Whether sunflower seeds, titballs or millet: Especially in winter and spring, bird feed should help our garden and songbirds through the barren times. However, many of these feed mixtures apparently contain unwanted accessories, as a study now reveals: seeds of arable weeds and even invasive plant species such as mugwort anvil. Bird feed could thus be an underestimated path of propagation for such plants.
Millions of garden owners and bird lovers want to do something good for the feathered inhabitants of our parks, gardens and suburbs by laying out bird feed. Mostly these are ready-to-buy grain and seed mixtures that contain millet, sunflower seeds and other nutritious seeds. In the case of titballs, these grains are additionally mixed with fat and sebum. Studies show that at least some bird species do benefit from such feeding, for example by producing more offspring in the following spring.
Weed seeds in almost all mixtures
However, these feed mixtures do not always contain what is on the package. Studies have often found that in addition to sunflower seeds, millet and the like, seeds of arable weeds or even potentially invasive alien plants are contained in these mixtures. Erik Oseland from the University of Missouri and his colleagues have now investigated how high the proportion of these undesirable seeds is and which seeds are particularly often represented. For their study, they ordered and bought 98 different varieties of bird feed commercially available in the United States and examined its composition.
It turned out that almost all grain mixtures contained foreign seeds as contamination – in total, the researchers found seeds from 29 different plants classified as arable weeds. In addition to common species such as knotweed (Fallopia convolvulus), blood red finger millet (Digitaria sanguinalis) or amaranth species, these also included invasive species such as the broom-wheel alarm (Bassia scopari) or the mugwort ambrosia (Ambrosia artemisiifolia). This species, imported from North America to Europe, produces highly allergenic pollen and is now also spreading rapidly in Germany. Oseland and his team found the seeds of this species particularly often in feed mixtures that contained many sunflower seeds.
Path of propagation for invasive species
Additional germination tests showed that 19 percent of the amaranth and foxtail seeds from the bird feed were germinable. Overall, Oseland and his team found weed seeds in 98 percent of the bird feed mixtures. Such arable weeds are rarely a problem in the home garden or in the park, but they are, however, if the birds carry these seeds, for example, on organically farmed fields. “It is difficult to accurately assess the role of commercial bird feed,” says Oseland. “But it is very likely that this is a previously overlooked path for unwanted and problematic weeds to spread to new areas.
Source: Cambridge University Press; Technical article: Invasive Plant Science and Management, doi: 10.1017 / inp.2020.2