Between-population Variation Research Articles

Rapid increase in growth and productivity can aid invasions by a non-native tree.

Research on biological invasions has produced detailed theories describing range expansions of introduced populations. However, current knowledge of evolutionary factors associated with invasive range expansions, especially those related to rapid evolution of long-lived organisms, is still rudimentary. Here, we used a system of six 40-year-old invasive pine populations that originated from replicated introduction events to study evolution in productivity, growth, and chemical defence traits. We tested the hypotheses that invasive populations were undergoing rapid phenotypic change as populations spread, that populations exhibit trade-offs between evolution in growth and chemical defences, and that rates of rapid evolution in plant growth and productivity effect rates of invasion. Although all invasions started from replicated pools of genetic material and equal propagule pressure, we found divergence in mean values for the six invasive populations in the six traits measured. Not only were there between-population variations but also invasive populations were also rapidly changing along each invasive population expansion. Two populations displayed greater leaf areas (LAs) and smaller specific LAs (SLAs) during range expansion. Four populations had faster growth rates at the leading edge of the invasion front in comparison with plants at the rear edge. In terms of total plant defences, non-volatile resin increased in plants along one invasion gradient and decreased in a second, total needle phenolics increased in plants along one invasion gradient and total wood phenolics increased in plants along the one invasion gradient and decreased in a second. We found no trade-offs between investments in growth and chemical defence. Also, faster rates of change in growth rate and LA were positively associated with greater dispersal distances of invasive populations, suggesting rapid evolution may increase invasiveness. Understanding the roles of both natural and human-mediated ecological and evolutionary processes in population-level dynamics is key to understanding the ability of non-native species to invade.

Between-population variability of common carotid intima-media thickness in industrially developed regions in Germany and Russia

Between-population variability of common carotid intima-media thickness in industrially developed regions in Germany and Russia

Factors Driving Variation in Biparental Incubation Behaviour in the Reed WarblerAcrocephalus scirpaceus

Biparental incubation is a common pattern of parental care in birds. Within species with biparental incubation, the contribution of each sex can vary widely. Many studies have addressed the factors that influence variation in female incubation behaviour, but the underlying causes of within-species variation in male incubation behaviour remain poorly understood. In this paper we analyse incubation behaviour in the Reed Warbler, a small, predominantly socially monogamous passerine nesting in reed beds. We examined the impact of time of day, weather conditions (ambient temperature, wind speed, rainfall) and progress of the breeding season on male and female incubation behaviour in 81 pairs of Reed Warbler breeding in the Barycz Valley (SW Poland). We found that females had on average higher nest attentiveness (total time spent incubating per hour) than their partners (47% vs. 29%) but mean incubation bout length (a single, uninterrupted stay at the nest) did not differ significantly between the sexes (9 min vs. 7 min). The two parents responded differently to changing environmental parameters. Female nest attentiveness was unaffected by date, time of the day, advancement of incubation and weather conditions, while males spent more time on the nest at higher wind speeds and lower temperatures. In contrast, male incubation bout length was not affected by these factors, whereas female bout length increased throughout the breeding season and was longer at lower temperatures. Incubation recesses (periods where both parents were off the nest) were longer during favourable weather conditions (at high temperatures and low wind), probably because cooling of the eggs takes much longer under such conditions and parents can spend more time foraging. A comparison of our results with those from other populations revealed important betweenpopulation and sex-specific differences in nest attentiveness expressed as the percentage of male/female time spent on the nest. We suggest that betweenpopulation variation may result from differences in habitat quality and/or food resources affecting the necessity of male contribution to parental care, variation in breeding synchrony and densities influencing male engagement in extra-pair copulations, differences in predation levels, or methodological differences.

Evidence of between-population variation in morphology and thermal plasticity of agonistic behavior in two genetically distinct populations of steelhead (Oncorhynchus mykiss)

Morphological and behavioral traits affect an individual’s fitness and can reflect both evolutionary adaptations and phenotypic responses to environmental conditions. We conducted a reciprocal transplant ‘common garden’ experiment at two temperature regimes to test for phenotypic plasticity in morphological and behavioral traits between and within two populations of steelhead Oncorhynchus mykiss from Hood Canal, WA, USA. The Dewatto River and Duckabush River populations exhibited asymmetric changes in body morphology in response to the two temperature regimes, suggesting both between- and within-population variation in morphological plasticity. In most cases, within population variation in body shapes was less than between temperature regimes. Most notably the populations differed in dorso-ventral and caudal regions, body depth, and head shape, with some differences on the anterior-posterior placement of the dorsal and ventral fins. The warm temperature regime caused more exploratory behavior, more charging behavior, and higher fin erosion, and population effects included slight differences in feeding aggression frequency. Morphology appeared to vary more between populations than between temperature regimes, and behavioral traits varied more between temperature regimes than between populations. Morphological variation may reflect adaptations to variation in freshwater habitat conditions, and both populations show behavioral plasticity in response to temperature. This study sheds new light on the role of genetic and environmental influence on morphology and behavior in juvenile steelhead.

Toward body composition reference data for infants, children, and adolescents.

Growth charts for weight and height have provided the basis for assessment of children’s nutritional status for over half a century, with charts for body mass index (BMI) introduced in the 1990s. However, BMI does not provide information on the proportions of fat and lean mass; and within the past decade, growth charts for children’s body composition have been produced by using techniques such as skinfold thicknesses, body circumferences, bioelectrical impedance analysis (BIA), and dual-energy X-ray absorptiometry (DXA). For public health research, BIA and skinfold thicknesses show negligible average bias but have wider limits of agreement than specialized techniques. For patients, DXA is the best individual method, but multicomponent models remain ideal because they address perturbations in lean mass composition. Data can be expressed in age- and sex-specific SD scores, in some cases adjusting for height. Most such reference data derive from high-income countries, but techniques such as air-displacement plethysmography allow infant body composition growth charts to be developed in low- and middle-income settings, where the data may improve understanding of the effects of low birth weight, wasting, and stunting on body composition. Recent studies suggest that between-population variability in body composition may derive in part from genetic factors, suggesting a universal human body composition reference may not be viable. Body composition growth charts may be extended into adult life to evaluate changes in fat and lean mass through the entire life course. These reference data will improve the understanding of the association between growth, body composition, health, and disease.

Ecological Correlates of Group-Size Variation in a Resource-Defense Ungulate, the Sedentary Guanaco

For large herbivores, predation-risk, habitat structure and population density are often reported as major determinants of group size variation within and between species. However, whether the underlying causes of these relationships imply an ecological adaptation or are the result of a purely mechanistic process in which fusion and fragmentation events only depend on the rate of group meeting, is still under debate. The aim of this study was to model guanaco family and bachelor group sizes in contrasting ecological settings in order to test hypotheses regarding the adaptive significance of group-size variation. We surveyed guanaco group sizes within three wildlife reserves located in eastern Patagonia where guanacos occupy a mosaic of grasslands and shrublands. Two of these reserves have been free from predators for decades while in the third, pumas often prey on guanacos. All locations have experienced important changes in guanaco abundance throughout the study offering the opportunity to test for density effects. We found that bachelor group size increased with increasing density, as expected by the mechanistic approach, but was independent of habitat structure or predation risk. In contrast, the smaller and territorial family groups were larger in the predator-exposed than in the predator-free locations, and were larger in open grasslands than in shrublands. However, the influence of population density on these social units was very weak. Therefore, family group data supported the adaptive significance of group-size variation but did not support the mechanistic idea. Yet, the magnitude of the effects was small and between-population variation in family group size after controlling for habitat and predation was negligible, suggesting that plasticity of these social units is considerably low. Our results showed that different social units might respond differentially to local ecological conditions, supporting two contrasting hypotheses in a single species, and highlight the importance of taking into account the proximate interests and constraints to which group members may be exposed to when deriving predictions about group-size variation.

Sex-specific differences in desiccation resistance and the use of energy metabolites as osmolytes in Drosophila melanogaster flies acclimated to dehydration stress

In the Indian subcontinent, there are significant between-population variations in desiccation resistance in Drosophila melanogaster, but the physiological basis of adult acclimation responses to ecologically relevant humidity conditions is largely unknown. We tested the hypothesis that increased desiccation resistance in acclimated flies is associated with changes in cuticular permeability and/or content of energy metabolites that act as osmolytes. Under an ecologically relevant humidity regime (~50 % relative humidity), both sexes showed desiccation acclimation which persisted for 2-3 days. However, only females responded to acclimation at ~5 % relative humidity (RH). Acclimated flies exhibited no changes in the rate of water loss, which is consistent with a lack of plastic changes in cuticular traits (body melanization, epicuticular lipid). Therefore, changes in cuticular permeability are unlikely in drought-acclimated adult flies of D. melanogaster. In acclimated flies, we found sex differences in changes in the content of osmolytes (trehalose in females versus glycogen in males). These sex-specific changes in osmolytes are rapid and reversible and match to corresponding changes in the increased desiccation resistance levels of acclimated flies. Further, the increased content of trehalose in females and glycogen in males support the bound-water hypothesis for water retention in acclimated flies. Thus, drought acclimation in adult flies of D. melanogaster involves inducible changes in osmolytes (trehalose and glycogen), while there is little support for changes in cuticular permeability.

Limited genetic variability and phenotypic plasticity detected for cavitation resistance in a Mediterranean pine

Resistance to cavitation is a major determinant of plant survival under severe drought and can be used to quantify species adaptive potential. Interspecific variation in this key trait is well defined in woody species, but intraspecific variation (level and structure) resulting from standing genetic variation and phenotypic plasticity has never been determined. Combining for the first time in situ characterization of natural populations and two reciprocal common gardens in dry and wet sites, we estimated variance components (phenotypic, genetic, environmental, and genetic×environmental) of cavitation resistance based on 513 genotypes of a Mediterranean pine, Pinus pinaster. Despite the selected populations being climatically contrasted, phenotypic plasticity in resistance to cavitation remained low and was essentially attributed to family level. Between-population variation in cavitation resistance for both phenotypic and genetic variation was limited. These results strongly suggest that cavitation resistance is buffered against genetic and to a lesser extent environmental variation (canalization) in maritime pine. Consequently, in a drier world, the increasing drought tolerance of Pinus species might be severely constrained by the low level of cavitation resistance variation, resulting in a large-scale loss of productivity.

Spatial variation buffers temporal fluctuations in early juvenile survival for an endangered Pacific salmon

Spatial, phenotypic and genetic diversity at relatively small scales can buffer species against large-scale processes such as climate change that tend to synchronize populations and increase temporal variability in overall abundance or production. This portfolio effect generally results in improved biological and economic outcomes for managed species. Previous evidence for the portfolio effect in salmonids has arisen from examinations of time series of adult abundance, but we lack evidence of spatial buffering of temporal variability in demographic rates such as survival of juveniles during their first year of life. We therefore use density-dependent population models with multiple random effects to represent synchronous (similar among populations) and asynchronous (different among populations) temporal variability as well as spatial variability in survival. These are fitted to 25 years of survey data for breeding adults and surviving juveniles from 15 demographically distinct populations of Chinook salmon (Oncorhynchus tshawytscha) within a single metapopulation in the Snake River in Idaho, USA. Model selection identifies the most support for the model that included both synchronous and asynchronous temporal variability, in addition to spatial variability. Asynchronous variability (log-SD = 0·55) is approximately equal in magnitude to synchronous temporal variability (log-SD = 0·67), but much lower than spatial variability (log-SD = 1·11). We also show that the pairwise correlation coefficient, a common measure of population synchrony, is approximated by the estimated ratio of shared and total variance, where both approaches yield a synchrony estimate of 0·59. We therefore find evidence for spatial buffering of temporal variability in early juvenile survival, although between-population variability that persists over time is also large. We conclude that spatial variation decreases interannual changes in overall juvenile production, which suggests that conservation and restoration of spatial diversity will improve population persistence for this metapopulation. However, the exact magnitude of spatial buffering depends upon demographic parameters such as adult survival that may vary among populations and is proposed as an area of future research using hierarchical life cycle models. We recommend that future sampling of this metapopulation employ a repeated-measure sampling design to improve estimation of early juvenile carrying capacity.

Conditional cold avoidance drives between-population variation in germination behaviour in Calluna vulgaris

Across their range, widely distributed species are exposed to a variety of climatic and other environmental conditions, and accordingly may display variation in life history strategies. For seed germination in cold climates, two contrasting responses to variation in winter temperature have been documented: first, an increased ability to germinate at low temperatures (cold tolerance) as winter temperatures decrease, and secondly a reduced ability to germinate at low temperatures (cold avoidance) that concentrates germination towards the warmer parts of the season. Germination responses were tested for Calluna vulgaris, the dominant species of European heathlands, from ten populations collected along broad-scale bioclimatic gradients (latitude, altitude) in Norway, covering a substantial fraction of the species' climatic range. Incubation treatments varied from 10 to 25 °C, and germination performance across populations was analysed in relation to temperature conditions at the seed collection locations. Seeds from all populations germinated rapidly and to high final percentages under the warmer incubation temperatures. Under low incubation temperatures, cold-climate populations had significantly lower germination rates and percentages than warm-climate populations. While germination rates and percentages also increased with seed mass, seed mass did not vary along the climatic gradients, and therefore did not explain the variation in germination responses. Variation in germination responses among Calluna populations was consistent with increased temperature requirements for germination towards colder climates, indicating a cold-avoidance germination strategy conditional on the temperature at the seeds' origin. Along a gradient of increasing temperatures this suggests a shift in selection pressures on germination from climatic adversity (i.e. low temperatures and potential frost risk in early or late season) to competitive performance and better exploitation of the entire growing season.

Parturition date for a given female is highly repeatable within five roe deer populations

Births are highly synchronized among females in many mammal populations in temperate areas. Although laying date for a given female is also repeatable within populations of birds, limited evidence suggests low repeatability of parturition date for individual females in mammals, and between-population variability in repeatability has never, to our knowledge, been assessed. We quantified the repeatability of parturition date for individual females in five populations of roe deer, which we found to vary between 0.54 and 0.93. Each year, some females gave birth consistently earlier in the year, whereas others gave birth consistently later. In addition, all females followed the same lifetime trajectory for parturition date, giving birth progressively earlier as they aged. Giving birth early should allow mothers to increase offspring survival, although few females managed to do so. The marked repeatability of parturition date in roe deer females is the highest ever reported for a mammal, suggesting low phenotypic plasticity in this trait.

Rapid Development of Adaptive, Climate-Driven Clinal Variation in Seed Mass in the Invasive Annual Forb Echium plantagineum L.

We examined adaptive clinal variation in seed mass among populations of an invasive annual species, Echium plantagineum, in response to climatic selection. We collected seeds from 34 field populations from a 1,000 km long temperature and rainfall gradient across the species' introduced range in south-eastern Australia. Seeds were germinated, grown to reproductive age under common glasshouse conditions, and progeny seeds were harvested and weighed. Analyses showed that seed mass was significantly related to climatic factors, with populations sourced from hotter, more arid sites producing heavier seeds than populations from cooler and wetter sites. Seed mass was not related to edaphic factors. We also found that seed mass was significantly related to both longitude and latitude with each degree of longitude west and latitude north increasing seed mass by around 2.5% and 4% on average. There was little evidence that within-population or between-population variation in seed mass varied in a systematic manner across the study region. Our findings provide compelling evidence for development of a strong cline in seed mass across the geographic range of a widespread and highly successful invasive annual forb. Since large seed mass is known to provide reproductive assurance for plants in arid environments, our results support the hypothesis that the fitness and range potential of invasive species can increase as a result of genetic divergence of populations along broad climatic gradients. In E. plantagineum population-level differentiation has occurred in 150 years or less, indicating that the adaptation process can be rapid.

Plasticity in Human Life History Strategy

The life history of Homo sapiens is characterized by a lengthy period of juvenile dependence that requires extensive allocare, short interbirth intervals with concomitantly high fertility rates, and a life span much longer than that of other extant great apes. Although recognized as species-defining, the traits that make up human life history are also notable for their extensive within- and between-population variation, which appears to trace largely to phenotypic and developmental plasticity. In this review, we first discuss the adaptive origins of plasticity in life history strategy and its influence on traits such as growth rate, maturational tempo, reproductive scheduling, and life span in modern human populations. Second, we consider the likely contributions of this plasticity to evolutionary diversification and speciation within genus Homo. Contrary to traditional assumptions that plasticity slows the pace of genetic adaptation, current empirical work and theory point to the potential for plasticity...

Variation in eggshell traits between geographically distant populations of pied flycatchers Ficedula hypoleuca

The expression and impact of maternal effects may vary greatly between populations and environments. However, little is known about large‐scale geographical patterns of variation in maternal deposition to eggs. In birds, as in other oviparous animals, the outermost maternal component of an egg is the shell, which protects the embryo, provides essential mineral resources and allows its interaction with the environment in the form of gas exchange. In this study, we explored variation of eggshell traits (mass, thickness, pore density and pigmentation) across 15 pied flycatcher populations at a large geographic scale. We found significant between‐population variation in all eggshell traits, except in pore density, suggesting spatial variation in their adaptive benefits or in the females’ physiological limitations during egg laying. Between‐ population variation in shell structure was not due to geographic location (latitude and longitude) or habitat type. However, eggshells were thicker in populations that experienced higher ambient temperature during egg laying. This could be a result of maternal resource allocation to the shell being constrained under low temperatures or of an adaptation to reduce egg water loss under high temperatures. We also found that eggshell colour intensity was positively associated with biliverdin pigment concentration, shell thickness and pore density. To conclude, our findings reveal large‐ scale between‐population variation of eggshell traits, although we found little environmental dependency in their expression. Our findings call for further studies that explore other environmental factors (e.g. calcium availability and pollution levels) and social factors like sexual selection intensity that may account for differences in shell structure between populations.

SNP variation in ADRB3 gene reflects the breed difference of sheep populations

The β3-adrenergic receptor (ADRB3), a G-protein coupled receptor, plays a major role in energy metabolism and regulation of lipolysis and homeostasis. We detect the single nucleotide polymorphism (SNP) variation in full-length sequence of ovine ADRB3 gene in 12 domestic sheep populations within four types by polymerase chain reaction-single strand conformation polymorphism and sequencing to reveal the breed difference. Twenty-two SNPs, 12 of which in the exon 1 and ten in the intron, were detected, and 12 new exonic and four new intronic SNPs were found. Most SNPs presented in Shanxi Dam Line and least ones in Dorset. The average SNP number in both meat and dual purpose for meat and wool breeds was significantly higher than general and dual purpose breeds for wool and meat. Frequency of each SNP in studied breeds or types was different. The 18C Del and 1617T Ins majorly existed in dual purpose breeds for wool and meat. The 25A Del, 119C>G and 130C>T were mostly found in the meat and dual purpose for meat and wool breeds. The 1764C>A more frequently presented in meat than in other types. The majority of variations came from within the populations as suggested by analysis of molecular variance. Close relationship presented among the Chinese and western breeds, respectively. In conclusion, SNPs of ovine ADRB3 gene can reflect the breed difference and within- and between-population variations, and to a great extent, the breed relationship.

Detection of genetic and phytochemical differences between and within populations of Plantago major L. (plantain)

Leaves of Plantago major L. (common plantain) have been used for centuries almost all over the world as a wound healing remedy, and for other medicinal purposes. Polyphenols have been proposed to be responsible for many of the medicinal properties attributed to P. major. In this study, plants from 19 subpopulations (offspring families), derived from a total of five populations of P. major collected from different parts of Sweden, were screened for genetic variation with molecular markers, and for contents of major polyphenolic compounds. In addition to plantamajoside, four novel poplyphenolic compounds were identified and quantified. RAPD analysis demonstrated considerable between-population variation and very sparse within-population and within-subpopulation variation, just as expected for inbreeding species. Differentiation among populations was associated with the geographical distances between the sampled populations. Differences between populations and subpopulations in polyphenol contents did not reflect the DNA-marker based pattern, suggesting that the expression of the studied polyphenols was affected by, e.g., epigenetic inheritance or a very early environmental influence on the newly germinated seedlings, before potting and randomization.

Genetic variability in tench (Tinca tinca L.) as revealed by PCR-RFLP analysis of mitochondrial DNA

Four mitochondrial DNA segments, ND1, ND6, cyt b and D-loop, were analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 14 tench (Tinca tinca L.) populations located in Europe and Asia; data on 5 Italian populations previously analyzed for the same mtDNA segments were also included in the study. All the considered segments were polymorphic and originated a total of 9 composite haplotypes which were clustered into 2 haplogroups, A and B, possibly corresponding to the Western and Eastern phylogroups previously described in tench. Nine out of 19 populations showed polymorphism, with haplotype diversity ranging from 0.246 to 0.643 and nucleotide diversity from 0.009 to 0.078. Seventy-five percent of the pairwise comparisons were significant, indicating a high between-population variability. The Neighbour-Joining tree revealed the presence of 3 clusters, including pure populations, with only a A or B haplogroup, and mixed populations, with both haplogroups. The possibility of identifying populations with different haplotypes has practical implications for both conservation and supportive stocking.

Mating patterns and pollen dispersal in four contrasting wild cherry populations (Prunus avium L.)

Although pollen dispersal has been extensively studied in trees, parameters influencing between-population variation are still poorly understood. In this study, we conducted paternity analyses on open-pollinated seeds in four natural populations of wild cherry (Prunus avium) with contrasting density and clonal propagation, using eight microsatellite loci and one self-incompatibility system locus. We also measured four quantitative traits and spatial positions as potential correlates of reproductive success. Levels of polyandry differed among populations and 30% of the seed families exhibited unequal paternal contributions, suggesting variation in reproductive success rather than variation in mate availability. Mating occurred preferentially among neighbours in all populations, suggesting that it is a common pattern in wild cherry and probably results from pollinator behaviour. Paternal success was positively correlated with diameter at breast height, as indicated in previous studies and tree dominance only resulted in higher paternal success in low density plots. Mating patterns were thus also affected by both density and tree size. Large-scale studies are needed to disentangle relative influences of these factors on the mating system and pollination success.

The parallel evolution of dwarfism in Arctic charr is accompanied by adaptive divergence in mTOR-pathway gene expression

Ecological factors have a major role in shaping natural variation in body size, although the underlying mechanisms are poorly understood. Icelandic Arctic charr (Salvelinus alpinus L.) populations represent an ideal model to understand body-size evolution, because adult dwarfism has arisen independently on multiple occasions in response to parallel environmental pressures. The mechanistic target of rapamycin (mTOR) pathway transmits signals from the environment to control cellular growth and is a primary candidate to be under selection for the dwarf phenotype. To test this hypothesis, we modified 'inputs' to this pathway in five dwarf and two generalist populations (with ancestral life history and body-size traits), using a standardized manipulation of food intake in a common environment. The skeletal muscle transcript levels of 21 mTOR-pathway genes were quantified in 274 individuals (∼6000 datapoints), and statistical modelling was used to elucidate sources of variation. Constitutive expression differences between populations were the main component of variation for around three-quarters of the studied genes, irrespective of nutritional-state and body-size phenotype. There was evidence for stabilizing selection acting among populations, conserving the nutritionally dependent regulation of pathway genes controlling muscle atrophy. There were three genes (mTOR, 4E-BP-1 and IGFBP4), where the expression variation between dwarf and generalist populations exceeded the between-population variation. Divergence in the expression of these candidate adaptive genes was most evident during a period of rapid growth following sustained fasting and was directionally consistent with their functions regulating growth and protein synthesis. We concluded that selection has operated efficiently to shape gene expression evolution in Icelandic charr populations and that the regulation of certain mTOR-pathway genes evolved adaptively in locations favouring dwarfism, resulting in reduced muscle protein accretion under growth-favouring conditions.

Limited predatory capacity of introduced black rats on bird eggs: an experimental approach

Invasive rats (Rattus spp.) are renowned bird predators and have been identified as a leading cause of island bird population declines and extinctions. Recently, new questions have been raised regarding the mechanisms and the severity of impact of invasive rat predation on bird populations. We investigated the predatory capacity of the invasive black rat Rattus rattus on bird eggs using captive trials on wild-trapped individuals. Five factors were specifically tested for their influence on egg predation success: egg size, egg state, rat body mass, gender and habitat. Our results showed that rats only managed to prey on intact eggs when these were small (canary) and that they had great difficulty preying on medium-sized (hen) and even small (quail)-sized intact eggs, regardless of the rat' body mass, gender and habitat. Conversely, rats preyed extensively on previously damaged eggs of all sizes. Our findings suggest that preying on intact bird eggs without specific learning skills, such as rolling an egg to break it, may be challenging for the black rats. Moreover, our findings strongly indicate that bird susceptibility to egg predation by rats varies with island contexts and may depend on a combination of multiple additive and synergic factors. Experiments that allow for testing the multiple evolutionary and ecological factors explaining between-island or between-population variation in rodent impacts are needed to promote a better overview of the processes involved in bird population declines.