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Changes in body size have been documented across taxa in response to human activities and climate change. Body size influences many aspects of an individual's physiology, behavior, and ecology, ultimately affecting life history performance and resilience to stressors. In this study, we developed an analytical approach to model individual growth patterns using aerial imagery collected via drones, which can be used to investigate shifts in body size in a population and the associated drivers. We applied the method to a large morphological dataset of gray whales (Eschrichtius robustus) using a distinct foraging ground along the NE Pacific coast, and found that the asymptotic length of these whales has declined since around the year 2000 at an average rate of 0.05-0.12 m/y. The decline has been stronger in females, which are estimated to be now comparable in size to males, minimizing sexual dimorphism. We show that the decline in asymptotic length is correlated with two oceanographic metrics acting as proxies of habitat quality at different scales: the mean Pacific Decadal Oscillation index, and the mean ratio between upwelling intensity in a season and the number of relaxation events. These results suggest that the decline in gray whale body size may represent a plastic response to changing environmental conditions. Decreasing body size could have cascading effects on the population's demography, ability to adjust to environmental changes, and ecological influence on the structure of their community. This finding adds to the mounting evidence that body size is shrinking in several marine populations in association with climate change and other anthropogenic stressors. Our modeling approach is broadly applicable across multiple systems where morphological data on megafauna are collected using drones., (© 2024 The Author(s). Global Change Biology published by John Wiley & Sons Ltd.)

Individual specialization may occur relative to diet, behaviour or spatial distribution, potentially leading to differential resource and space use within a population. While specializations have been documented across many animal populations, the underlaying causes of individual specialization (e.g. morphology, age or sex) are not always identified. Causes of specialization can be especially challenging to uncover for large, long-lived marine animals. We used a Bayesian multilevel, multinomial logistic regression model to study the relationships between grey whale, Eschrichtius robustus , use of foraging tactics and morphology (body length and condition), while accounting for habitat characteristics and individual variation in tactic use. The model was informed by a 7-year longitudinal data set of concurrent morphology and foraging behaviour collected using drones. We found evidence of an ontogenetic shift in the use of foraging tactics associated with body length (a proxy for age). Individual specialization in behaviour was also associated with water depth and habitat. After accounting for the effects of these covariates, there was some residual individual level variation in the use of different foraging tactics. Our findings demonstrate variation in resource and habitat use within a baleen whale population at the individual level relative to body length and habitat, suggesting that individual spatial distribution and access to prey may vary by age class. Our results can be applied to investigate whether juveniles and adults differ in their foraging success and resilience to stressors. • We used drones to measure behaviour and morphology of eastern grey whales. • Whale foraging behaviours were associated with morphology and habitat type. • Different foraging tactics were associated with different depths and habitat types. • Whales switched foraging tactics as they grew in length. • Morphology affected habitat and prey accessibility and consequently tactic use. [ABSTRACT FROM AUTHOR]