Common buckthorn (\(Rhamnus\) \(cathartica\)) is a shrub known for its small, dark, berry-like fruits that persist on branches long after leaves have fallen. This non-native plant, originally from Eurasia, has spread aggressively across North American landscapes. The presence of these abundant, persistent fruits naturally leads to a question about their interaction with local wildlife. The central question is whether birds consume these dark berries and what the biological and ecological consequences of that consumption are for the birds and the environment.
The Direct Answer: Consumption and Specific Species
Birds do consume buckthorn berries, especially the ripe, black fruits of both common and glossy buckthorn (\(Frangula\) \(alnus\)). Consumption increases notably in the late fall and winter months, coinciding with the depletion of preferred native fruit sources that offer higher nutritional rewards.
Several bird species are known to feed on buckthorn, often in large flocks when other food is scarce. The most frequently observed consumers are the American Robin, the Cedar Waxwing, and the European Starling. These highly mobile species are attracted to the dense clusters of fruit that remain available throughout the cold season.
Other species, including the Gray Catbird and various thrushes, also forage on the berries. The birds effectively act as dispersal agents for the plant, a key factor in the species’ widespread establishment. The consumption is driven largely by the availability of the fruit when high-fat, high-protein native berries are no longer present.
Physiological Effects on Avian Consumers
The berries of the buckthorn species are characterized by a poor nutritional profile for birds, particularly migrating species that require high-fat food sources. These fruits are predominantly composed of water and carbohydrates in the pulp. While they provide some immediate caloric intake, they lack the lipids and proteins necessary to fuel long-distance migration or sustain body mass through cold winter nights.
The biological impact is largely attributed to a group of chemical compounds present in the fruit called anthraquinones, specifically emodin. This compound is known to function as a laxative. While the berries are not acutely toxic to most bird species when ripe, the emodin accelerates the passage of the fruit through the bird’s digestive tract.
The rapid transit time prevents the bird from efficiently extracting and utilizing the limited nutrients available in the fruit. This fast-moving, low-energy food source means the bird must continuously forage to meet its energy demands. The presence of emodin in the berries is a defining factor in this accelerated digestion.
The Mechanism of Seed Dispersal
The consumption of buckthorn berries by birds is the primary mechanism for the plant’s successful seed dispersal. The seeds, which are not destroyed in the bird’s gut, are passed rapidly due to the laxative action of the anthraquinones.
The mechanical action of moving through the bird’s digestive tract helps to physically scarify the hard seed coat. This scarification breaks the seed’s dormancy, preparing it for germination. The seed is then deposited far from the parent shrub, often in a new, uncolonized location.
The seed is expelled within a small, nutrient-rich package of bird droppings, providing a ready-made fertilizer source. This combination of scarification, distant transport, and nutrient-rich deposition is why buckthorn is such an ecologically successful invasive species, contributing significantly to its spread across forest understories.