Canvasback Study Seeks Population ‘Sources’ and ‘Sinks’

Researchers began fitting hens with geolocator devices this spring

Josiah Gritter sprinted across a rocky flat toward the makeshift wire pen, the long-handled fishing net he carried whiplashing on every stride. Mud flew from the soles of his waders as he plunged thigh-deep into the receding California tidal basin to trudge the final 50 yards to reach the trap.

A trio of canvasbacks—wintering ducks that had earlier dived to feed on baited grain and found their way inside the cage—desperately searched for the exit as Gritter closed the distance. Two drakes and a hen remained inside as the net man sealed the escape chute with his boot. Gritter looked back as Hannah Sabatier, a graduate student researcher from the University of Wisconsin-Stevens Point, approached the pen. Three scoops later, Gritter had captured the ducks and carefully handed each one to Sabatier.

“Don’t drop the hen,” Gritter joked as Sabatier cradled the ducks across her midsection for the short wade to a nearby holding crate.

Moments later, Gritter held the hen again as Sabatier secured a stainless-steel band on the duck’s left leg, then applied another special red marker on the right foot. The red plastic sleeve serves as a carrier for a fingernail-sized data-logging device called a geolocator that was tightly attached with a zip-tie. Geolocators, which weigh just 2 grams (less than a U.S. penny), record the intensity of light and the date/time. From that data, researchers can determine where the hen traveled, as well as whether she nested, how many eggs she laid, and even if the nest likely hatched.

“Geolocators record light with a timestamp, which allows us to monitor the duck’s movements and nesting behaviors so we can better understand their source/sink dynamics,” Sabatier explained before setting the canvasback hen free to fly back toward San Francisco Bay.

Waterfowl biologists use the terms “source” and “sink” to describe whether a specific area of habitat is contributing to the overall population. A source is characterized by good reproductive effort that adds ducks to the fall flight. Generally, source areas are strong duck breeding and brood-rearing habitat. Sinks, on the other hand, are habitat areas that attract breeding ducks but where hatching a nest is difficult because of predation and a lack of cover. In sink areas, nest success is often so poor that few ducks are produced. Worse, some of the hens will die during the nesting effort, so a sink can yield a net loss to the duck population.

“We are interested in looking at canvasback breeding ecology and, more broadly, the amount of time canvasbacks spend at staging or stopover sites, especially during spring migration and how that might be affecting breeding success,” said Dr. Ben Sedinger, the UWSP Kennedy-Grohne Chair in Waterfowl and Wetlands Conservation who is directing the research. “The overarching objective of the project is to look at source and sink dynamics in North American canvasback populations. Are there important areas that are acting like sources for the continental canvasback population where we’re producing a lot of offspring vs. areas where a lot of females are going to breed and it’s acting like a sink and we’re not seeing as much production from those areas?”

Ultimately, the study could influence future management decisions that boost duck populations.

“Maybe there’s a lot of potential in a certain region to produce canvasbacks but there’s something going on that’s affecting production during the summer,” Sedinger said. “This work could inform future research as we try to figure out how to produce more canvasbacks at a continental scale.”

Sedinger and UWSP are undertaking the research in collaboration with Delta Waterfowl. With 2025 as a pilot year for the study, Sabatier and Gritter attached geolocators at three locations this winter and spring. The team began in North San Francisco Bay in January and moved all the way across the country to Chesapeake Bay in Maryland during February, before finishing on Pool 19 of the Mississippi River near Nauvoo, Illinois.

“Our goal is to mark as many hen canvasbacks as possible but also to help figure out the locations and techniques for technicians who follow us next year to catch more canvasbacks,” said Sabatier, who recently earned her master’s degree for a breeding habitat selection and nest success study on Saskatchewan puddle ducks.

Sabatier and Gritter fitted 66 hen canvasbacks with geolocators to jump-start the study. They also marked a handful of drake canvasbacks in California.

Sedinger anticipates that another year of capturing and fitting canvasbacks with geolocators will be necessary in 2026 before a graduate student takes over the project in 2027 to begin assessing the data.

Importantly, hunters will play a key role in the study.

The geolocators can capture and store up to two years of time and location information, but they do not transmit the data like many other modern devices used in waterfowl research.

“We need to get the bird back (to download the data),” Sedinger said. “We get them back by either recapturing them during a future banding operation after they’ve been deployed for a year or two, or by hunter recoveries. With banding data in the waterfowl world, we have the best citizen science wildlife project in the world. We’re relying on hunters to get those geolocators back.”

The plastic band carrying the geolocator is engraved with Delta Waterfowl’s phone number.

“If you’re hunting diving ducks and come across a canvasback with a red plastic band on its leg with a phone number on it, we’d appreciate if you’d get in touch,” Sedinger said. “We realize that’s a special thing to take a bird with a geolocator. We just want the data. We’re happy to return the geolocator to the hunter once we retrieve the data from it.” —Paul Wait