With waterfowl habitat continually changing and wetland loss occurring on a regular basis, it is imperative for researchers to see if landscapes provide enough habitat to support waterfowl populations at ideal levels.

A habitat’s carrying capacity is the number of living organisms that a region can support without environmental degradation. Researchers at the University of Delaware recently partnered with the United States Geological Survey’s Patuxent Wildlife Research Center (USGS PWRC) and Ducks Unlimited (DU) to piece together a part of the carrying capacity puzzle, looking at how much energy ducks burn during a given day.

The research was led by Jake McPherson, a master’s level student in the Department of Entomology and Wildlife Ecology as well as a regional biologist for Ducks Unlimited, and Chris Williams, professor of wildlife ecology who also oversees a waterfowl and upland game bird research program in UD’s College of Agriculture and Natural Resources.

Supply and demand

McPherson said that there is a question of energy supply — how much energy a habitat is able to provide a certain number of waterfowl — and of energy demand, which is where his research comes into play.

“On the energy demand side, you need to know how much energy a duck uses in a given day and you can scale that up and, for example, say, ‘One duck uses this many calories a day, and it’s going to be in this region for 60 days and we want to support 100,000 ducks’ so you can come up with a total energetic need for those birds,” said McPherson.

In order to investigate how much energy non-breeding waterfowl use in a day, past waterfowl graduate students under Williams first had to determine what specific activities make up the normal day of a duck. But after that, McPherson has come in to estimate the energy expenditure for some of those behaviors.

“It swims, flies, dives, feeds and each of those activities have different energy requirements. I’m looking at the specific energetic cost of each of those behaviors,” said McPherson.

The study used American black ducks and a lesser scaup in order to represent the two guilds of ducks: divers—ducks who dive for their food—and dabblers—those who dabble for food in shallow water or on the surface.

Using respirometry equipment for the study, McPherson put individual ducks in a sealed chamber. Whenever the duck would perform an action, whether it be swimming or diving, the respirometry machine would read the changes in oxygen and carbon dioxide levels within the chamber.

“As energetic activity increases in the chamber, that bird’s going to be consuming more oxygen than it would be if it was resting,” said McPherson. “We can use the oxygen consumption rate observed inside the chamber during that behavior to come up with an estimate of calories burned per time.”

McPherson said that while the size of the chamber can affect the accuracy of the readings, the researchers were able to develop a pyramid shaped chamber big enough that the ducks could do their normal activities without restriction but also small enough that they could get accurate readings.

In order to determine what the ducks were doing when they observed changes in the amount of oxygen in the chamber, they also videotaped the ducks during two-hour periods and cross referenced the data with the videos.

“We had to videotape these birds and time-synch the video to the respirometry output. I could look at the respirometry output and say, ‘I can see there was an increase in oxygen consumption and therefore energy expenditure in this period, let me go back and see exactly what the bird was doing during that period.’ That’s how we can correlate calories burned to a specific activity,” said McPherson.

One of the biggest challenges they faced in their research is that they were unable to observe what is perhaps the biggest energetic cost for waterfowl: flying.

“You can’t really measure flying in my set up so we said, ‘We’re going to try and get all of these other behaviors and we’ll accept that the energetic cost of flying is beyond the scope of this project,” said McPherson.

Previous study

Currently, when wildlife researchers are determining how many calories waterfowl are burning in a certain habitat, they are using numbers from a study in the 1970’s where researchers surgically implanted heart monitors onto birds in a semi-wild setting and then correlated the heart rate monitor with their observations in the field.

McPherson said that there are couple of challenges with this study, beginning with the surgically invasive implants which could affect the behavior of a wild duck.

“Certainly, it could be said of respirometry as well but surgical implants tend to be more invasive,” said McPherson. “Then, with monitoring heart rate, you can see an increase in heart rate and it wouldn’t be associated with behavior. If a predator flies over, that duck may just be sitting on the water but its heart rate may elevate exponentially and so these are some of the things we were thinking about in terms of the design set up of that previous study.”

McPherson said they are hoping to compare some of these older numbers to the ones they discover.

“Maybe we can confirm them or maybe we’ll find out those numbers were off a bit,” said McPherson.

Williams said that one of the ultimate goals of his lab is to be able to create shortcuts for researchers so that they can estimate carrying capacity without doing costly research in the field.

“It takes a lot of time and money to watch ducks in the field and record their behaviors as well as go out in the field and collect the amount of food that’s on the landscape. If we can get ourselves to a place where we feel like we’ve exhausted the data collection and there are no surprises, we could find shortcuts to make these estimates in the future,” said Williams. “Certainly, that would be a gold standard for us, especially for the state or federal agencies, who could use broad summaries of the data and extrapolate that to where their conservation goals are for the future.”

McPherson, who grew up hunting and fishing in eastern Virginia, said he is looking to determine these carrying capacity estimates in order for future generations to understand and appreciate wildlife.

“My interest in conserving waterfowl populations is to ensure that not only can I continue to enjoy this sport but future generations can enjoy it as well,” said McPherson.

In addition to support from DU and USGS PWRC, the research was also supported by the Black Duck Joint Venture, the Upper Mississippi/Great Lakes Joint Venture and the Waterfowl Research Foundation.