Who would've thunk it? First, that a program about thrushes would turn out to have some very interesting information on plants, and secondly, that highly invasive non-native plants might turn out to be beneficial.
Tonight I attended a program sponsored by Jackson Audubon. The speaker was Julie Craves, a researcher at the Rouge River Bird Observatory (RRBO), which is located in Dearborn, MI, which is near Detroit. The topic was "Birds and Berries," and it was about her nearly twenty years of research on the importance of urban spaces in the stop-over ecology of birds during the fall migration.
A weighty topic that could literally encompass tens of species of birds (her nets caught 111 species, but not all of those were migratory), Julie chose to focus on three species: grey-cheeked thrushes, Swainson's thrushes, and hermit thrushes.
To sum up ever so briefly her many years of research, here is what she did and what she has discovered.
During the fall, she and some students set up mist nests in a 300 acre spot of green space there in the heart of a highly developed urban environment. Birds are captured daily during the designated research period. Once free from the mist nets, the birds are placed in cotton bags and hung quietly to await their turns for "processing." Processing involves determining how much fat is on the birds, assessing visual evidence of diet (e.g., purple-stained feathers around the vent indicated a diet of purple berries, such as wild grapes), and banding each specimen that is to be included in the study. If the birds cooperatively leave a stool sample behind in the bags, all the better, for each sample is taken to a lab and dissected. Why? Because when birds eat the fruits available to them in the autumn landscape, most of the consumed seeds pass right through their digestive tracts. The seeds are separated from the guano, identified (apparently easily done), and counted.
What Julie expected to find out was just which native plants were vital to the birds' diets. What she actually found out, much to her surprise, was how important the non-native plants are, primarily Amur honeysuckle, common buckthorn and glossy buckthorn.
Anyone who has even a passing interest in invasive species knows that these are three of the top invasives that are threatening American ecosystems. Here we are (we being biologists, ecologists, nature enthusiasts, et al), trying to eradicate, or at the very least control, these species, and suddenly we learn that they are important for the birds.
The big question is: why are these non-native plants preferred in these birds' diets over the fruits of the native plants, plants that evolved along with the birds for millenia?
Here are some of the current thoughts. Many of the native fruits are high in fats (lipids), which, while high energy foods, also require more energy to process. The non-native plants, on the other hand, are producing high-sugar fruits. Sugar is readily used by the birds' bodies, providing a quicker source of energy. They also pass through the birds' systems pretty quickly, allowing them to eat even more.
When the birds stop-over somewhere while on migration, the purpose is mostly to refuel. They have to consume approximately 4% of their body weight in food every day, often putting on 4-6 grams before they take off again.
For years, the common "wisdom" was that the non-native fruits had little nutritional value for birds. The fruits passed through the birds' systems pretty darn fast. Now it seems that this is all part of the strategy to store quick energy.
So, what is the take-home message here? Should we all go out and preserve stands of invasive plants? Not necessarily. What we need to do, however, is start to consider the value of these plants in the changing landscape. A new ecosystem is developing in the wake of this vegetative invasion, and it seems that at least some animals have adapted to the change, making the most of what these plants have to offer.
Bottom line: we still have a lot to learn about this world in which we live. Food for thought.