Variation in the diet composition of the stone marten Martes foina in a rural area of western Poland

In the patchwork landscapes of Europe, where forests give way to vast fields of wheat and corn, a quiet drama of survival plays out every day. Here, the stone marten, a sleek, cat-sized predator with a coat the color of old parchment, navigates a world shaped by human hands. Unlike its forest-dwelling cousin, the pine marten, the stone marten has learned to thrive in the spaces between human settlements and the open countryside. It is an opportunistic hunter, a generalist that does not rely on a single food source but instead shifts its menu based on what is available. This flexibility is crucial in agricultural regions, where the populations of many small animals, from birds to rodents, fluctuate wildly due to farming practices and climate. Understanding what these animals eat is more than a matter of biological curiosity; it is a way to gauge the health of the entire ecosystem. If a predator's diet changes, it often signals a shift in the abundance of its prey, which can have ripple effects on the survival of other species, including game animals like the brown hare that farmers and conservationists watch closely.

For over a decade, researchers in western Poland set out to uncover the daily reality of the stone marten's life in this specific rural setting. They did not track the animals with collars or cameras, but rather followed a different, more indirect trail: the marten's droppings. Over ten years, from 2007 to 2017, the team collected nearly 2,400 fresh scats from roadsides and paths that cut through both small villages and the open farmland. By carefully washing and sorting the contents of these samples, they could identify exactly what the martens had eaten, from the tiny bones of mice to the seeds of fruit and the feathers of birds. The goal was to see how the diet changed with the seasons and whether the animals living near human homes ate differently from those roaming the fields.

The story the data told was one of a predator that is a master of seasonal adaptation. For most of the year, the stone marten is a fruit-eater. From late summer through autumn, the landscape is rich with apples, cherries, plums, and grapes, and the martens feast on them. During these months, fruit makes up the vast majority of what they consume, sometimes accounting for nearly seventy percent of their diet by weight. It is a time of plenty, where the predator acts more like a forager than a hunter. However, as winter arrives and the fruit disappears, the marten's role shifts dramatically. From January through May, the diet turns almost entirely to meat. Small mammals, such as voles and mice, and birds become the primary fuel for the animal, making up the bulk of what is eaten during the cold months.

This shift is not random; it is a direct response to the rhythm of the countryside. The researchers found that the availability of food dictates the menu. In the summer and autumn, when the numbers of common voles—small, mouse-like rodents that burrow in the fields—are at their highest, the martens eat more of them. But this relationship is specific to the warmer months. In the winter and spring, when voles are harder to find or when the martens are focused on other prey, the number of voles in the fields does not seem to change what the martens eat. Instead, during the colder half of the year, the martens rely heavily on birds and small mammals regardless of how many voles are currently scurrying through the grass.

The location of the hunt also matters. The study revealed a clear difference between what martens eat in the quiet villages and what they eat out in the open fields. In the winter and spring, martens living in the villages eat more fruit and birds, likely because they have access to gardens, compost piles, and the birds that nest near human structures. Those roaming the agricultural land, far from the houses, eat more small mammals and insects. This suggests that even though the same animal species is involved, the local environment shapes its behavior. A marten in a village is a different kind of eater than a marten in a cornfield, simply because the opportunities available to them are different.

One of the most pressing questions the researchers hoped to answer was whether the stone marten is a major threat to the brown hare, a species that has seen its numbers decline across Europe. Farmers and conservationists often worry that predators are the cause of these drops. The study looked for evidence of hare remains in the droppings, but the results were surprisingly reassuring for the hare, though perhaps not for the predator's reputation. Hare remains were found in only a tiny fraction of the samples, appearing in just over one percent of the year-round diet and slightly more in the spring. This indicates that the stone marten is not a primary hunter of adult hares. The researchers noted that when hares do appear in the diet, it is often during the spring when young hares are present, suggesting the martens might be taking advantage of vulnerable leverets or scavenging carcasses rather than actively hunting healthy adults.

The paper concludes that while the stone marten is not the main villain in the decline of the brown hare, it is part of a larger group of predators that collectively put pressure on young hares. In the complex web of the agricultural landscape, no single predator may be responsible for a population crash, but the combined effect of foxes, badgers, and martens hunting the young can be significant. The stone marten, with its ability to switch from fruit to meat and to adapt to both villages and fields, remains a resilient survivor. Its diet tells a story of an animal that is perfectly tuned to the changing seasons and the human-altered world it calls home, eating what is there when it is there, and surviving the winter by turning its attention to the small, abundant creatures of the fields.