Ducks and seeds

Seeds from aquatic plants have been known to successfully germinate even after passing through the digestive tract of a bird – this is one method by which plants can spread from one body of water to another.

Sometimes, though, seeds don’t make the full, daunting journey through a bird’s bowls.

“Regurgitation, or vomiting, is a common behavior in the daily life of many bird species,” write the authors of a paper recently published in the journal Aquatic Botany. If a plant seed is thrown up before passing all the way through a bird’s body, thereby “circumvent[ing] many of the damaging digestive processes,” so much the better for the seed.

The scientists, interested in whether or not the seeds of aquatic plants might spread via bird vomit, fed ducks seeds collected from 10 species of wetland plants. The ducks regurgitated seeds in about half of the 64 feeding trials the researchers conducted, seemingly in response to one of two conditions: overfeeding, or eating large, indigestible seeds.

If the birds ate a lot of food in a short time (as they sometimes do in the wild, when they happen upon an abundant food source), they threw up some of the seeds they had recently eaten, regardless of seed size, within three hours of eating. Large (more than 10 millimeters, or a little bit less than half an inch), tough seeds were sometimes regurgitated early on in response to overeating, but also between 11 and 24 hours post feeding, apparently after the seeds were rejected from the birds’ gizzards due to their size.

“As regurgitation in birds requires a suffocate movement which is impossible during flight,” the scientists note, “regurgitation most likely occurs after landing in wetland habitat,” meaning that bird vomit may be an important way for aquatic plants to spread.

Seeds from the aquatic plant Iris pseudacorus were among the largest seeds fed to ducks (and later regurgitated by them) during feeding trials. 

(Original image by Paul van de Velde via Flickr/Creative Commons license)

Edible flowers

Flowers exist in most terrestrial ecosystems, and though humans have appropriated flowers for our own decorative and aromatic purposes, the plants that produce them use them for one thing – reproduction. When flowers are pollinated, they produce fertilized seeds that, given the right circumstances, develop into the next generation of the plant.

Though most Valentines Day bouquets are composed of terrestrial flowers, many aquatic plants produce flowers, too. These flowers typically grow at the tip of a long stem stretching to the surface of the water, so that pollination via the normal vectors (wind, insects, other animals) can occur. (Plants, including submerged vegetation, also have a few other options for reproduction, including fragmentation and root branching [pdf].)

In a study recently published in the journal Aquatic Botany, a team of Spanish scientists investigated the seasonal dynamics of waterfowl feeding on aquatic plants in a coastal lagoon in the northwestern Mediterranean Sea; they found that flowers appeared to be a particularly appealing meal for the birds.

The researchers suspected that waterfowl – ducks and coots – might consume more aquatic vegetation as a group during the autumn and winter because, due to their migratory patterns, they are much more abundant during those seasons than in the summer in the coastal lagoon the scientists studied. (During the year the study occurred, there were about five to six times as many individual birds present during the autumn and winter than in the summer.)

To test their idea, the scientists created “exclusion cages,” to protect aquatic plants from waterfowl, then compared vegetation height and mass between the exclusion plots and other plots where birds could feed freely. Contrary to their expectations, the scientists found that there were no differences in vegetation between the two types of plots during the autumn and winter months; the ducks and coots did not appear to be eating more vegetation during those seasons.

During the summer, however, the waterfowl did eat a significant amount of one species of aquatic plant, Ruppia cirrhosa, or spiral ditch grass (the plants were shorter, and their biomass smaller, in the plots were waterfowl were present). The birds also ate the flowers of the plant – there were approximately eight times fewer Ruppia cirrhosa flowers in the plots where ducks and coots were present and able to eat them than in the exclusion cages.

The scientists suggest that ducks and coots in the lagoon may have focused their feeding on other food resources, like algae, insects, and seeds, during the autumn and winter, when aquatic plants stopped growing. “[T]he strongest waterfowl impacts on the submerged vegetation within brackish Mediterranean lagoons do not occur when abundance of individuals is higher,” they write, “but in summer when plants and flowers are largely available.”

For plants, flowers are a practical way to reproduce; for humans, they’re a way to send a message of love or congratulations, or a fragrant way to decorate a counter; for waterfowl, they appear to be a convenient way to make a meal.

In the plots free of waterfowl, Ruppia cirrhosa produced about 10 times more flowers than Potamogeton pectinous (the plant shown here). 

(Image by Ruppia2000 via Wikimedia Commons)