Demographic modeling to fine-tune conservation targets: importance of pre-adults for the decline of an endangered raptor.

Large, long-lived species with slow life histories and protracted pre-breeding stages are particularly susceptible to declines and extinction, often for unknown causes. Here, we show how demographic modeling of a medium-sized raptor, the Red Kite Milvus milvus, can aid to refocus conservation research and attention on the most likely mechanisms driving its decline. Red Kites' survival and reproduction increased through three sequential stages for 1-2, 3-6, and 7-30 yr of age, mainly corresponding to individuals that are dispersing, attempting to gain a territory, and breeding. As typical of long-lived species, elasticities were highest for adult (≥7 yr old) survival, but this was high, with little scope for improvement. Instead, the declines were driven by an extremely low survival of pre-adults in their first years of life, which weakened the whole demographic system by nullifying the offspring contribution of adults and curtailing their replacement by recruits. For example, 27 pairs were necessary to generate a single prime age adult. Simulation of management scenarios suggested that the decline could be halted most parsimoniously by increasing pre-adult survival to the mean levels recorded for other areas, while only the synergistic, simultaneous improvement of breeding success, adult and pre-adult survival could generate a recovery. We propose three actions to attain such goals through selective supplementary feeding of both breeding and non-breeding individuals, and through mortality improvement by GPS remote-sensing devices employed as surveillance monitoring tools. Our results show how improving demographic models by using real, local vital rates rather than "best guess" vital rates can dramatically improve model realism by refocusing attention on the actual stages and mortality causes in need of manipulation, thus building precious time and resources for conservation management. These results also highlight the frequent key role of pre-adult survival for the management of long-lived species, coherent with the idea of demographic systems as integrated chains only as strong as their weakest link.

Individual improvements and selective mortality shape lifelong migratory performance.

Billions of organisms, from bacteria to humans, migrate each year and research on their migration biology is expanding rapidly through ever more sophisticated remote sensing technologies. However, little is known about how migratory performance develops through life for any organism. To date, age variation has been almost systematically simplified into a dichotomous comparison between recently born juveniles at their first migration versus adults of unknown age. These comparisons have regularly highlighted better migratory performance by adults compared with juveniles, but it is unknown whether such variation is gradual or abrupt and whether it is driven by improvements within the individual, by selective mortality of poor performers, or both. Here we exploit the opportunity offered by long-term monitoring of individuals through Global Positioning System (GPS) satellite tracking to combine within-individual and cross-sectional data on 364 migration episodes from 92 individuals of a raptorial bird, aged 1-27 years old. We show that the development of migratory behaviour follows a consistent trajectory, more gradual and prolonged than previously appreciated, and that this is promoted by both individual improvements and selective mortality, mainly operating in early life and during the pre-breeding migration. Individuals of different age used different travelling tactics and varied in their ability to exploit tailwinds or to cope with wind drift. All individuals seemed aligned along a race with their contemporary peers, whose outcome was largely determined by the ability to depart early, affecting their subsequent recruitment, reproduction and survival. Understanding how climate change and human action can affect the migration of younger animals may be the key to managing and forecasting the declines of many threatened migrants.

Integrating population connectivity into pollution assessment: Overwintering mixing reveals flame retardant contamination in breeding areas in a migratory raptor.

Determining the exposure and magnitude at which various pollutants are differentially assimilated at the breeding and non-breeding grounds of migratory wildlife is challenging. Here, the possibility of applying the migratory connectivity framework to understanding contamination in birds is illustrated by considering flame retardants in inviable eggs of a migratory raptor, the black kite (Milvus migrans). The occurrence and concentration of legacy and emerging compounds in eggs from the southeastern peri-urban area of Madrid city, central Spain, were compared with those from Doñana National Park in southern Spain. A much higher occurrence and concentration of multiple polybrominated diphenyl ethers and Dechlorane 602 were found in Madrid than Doñana, but the opposite patterns were found for Dechlorane Plus. Individuals from these and other breeding areas in western Europe showed a strong intermixing pattern over widespread wintering areas in Africa, as assessed by ringing recoveries and movements tracked by satellite devices. This diffuse migratory connectivity reveals breeding areas as the main contamination grounds. High contamination burdens sequestered in eggs point to rapid assimilation of these compounds before laying, associated with important emission sources in Madrid, especially landfills of partially incinerated urban refuse, and other anthropogenic operations. Diet composition regarding aquatic vs. terrestrial prey, and bioaccumulation and biomagnification processes are suggested to explain differential assimilation of some compounds, especially Dechlorane Plus in Doñana, although a local emission source polluting this area cannot be ruled out. Insight from the migratory connectivity framework can help to disentangle large-scale patterns of contaminant uptake and refocus attention on key regions and potential causes of chemical hazards in declining migratory species and human populations.

Lifetime variation in feather corticosterone levels in a long-lived raptor.

In long-lived animals, the challenges that threaten individual homeostasis, and the way they are dealt with, are expected to vary in an age-related manner, encompassing the progressive selection of superior phenotypes and the acquisition and improvement of key skills (e.g. foraging, breeding and fighting abilities). Since exposure to homeostatic challenges typically elevates circulating glucocorticoid (GC) levels in vertebrates (modulating the behavioural and physiological responses that mediate allostasis), we may expect concomitant age-related changes in these hormones. Here, we investigated whether the level of corticosterone (the main avian GC) deposited in feathers during regular moult reflected the expected lifelong progression of energetic challenges in a long-lived raptor, the black kite (Milvus migrans). Feather corticosterone values were highest in the youngest birds, gradually declined to reach minimum levels in prime age, 7- to 11-year-old birds, and then increased again slightly among the oldest, senescent birds (≥12 years old). This pattern mirrored the age-related changes in reproductive success and survival rates previously reported for this population, suggesting that feather corticosterone levels captured the most vulnerable and challenging periods experienced by these birds as they proceeded through life. Moreover, feather corticosterone levels were negatively related to body size, suggesting that larger birds either experienced fewer homeostatic challenges, or were better able to cope with them. Feather corticosterone measures thus provided a valuable snapshot of how allostatic loads vary along the life of individuals, supporting the idea of a tight, long-term link between cumulative physiological responses to ecological challenges and demographic performance.

Hardship at birth alters the impact of climate change on a long-lived predator.

Climate change is increasing the frequency of extreme events, such as droughts or hurricanes, with substantial impacts on human and wildlife communities. Extreme events can affect individuals through two pathways: by altering the fitness of adults encountering a current extreme, and by affecting the development of individuals born during a natal extreme, a largely overlooked process. Here, we show that the impact of natal drought on an avian predator overrode the effect of current drought for decades, so that individuals born during drought were disadvantaged throughout life. Incorporation of natal effects caused a 40% decline in forecasted population size and a 21% shortening of time to extinction. These results imply that climate change may erode populations more quickly and severely than currently appreciated, suggesting the urgency to incorporate "penalties" for natal legacies in the analytical toolkit of impact forecasts. Similar double impacts may apply to other drivers of global change.

Compensation for wind drift during raptor migration improves with age through mortality selection.

Each year, billions of flying and swimming migrants negotiate the challenging displacement imposed by travelling through a flowing medium. However, little is known about how the ability to cope with drift improves through life and what mechanisms drive its development. We examined 3,140 days of migration by 90 GPS-tagged raptorial black kites (Milvus migrans) aged 1-27 years to show that the ability to compensate for lateral drift develops gradually through many more years than previously appreciated. Drift negotiation was under strong selective pressure, with inferior navigators subject to increased mortality. This progressively selected for adults able to compensate for current cross flows and for previously accumulated drift in a flexible, context-dependent and risk-dependent manner. Displacements accumulated en route carried over to shape the wintering distribution of the population. For many migrants, migratory journeys by younger individuals represent concentrated episodes of trait selection that shape adult populations and mediate their adaptation to climate change.

Simultaneous analysis of multiple PCR amplicons enhances capillary SSCP discrimination of MHC alleles.

Major histocompatibility complex (MHC) genotyping still remains one of the most challenging issues for evolutionary ecologists. To date, none of the proposed methods have proven to be perfect, and all provide both important pros and cons. Although denaturing capillary electrophoresis has become a popular alternative, allele identification commonly relies upon conformational polymorphisms of two single-stranded DNA molecules at the most. Using the MHC class II (beta chain, exon 2) of the black kite (Aves: Accipitridae) as our model system, we show that the simultaneous analysis of overlapping PCR amplicons from the same target region substantially enhances allele discrimination. To cover this aim, we designed a multiplex PCR capable to generate four differentially sized and labeled amplicons from the same allele. Informative peaks to assist allele calling then fourfold those generated by the analysis of single PCR amplicons. Our approach proved successful to differentiate all the alleles (N=13) isolated from eight unrelated birds at a single optimal run temperature and electrophoretic conditions. In particular, we emphasize that this approach may constitute a straightforward and cost-effective alternative for the genotyping of single or duplicated MHC genes displaying low to moderate sets of divergent alleles.

Reproductive endocrinology of wild, long-lived raptors.

The last decades have witnessed a surge of studies analyzing the role of sex hormones on the behavior and ecology of wild bird populations, allowing a more integrated view of the evolution of avian physiology and life histories. Despite a marked progress, field studies show a considerable bias towards research on specific phylogenetic groups, neglecting a significant fraction of the class Aves. Here we analysed changes in the circulating levels of sex steroids in relation to reproductive behaviour in wild black kites (Milvus migrans), a long-lived and socially monogamous Accipitridae raptor. Males and females displayed a single seasonal peak of circulating testosterone (males) and estradiol (females) during pre-laying and laying. Absolute male testosterone levels were low even at the seasonal maximum and remained below detection limits in females. The latter results supports the idea that avian species establishing long-term pair bonds require lower amounts of circulating androgens for reproduction. Circulating progesterone showed a single seasonal peak in females and males, but their timing (during Incubation and Post-brooding respectively) did not overlap. The fact that females black kites perform the majority of incubation and males provide the majority of care to fledglings suggests that progesterone is involved in the expression of parental behaviors.

When and where mortality occurs throughout the annual cycle changes with age in a migratory bird: individual vs population implications.

The annual cycle of most animals is structured into discrete stages, such as breeding, migration and dispersal. While there is growing appreciation of the importance of different stages of an organism's annual cycle for its fitness and population dynamics, almost nothing is known about if and how such seasonal effects can change through a species lifespan. Here, we take advantage of the opportunity offered by a long-term satellite/GPS-tracking study and a reliable method of remote death-detection to show that certain stages of both the annual and life cycle of a migratory long-lived raptor, the Black kite Milvus migrans, may represent sensitive bottlenecks for survival. In particular, migratory journeys caused bursts of concentrated-mortality throughout life, but the relative importance of stage-specific survival changed with age. On the other hand, the balance between short-stages of high mortality and long-stages of low mortality made population-growth similarly dependent on all portions of the annual cycle. Our results illustrate how the population dynamics of migratory organisms can be inextricably linked to ecological pressures balanced over multiple stages of the annual cycle and thus multiple areas of the globe, suggesting the frequent need for challenging conservation strategies targeting all portions of a species year-round range.

Effects of Ontogeny, Diel Rhythms, and Environmental Variation on the Adrenocortical Physiology of Semialtricial Black Kites (Milvus migrans).

We examined whether hypothalamic-pituitary-adrenal axis activity in the nestlings of a semialtricial raptor, the black kite (Milvus migrans), varied with advancing age, throughout the day, and in response to a number of socioecological factors presumed to affect allostatic load. Both baseline corticosterone (CORT) titers and maximum CORT levels during 30 min of handling and restraint augmented across all sampled ages, suggesting that nestlings' energetic demands and capacity to respond to perturbations increase progressively throughout development. CORT secretion also peaked in the early morning, reached minimum levels in the central hours of the day, and increased again before dusk, suggesting a possible role of CORT in the regulation of activity-inactivity patterns. Finally, nestlings raised in a year of low marsh inundation, implying lower food availability and heightened allostatic loads, exhibited higher adrenocortical responsiveness to stress than nestlings raised in years of intermediate or high flooding. The nondetectable effect of other socioecological variables, such as territory quality, temperature, or brood order, suggests that parents may be able to buffer their nestlings from adverse environmental conditions or that the effect of such factors may have been obscured by selective mortality operating before sampling. We propose that future studies increase the simultaneous use of complementary techniques (fecal sampling, feather analysis) to reach firmer and more comprehensive conclusions, especially for planning the management and conservation of sensitive species.

Different location sampling frequencies by satellite tags yield different estimates of migration performance: pooling data requires a common protocol.

BACKGROUND: Migration research is in rapid expansion and increasingly based on sophisticated satellite-tracking devices subject to constant technological refinement, but is still ripe with descriptive studies and in need of meta-analyses looking for emergent generalisations. In particular, coexistence of studies and devices with different frequency of location sampling and spatial accuracy generates doubts of data compatibility, potentially preventing meta-analyses. We used satellite-tracking data on a migratory raptor to: (1) test whether data based on different location sampling frequencies and on different position subsampling approaches are compatible, and (2) seek potential solutions that enhance compatibility and enable eventual meta-analyses. METHODOLOGY/PRINCIPAL FINDINGS: We used linear mixed models to analyse the differences in the speed and route length of the migration tracks of 36 Black kites (Milvus migrans) satellite-tagged with two different types of devices (Argos vs GPS tags), entailing different regimes of position sampling frequency. We show that different location sampling frequencies and data subsampling approaches generate large (up to 33%) differences in the estimates of route length and migration speed of this migratory bird. CONCLUSIONS/SIGNIFICANCE: Our results show that the abundance of locations available for analysis affects the tortuosity and realism of the estimated migration path. To avoid flaws in future meta-analyses or unnecessary loss of data, we urge researchers to reach an agreement on a common protocol of data presentation, and to recognize that all transmitter-based studies are likely to underestimate the actual distance traveled by the marked animal. As ecological research becomes increasingly technological, new technologies should be matched with improvements in analytical capacity that guarantee data compatibility.