How does the abundance of rabbits affect the eagle owl's selection of the size of its prey?

    A study reveals that the eagle owl is capable of adapting its predatory capacity on prey of various sizes to maximize its energy efficiency based on its abundance


    Understanding how predators and their prey are related to each other within an ecosystem is fundamental to understanding the ecology of the fauna species involved and their populations. One of the main predator-prey relationships derives from the selection of prey by predators. In this field of trophic ecology, optimal diet theory predicts that predators prefer profitable prey in terms of energy efficiency, based on the balance between the energy they provide and the energy needed to find, capture, and consume them.

    But this predator-prey relationship is modulated by various factors that can lead to a greater or lesser specialization of the predator on a specific food resource or prey, maintaining a constant coevolution. These factors include the abundance of prey and its body size..

    Variations in the abundance of prey can trigger behavioral changes related to the trophic ecology of the predator, which could condition the selection of prey based on their body size. Thus, when prey is in high abundance and easy to hunt, predators tend to concentrate on the largest individuals, but when prey abundance is low, predators must use a broader range of prey types and sizes. , including the smallest individuals.

     

    the eagle owl (vulture vulture) has been described as a facultative predator of rabbits in the central Iberian Peninsula, meaning that it switches to alternative prey, with body sizes similar to or larger than adult rabbits, when rabbit abundance decreases (Photo: © Javier Fernandez Vega).

    Recently, the eagle owl (vulture vulture) has been described as a facultative predator of rabbits in the center of the Iberian Peninsula., meaning it switches to alternative prey, with body sizes similar to or larger than adult rabbits, when rabbit abundance declines. Therefore, it would be expected that in areas with low rabbit abundance, the eagle owl would increase the size of the rabbits hunted to compensate for the decrease in available food.

    However, it has been suggested that great owls select the larger size categories when prey abundance is high due to their higher profitability, while in areas with low prey abundance they tend to select smaller prey in order to maximize profit. Energy efficiency. Therefore, in relation to the trophic ecology of owl species, the relationship between prey abundance and selection for body size is unclear.

    To answer this question, researchers from the Research Group in Game Resources and Wildlife Management of the Instituto de Investigación en Recursos Cinegéticos (IREC – CSIC, UCLM, JCCM), in collaboration with scientists from the University of Córdoba, the University of Málaga and the Institute for Advanced Social Studies (IESA-CSIC), have analyzed the variation in the eagle owl's diet, and in particular the size of hunted rabbits, in a controlled semi-natural population (Los Melonares) with high availability of rabbits, comparing it with two natural populations (Ruidera and Ossa de Montiel) with low to high abundance of rabbits.

    Results in the controlled population have shown that the eagle owl positively selects for the smallest rabbits (weighing less than 300 grams) throughout the year, regardless of the size distribution of the rabbits within the population. However, this selection was only significant when the abundance of rabbits reached the highest levels (Spring-Autumn).

    On the contrary, in the controlled population, owls tended to avoid larger rabbits (weighing more than 900 grams) throughout the year, and this observation was not significant in scenarios with a low abundance of rabbits (Winter). For his part, in natural populations, smaller rabbits were included in the eagle owl's diet more frequently as rabbit abundance increased.

    These results show that the optimal diet theory does not always hold, there being a great variability in the response of predators in the selection of prey based on its abundance. Therefore, although adult rabbits are apparently easy for the eagle owl to capture and it is presumed that it should select the largest ones in conditions of high abundance, the data obtained in this work show that, in these situations of high abundance , the owl prefers to select the most vulnerable and easy to detect individuals, which are the juvenile individuals.

     

    The graph on the left shows the proportion of rabbits in the eagle owl's diet and the rabbit population distribution by size in the different seasons of the year in the semi-natural rabbit population (Los Melonares). The graph on the right reflects the negative relationship between rabbit size and abundance in natural rabbit populations (Ruidera and Ossa de Montiel).

    Ultimately, the eagle owl seems to adapt its predatory ability on prey of various sizes to maximize its energy efficiency based on its abundance. In situations of high rabbit abundance, with high availability of small individuals, the eagle owl selects smaller rabbits because of their predictable location in areas surrounding burrows, their greater vulnerability and ease of capture, lower risk of causing injury, and easier to feed the chickens. On the other hand, the eagle owl may adapt and switch to larger prey when rabbit abundance decreases and the lower prey encounter rate increases the energy expenditure required to forage.

    The scientific publication of this research is available at: