A meta‐analysis of global avian survival across species and latitude

The role of tropical rainfall in driving range dynamics for a long-distance migratory bird

Predicting how the range dynamics of migratory species will respond to climate change requires a mechanistic understanding of the factors that operate across the annual cycle to control the distribution and abundance of a species. Here, we use multiple lines of evidence to reveal that environmental conditions during the nonbreeding season influence range dynamics across the life cycle of a migratory songbird, the American redstart (Setophaga ruticilla). Using long-term data from the nonbreeding grounds and breeding origins estimated from stable hydrogen isotopes in tail feathers, we found that the relationship between annual survival and migration distance is mediated by precipitation, but only during dry years. A long-term drying trend throughout the Caribbean is associated with higher mortality for individuals from the northern portion of the species' breeding range, resulting in an approximate 500 km southward shift in breeding origins of this Jamaican population over the past 30 y. This shift in connectivity is mirrored by changes in the redstart's breeding distribution and abundance. These results demonstrate that the climatic effects on demographic processes originating during the tropical nonbreeding season are actively shaping range dynamics in a migratory bird.

A global evaluation of the associations between long-term dynamics of seed falls and rodents

One classic system of pulsed resource and animal population is mast seeding and population dynamics of seed-eating rodents in forests. However, we still lack an understanding of the global patterns regarding the contributions of seed falls to rodent outbreaks or population dynamics. We analyzed a global dataset of coupled long-term time series of seed abundances and rodent populations from published literature, including 66 and 89 time series (156 rodent-seed pairs from 37 studies) for rodent and seed abundances, respectively. We found only half of the examined rodent populations showed statistically significant coincidence between rodent outbreak and mast-seeding years. Over all the coupled time series, seed abundance was found to positively correlate with rodent abundance with a one-year lag, and the relative importance of seed abundance was much lower than that of density dependence in affecting rodent population growth rates. We also found the relative importance of seed abundance decreased, but that of rodent density dependence increased with the latitude of study. For the first time, our work provides a global pattern on the associations between seed falls and rodent population dynamics mostly in mid- and high-latitude forests, and highlights the necessity of more long-term studies on this subject in more forest ecosystems.

Riskiness of Movement Lifestyle Varies Inversely with Adult Survival Probability among Species

AbstractWhy do species differ in their movement lifestyles? Animals that spend more time sitting motionless and acquire food using less conspicuous movements can be more vigilant and less obvious to predators. More active animals that use food types and sites that require more conspicuous behaviors increase vulnerability to predators. Life history theory predicts that aversiveness to mortality risk evolves inversely to adult survival probability. Consequently, we postulated that long-lived species evolved inconspicuous movement lifestyles, whereas shorter-lived species use more conspicuous movement lifestyles. We tested this hypothesis by quantifying the movement lifestyles of nine tropical songbird species. Use of conspicuous movement and foraging behaviors, such as flying and hovering, was greatest in shorter-lived species and decreased with increasing adult survival probability across species. Similarly, foraging speed decreased with increasing adult survival based on a meta-analysis of 64 songbird species. Faster and conspicuous movement lifestyles of shorter-lived species likely increase food acquisition rates, which fits with faster life history strategies that include more feeding trips for young and faster growth. Similarly, slow movement lifestyles of long-lived species fit with the reduced food needs of slower life history strategies. Movement lifestyles may have evolved as an integrated component of the slow-fast life history continuum.

Survival rates in the world's southernmost forest bird community

The Magellanic sub-Antarctic Forest is home to the world's southernmost avian community and is the only Southern Hemisphere analogue to Northern Hemisphere temperate forests at this latitude. This region is considered among the few remaining pristine areas of the world, and shifts in environmental conditions are predominantly driven by climate variability. Thus, understanding climate-driven demographic processes is critical for addressing conservation issues in this system under future climate change scenarios. Here, we describe annual survival patterns and their association with climate variables using a 20-year mark-recapture data set of five forest bird species in the Cape Horn Biosphere Reserve. We develop a multispecies hierarchical survival model to jointly explore age-dependent survival probabilities at the community and species levels in a group of five forest passerines. At the community level, we assess the association of migratory behavior and body size with survival, and at the species level, we investigate the influence of local and regional climatic variables on temporal variations of survival. We found a positive effect of precipitation and a negative effect of El Niño Southern Oscillation on juvenile survival in the white-crested Elaenia and a consistent but uncertain negative effect of temperature on survival in juveniles and 80% of adults. We found only a weak association of climate variables with survival across species in the community and no temporal trends in survival for any of the species in either age class, highlighting apparent stability in these high austral latitude forests. Finally, our findings provide an important resource of survival probabilities, a necessary input for assessing potential impacts of global climate change in this unique region of the world.

Similar regional-scale survival of tropical and southern temperate birds from the New World

The general assumption that the survival patterns of tropical and southern temperate birds are similar lacks empirical data from higher latitudes. Regional comparisons of New World species are rare, and this assumption has been based on data from African studies. Here, we estimate the survival rates of 88 tropical and southern temperate bird populations (69 species) from eight localities in South America to evaluate the hypothesis that the survival of these populations is homogeneous at the regional scale. We estimated survival based on the Cormack-Jolly-Seber model and compared values from different environments. The survival estimates ranged from 0.30 to 0.80 (0.56 ± 0.12). Apparent survival did not differ significantly between low-latitude tropical environments (03°S) and the other sites from high-latitudes (between 22° and 34°S). Despite a predicted positive trend, body size was not significantly related to survival among passerines. On the other hand, phylogenetic relationships explained more than a third of the variation in bird survival. Based on the largest available database on South American bird species, our findings support the hypothesis that bird survival is homogeneous, at the regional scale, along the southern hemisphere. In particular, we reinforce the hypothesis that climatic variation has a limited influence on bird survival in the southern hemisphere.

Decoupling pioneering traits from latitudinal patterns in a north American bird experiencing a southward range shift

Ecogeographic rules describe spatial patterns in biological trait variation and shed light on the drivers of such variation. In animals, a consensus is emerging that 'pioneering' traits may facilitate range shifts via a set of bold, aggressive and stress-resilient traits. Many of these same traits are associated with more northern latitudes, and most range shifts in the northern hemisphere indicate northward movement. As a consequence, it is unclear whether pioneering traits are simply corollaries of existing latitudinal variation, or whether they override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation. The tree swallow Tachycineta bicolor is a songbird undergoing a southward range shift in the eastern United States, in direct opposition of the poleward movement seen in most other native species' range shifts. Because this organic range shift countervails the typical direction of movement, this case study provides for unique ecological insights on organisms and their ability to thrive in our changing world. We sampled female birds across seven populations, quantifying behavioural, physiological and morphological traits. We also used GIS and field data to quantify a core set of ecological factors with strong ties to these traits as well as female performance. Females at more southern expansion sites displayed higher maternal aggression, higher baseline corticosterone and more pronounced elevation of corticosterone following a standardized stressor, contrary to otherwise largely conserved latitudinal patterning in these traits. Microhabitat variation explained some quantitative phenotypic variation, but the expansion and historic ranges did not differ in openness, distance to water or breeding density. This countervailing range shift therefore suggests that pioneering traits are not simply corollaries of existing latitudinal variation, but rather, they may override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation.

Susceptibility to Predation Varies with Body Mass, Foraging Niche, and Anti-Predator Responses among Bird Species

Predation is a major source of mortality for many avian species. Species that face more predators, and those with less effective anti-predator responses, are presumably more likely to die from predation over time. Predation rate, as a measure of susceptibility to predation, is difficult to measure in the field. Radio-tracking studies, however, allow researchers to determine the time and cause of death of marked individuals, making it possible to estimate predation rate. I used estimates of predation rates from a large number of published radio-tracking studies in birds to assess in a phylogenetic framework the effect of several potential determinants. I obtained 393 estimates of predation rates from 129 species. Predation rates were lower in areas with fewer predators, such as islands and aquatic habitats, and for species with fewer potential predators, such as larger species. The predation rate was also lower for prey species with effective anti-predator responses, such as those that forage in flocks. Radio-tracking studies provide a unique opportunity to estimate overall predation rate in the field. Broadening the range of species and the range of habitats involved in such studies will help to further elucidate the factors that affect susceptibility to predation.

Global drivers of variation in cup nest size in passerine birds

The size of a bird's nest can play a key role in ensuring reproductive success and is determined by a variety of factors. The primary function of the nest is to protect offspring from the environment and predators. Field studies in a number of passerine species have indicated that higher-latitude populations in colder habitats build larger nests with thicker walls compared to lower-latitude populations, but that these larger nests are more vulnerable to predation. Increases in nest size can also be driven by sexual selection, as nest size can act as a signal of parental quality and prompt differential investment in other aspects of care. It is unknown, however, how these microevolutionary patterns translate to a macroevolutionary scale. Here, we investigate potential drivers of variation in the outer and inner volume of open cup nests using a large dataset of nest measurements from 1117 species of passerines breeding in a diverse range of environments. Our dataset is sourced primarily from the nest specimens at the Natural History Museum (UK), complemented with information from ornithological handbooks and online databases. We use phylogenetic comparative methods to test long-standing hypotheses about potential macroevolutionary correlates of nest size, namely nest location, clutch size and variables relating to parental care, together with environmental and geographical factors such as temperature, rainfall, latitude and insularity. After controlling for phylogeny and parental body size, we demonstrate that the outer volume of the nest is greater in colder climates, in island-dwelling species and in species that nest on cliffs or rocks. By contrast, the inner cup volume is associated solely with average clutch size, increasing with the number of chicks raised in the nest. We do not find evidence that nest size is related to the length of parental care for nestlings. Our study reveals that the average temperature in the breeding range, along with several key life-history traits and proxies of predation threat, shapes the global interspecific variation in passerine cup nest size. We also showcase the utility of museum nest collections-a historically underused resource-for large-scale studies of trait evolution.

Latitudinal but not elevational variation in blood glucose level is linked to life history across passerine birds

Macrophysiological research is vital to our understanding of mechanisms underpinning global life history variation and adaptation to diverse environments. Here, we examined latitudinal and elevational variation in a key substrate of energy metabolism and an emerging physiological component of pace-of-life syndromes, blood glucose concentration. Our data, collected from 61 European temperate and 99 Afrotropical passerine species, revealed that baseline blood glucose increases with both latitude and elevation, whereas blood glucose stress response shows divergent directions, being stronger at low latitudes and high elevations. Low baseline glucose in tropical birds, compared to their temperate counterparts, was mainly explained by their low fecundity, consistent with the slow pace-of-life syndrome in the tropics. In contrast, elevational variation in this trait was decoupled from fecundity, implying a unique montane pace-of-life syndrome combining slow-paced life histories with fast-paced physiology. The observed patterns suggest that pace-of-life syndromes do not evolve along the single fast-slow axis.

Coevolution of relative brain size and life expectancy in parrots

Previous studies have demonstrated a correlation between longevity and brain size in a variety of taxa. Little research has been devoted to understanding this link in parrots; yet parrots are well-known for both their exceptionally long lives and cognitive complexity. We employed a large-scale comparative analysis that investigated the influence of brain size and life-history variables on longevity in parrots. Specifically, we addressed two hypotheses for evolutionary drivers of longevity: the cognitivebuffer hypothesis, which proposes that increased cognitive abilities enable longer lifespans, and the expensive brain hypothesis, which holds that increases in lifespan are caused by prolonged developmental time of, and increased parental investment in, large-brained offspring. We estimated life expectancy from detailed zoo records for 133 818 individuals across 244 parrot species. Using a principled Bayesian approach that addresses data uncertainty and imputation of missing values, we found a consistent correlation between relative brain size and life expectancy in parrots. This correlation was best explained by a direct effect of relative brain size. Notably, we found no effects of developmental time, clutch size or age at first reproduction. Our results suggest that selection for enhanced cognitive abilities in parrots has in turn promoted longer lifespans.

Flocking in birds increases annual adult survival in a global analysis

Adaptive hypotheses for the evolution of group foraging in animals typically invoke enhanced foraging efficiency and reduced predation risk. Net benefits of group foraging in different species should translate into a survival advantage for group members. Despite numerous interspecific studies in birds and mammals, few have documented a survival advantage for group foraging species. Using a large dataset in birds (> 1100 species worldwide), I investigated whether annual adult apparent survival was higher in species that forage in flocks than in solitary species. Using a phylogenetic framework to account for relatedness among species and controlling for known correlates of adult survival in birds such as body size, clutch size, latitude, and diet, I documented a positive effect of flocking on annual adult apparent survival. The increase in survival was less pronounced in occasionally flocking species suggesting that the benefits of group foraging can depend on the frequency of its use. The results highlight how group foraging can increase fitness in animals.

Bridging gaps in demographic analysis with phylogenetic imputation

Phylogenetically informed imputation methods have rarely been applied to estimate missing values in demographic data but may be a powerful tool for reconstructing vital rates of survival, maturation, and fecundity for species of conservation concern. Imputed vital rates could be used to parameterize demographic models to explore how populations respond when vital rates are perturbed. We used standardized vital rate estimates for 50 bird species to assess the use of phylogenetic imputation to fill gaps in demographic data. We calculated imputation accuracy for vital rates of focal species excluded from the data set either singly or in combination and with and without phylogeny, body mass, and life-history trait data. We used imputed vital rates to calculate demographic metrics, including generation time, to validate the use of imputation in demographic analyses. Covariance among vital rates and other trait data provided a strong basis to guide imputation of missing vital rates in birds, even in the absence of phylogenetic information. Mean NRMSE for null and phylogenetic models differed by <0.01 except when no vital rates were available or for vital rates with high phylogenetic signal (Pagel's λ > 0.8). In these cases, including body mass and life-history trait data compensated for lack of phylogenetic information: mean normalized root mean square error (NRMSE) for null and phylogenetic models differed by <0.01 for adult survival and <0.04 for maturation rate. Estimates of demographic metrics were sensitive to the accuracy of imputed vital rates. For example, mean error in generation time doubled in response to inaccurate estimates of maturation time. Accurate demographic data and metrics, such as generation time, are needed to inform conservation planning processes, for example through International Union for Conservation of Nature Red List assessments and population viability analysis. Imputed vital rates could be useful in this context but, as for any estimated model parameters, awareness of the sensitivities of demographic model outputs to the imputed vital rates is essential.

Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the intensity of biotic interactions increases from high to low latitudes. This hypothesis, which may partly explain latitudinal gradients in biodiversity, remains hotly debated, largely due to variable outcomes of published studies. We used meta-analysis to identify the scope of the LBIH in terrestrial ecosystems. For this purpose, we explored the sources of variation in the strength of latitudinal changes in herbivory, carnivory and parasitism (119 publications) and compared these gradients with gradients in the diversity of the respective groups of animals (102 publications). Overall, both herbivory and carnivory decreased towards the poles, while parasitism increased. The latitudinal gradient in herbivory and carnivory was threefold stronger above 50-60° than at lower latitudes and was significant due to interactions involving ectothermic consumers, studies using standardised prey (i.e. prey lacking local anti-predator adaptations) and studies aimed at testing LBIH. The poleward decrease in biodiversity did not differ between ectothermic and endothermic animals or among climate zones and was fourfold stronger than decrease in herbivory and carnivory. The discovered differences between the gradients in biotic interactions and biodiversity suggest that these two global macroecological patterns are likely shaped by different factors.

The macroecology of extra-pair paternity in birds

Extra-pair paternity (EPP) is a key aspect of the mating behaviour of birds and its frequency varies widely among populations and species. Several hypotheses predict patterns of geographical variation in the occurrence and frequency of EPP, but a global-scale study on variation in this trait is still lacking. We collected data on EPP from 663 populations of 401 avian species and explored the geographical variation in the frequency of EPP among populations, species and species assemblages. We modelled the variation in the frequency of EPP within the species' breeding range accounting for the specific ecological context of each population, and used the model predictions to compute frequencies of EPP at the level of species assemblages. A global map of assemblage-level EPP rates shows clear differences between zoogeographical realms, with the highest EPP values in the Nearctic realm. Our results show that the frequency of EPP (1) decreases with latitude and increases with the distance from the breeding range boundary within the species' breeding range, (2) is negatively associated with generation length and pair-bond duration among species, and (3) decreases with latitude at assemblage level. The latitudinal decline of EPP is consistent across zoogeographical realms.

Do avian species survive better on islands?

Island species are often predictably different from their mainland counterparts. Milder climates and reduced predation risk on islands have been involved to explain shifts in body size and a suite of life-history traits such as clutch size and offspring growth rate. Despite the key role of adult survival on risk taking and reproduction, the prediction that living on islands increases adult survival has yet to be tested systematically. I gathered data on adult annual apparent survival from the island and mainland year-round resident species of birds from around the world. With this large dataset (697 species), I found that species of birds living on islands showed higher apparent survival than their mainland counterparts in the two Hemispheres and at all latitudes, controlling for several known predictors of adult survival, including body size, clutch size and breeding system. These results shed light on the ecological factors that influence survival on islands and extend the life-history island syndrome to adult survival.