Climate change is intensifying extreme weather events like droughts and hurricanes, significantly disrupting ecosystems and impacting both human and wildlife populations. While research often focuses on the immediate effects of these events on adult organisms, the long-term consequences of experiencing such extremes during development (natal effects) are largely overlooked. This study investigates the dual pathways by which extreme events affect populations: (1) direct impacts on adult survival and reproduction, and (2) the long-term consequences for individuals born during these events. Two key hypotheses from life-history evolution and human medicine are relevant: the Predictive Adaptive Response (PAR) hypothesis, which suggests that early hardship might confer future advantages under similar conditions, and the Developmental Constraint (or "silver spoon") hypothesis, which posits that early hardship permanently impairs fitness regardless of later conditions. Understanding these pathways is crucial for accurate prediction of future climate impacts and effective conservation strategies. This research examines the demographic response of red kites (*Milvus milvus*) to drought, a long-lived avian predator, to determine the relative importance of natal and contemporary drought conditions on survival and population dynamics.

Numerous studies have documented the impacts of climate-driven disturbances on ecological communities. However, relatively few have explored the demographic mechanisms underlying these responses, especially the long-term effects of natal conditions. Existing research has demonstrated the influence of early-life conditions on later life history traits across various species, including fish, reptiles, birds, mammals, and humans. Individuals experiencing poor habitats, stressful environments, or low food availability during development often exhibit reduced survival and reproductive success as adults. The PAR and developmental constraint hypotheses offer contrasting predictions about how early hardship influences fitness later in life. The PAR hypothesis suggests that early adversity may prepare individuals to better cope with future challenges, while the developmental constraint hypothesis suggests that early hardship permanently reduces fitness. This study contributes to the field by explicitly comparing these hypotheses using a long-lived species and incorporates the complexities of both natal and contemporary conditions.

This study was conducted in Doñana National Park, Spain, a region experiencing increasing drought frequency. Researchers used a long-term dataset (1975-2018) of red kites, a long-lived raptor whose breeding success and survival are closely tied to the availability of prey in the seasonally flooded marshes. Drought years were defined as those falling below the 20th percentile of maximum marsh flooding on March 30th, based on satellite imagery. The study analyzed the impact of drought on various life stages and demographic parameters. Prey abundance was assessed using transects, and provisioning rates were measured using camera traps positioned near nests. Nestling body size and condition were measured upon ringing, and growth curves were constructed to estimate body condition. Capture-mark-recapture models were employed to estimate survival probabilities, incorporating birth year (natal conditions) and contemporary drought as explanatory variables. Brood rank (birth order) was also considered, reflecting the inherent nutritional differences between nestlings. The researchers used matrix population models to predict population trajectories under different drought frequency scenarios, both including and excluding natal effects. Statistical analyses included linear mixed models, generalized mixed models, and capture-mark-recapture models implemented in the program MARK.

Drought significantly reduced prey biomass and provisioning rates to nestlings. Breeding performance declined across multiple measures during drought years, including increased reproductive skipping, brood reduction, and nest predation, as well as reduced fledgling production. Nestling body condition was significantly lower during droughts, although body size was not affected. Natal drought had a more pronounced effect on survival than contemporary drought. Kites born during drought had substantially lower survival probabilities throughout their lives, even in the absence of subsequent drought events. For instance, even with ideal adult conditions, kites born in drought only had an average lifespan of 1.2 years, compared to 9.6 years for kites born in normal years. Incorporating natal effects into population models revealed a 40% decrease in projected population size and a 21% reduction in time to extinction compared to models considering only contemporary drought. Increased drought frequency further accelerated population decline and hastened extinction.

The findings demonstrate the significant and long-lasting impacts of natal drought on red kite populations. The strong negative influence of natal conditions, overriding the effects of contemporary conditions, emphasizes the importance of considering developmental legacies in assessing climate change impacts. The results strongly support the developmental constraint hypothesis, demonstrating that early hardship permanently impairs fitness even in the absence of later environmental challenges. The significant reduction in predicted population size and time to extinction when incorporating natal effects highlights the limitations of models that only account for contemporary environmental conditions. These findings suggest that current assessments of climate change impacts may underestimate the severity of population declines and the risk of extinction, particularly for long-lived species. The study’s focus on a long-lived species reveals the potential for delayed and insidious effects of climate change that could manifest as sudden crashes.

This study reveals that natal effects of climate change can profoundly influence population dynamics in long-lived species, causing more rapid and severe declines than previously anticipated. The inclusion of natal effects in population models is critical for accurate predictions and effective conservation strategies. Future research should investigate the mechanisms behind these effects and extend these findings to other species and types of environmental stressors. Implementing management strategies that mitigate the negative consequences of natal effects, such as supplemental feeding during droughts, may be crucial for conserving long-lived species threatened by climate change.

The study focused on a single species and region, limiting the generalizability of findings. While the methodology attempted to account for confounding factors, unforeseen interactions or other environmental pressures may have influenced the results. The model assumed a relatively simple relationship between drought severity and impacts, without exploring more nuanced drought intensity effects. Although the study's extensive dataset enhanced its reliability, future research could benefit from even longer term monitoring to strengthen these conclusions further. The prey abundance survey was broad, potentially missing some prey types that might have altered the outcome.