifer abundance after the LGM. Moreover, the structural organization of Beringian and European communities strongly differed: compared with Europe, species interactions in Beringian communities before—and possibly after—the LGM were highly modular. We suggest that this difference in modularity may have been driven by the geographical insularity of Beringian communities.     

Biofilm history and oxygen availability interact to affect habitat selection in a marine invertebrate

In marine systems, oxygen availability varies at small temporal and spatial scales, such that current oxygen levels may not reflect conditions of the past. Different studies have shown that marine invertebrate larvae can select settlement sites based on local oxygen levels and oxygenation history of the biofilm, but no study has examined the interaction of both. The influence of normoxic and hypoxic water and oxygenation history of biofilms on pre-settlement behavior and settlement of the bryozoan Bugula neritina was tested. Larvae used cues in a hierarchical way: the oxygen levels in the water prime larvae to respond, the response to different biofilms is contingent on oxygen levels in the water. When oxygen levels varied throughout biofilm formation, larvae responded differently depending on the history of the biofilm. It appears that B. neritina larvae integrate cues about current and historical oxygen levels to select the appropriate microhabitat and maximize their fitness.