When waterfowl return to Illinois in early spring on their way north, will they find enough food for a two-week layover? A limited food supply during spring migration and subsequent declines in duck populations can affect Illinois’ multimillion-dollar waterfowl hunting industry, say researchers from the Prairie Research Institute at Illinois.
More than 1 million ducks stop each spring in the backwater lakes and shallow wetlands of the Illinois River, dining on annual grass seeds, underwater vegetation and waste grain in crop fields.
Spring is important for duck populations because it precedes the breeding season. Healthy ducks in good condition typically have more ducklings with higher survival rates. Thus, more food in the spring can help ensure more abundant duck populations in the fall.
In 2010 and 2011, researchers studied ducks’ eating habits by identifying the amount of seed left over in artificial foraging patches and using computer models to consider various factors such as predator risks, seed depth in sediment and seed size.
“We want to know how many acres of wetlands to protect and restore for waterfowl populations for spring and fall flights,” said Heath Hagy, the director of the Forbes Biological Station at the Illinois Natural History Survey, a division of PRI. “To do this, we use energetic carrying capacity models. One aspect of this model is the giving-up density, or the point at which ducks stop foraging and move on to another patch.”
Hagy likened this term to a serving pan of cold, soggy macaroni and cheese that is left at the buffet after diners have filled their plates and their stomachs. If this unpalatable food is not discounted, there is a tendency to overestimate the amount of food that is available.
Hagy and colleagues buried 34 round dog-food bowls filled with rice and millet seeds underwater in locations throughout the central Illinois River Valley, and they later sorted seeds to figure out how much food the ducks left behind. They also tested effects of habitat characteristics on food use. For example, they erected a barrier to block the ducks’ surrounding view of the area, increasing the ducks’ perceived risk from predators.
“There was a small effect of predation risk on movement away from the feeding patch when food was plentiful, but it is perhaps more interesting that there was no effect of predation risk when food was limited,” Hagy said.
During the year with abundant rain, food was plentiful, and ducks could be picky about the food they chose. During the drier year, however, more food was consumed per plot because ducks had fewer dining choices.
Studies of this kind help land managers predict how many acres of wetlands are needed to sustain duck populations. A healthy population is important for the recreational waterfowl-hunting industry, which provides $453 to the Illinois economy for each bird harvested, according to an INHS survey.
This study was published in the Journal of Avian Biology, and was funded by the U.S. Fish and Wildlife Service.
