William & Mary Research Links Food Scarcity to Brood Failure in Chesapeake Bay’s High Salinity Areas

Shortly after 6 a.m., William & Mary Research Professor Bryan Watts guides a boat quietly past an osprey nesting platform. Layla Slone ’25 steadies a mirror on a 20 foot pole while Lizzie Arthur ’25 uses the reflection to peer into a nest big enough for a human to sit in. Arthur carefully records the number of chicks, chick age, parental activity and time before the team departs for the next of approximately 60 nests that they will survey by the end of the day.

These visits were the building blocks of a recent study performed by W&M’s Center for Conservation Biology (CCB) evaluating osprey populations in the main neck of the Chesapeake Bay. The study found that ospreys continued to experience poor breeding performance during the 2024 season, likely due to food scarcity.

Founded by Watts and Chancellor Professor Emeritus Mitchell A. Byrd in 1992, the CCB is a powerhouse of ground-breaking bird conservation research, carrying out 30 to 40 major studies per year and providing valuable information daily to local and global conservation communities. The center also provides opportunities for students like Slone and Arthur to participate in hands-on research as undergraduates, working closely with faculty like Watts as part of the university’s efforts to provide the most personal education of any public university in the United States.

“I have always loved nature,” Arthur said, “but this experience really deepened my love and desire to protect it. There are so many fascinating species out there with such unique and touching lives. By protecting them, we not only enrich our own lives, but also preserve the stunning and diverse natural world we are so lucky to live in.”

Why do ospreys matter?

Ospreys are top predators in coastal environments, making them important bioindicators of ecosystem health. Patterns within osprey populations can shed light on important environmental issues such as increased pollution levels and decreased prey availability.

For example, osprey populations across the United States and Canada experienced steep declines due to the widespread use of DDT from the 1940s to the 1960s. The birds’ cracked, nonviable eggs highlighted the detrimental effects of the pesticide by way of biomagnification.

In 1970, Byrd, then a professor in W&M’s biology department, and one of his students performed the first osprey survey of the Chesapeake Bay, kicking off decades of research projects to assist in the regional recovery of the species.

Ospreys recovered so well in the Chesapeake Bay that the area was considered a stronghold for the iconic fish-eating hawk, and the CCB helped other states reestablish their populations by successfully relocating ospreys from the bay to areas with less robust recovery rates.

With osprey populations in such good shape, the CCB was able to focus on other species in more dire need of research.

Several years ago, however, Watts began receiving reports from boaters and waterfront homeowners owners about low osprey nest productivity over several consecutive years.

“These people watch osprey nests every day during the breeding season, so it’s part of their life,” he said. “They can tell you what’s happened in a particular nest for the past 30 years.”

CCB researchers confirmed that ospreys were experiencing poor brood performance and noted signs that indicated food scarcity as a factor.

These observations spurred a 2021 CCB food addition study in which biologists supplemented experimental nests with fish. Supporting the food scarcity hypothesis, ospreys in experimental nests successfully raised enough young to rise above the population maintenance level, while those that did not receive supplemental fish continued to fail at a rate that put them well below maintenance levels.

The most recent study, performed in 2024, was designed to gain better spatial resolution of osprey breeding performance. Ospreys that nest in higher salinity areas rely on different prey species than ospreys that raise young in lower salinity environments. Thus, researchers chose to monitor nests in both high-salinity areas throughout the entire main stem of the bay and low-salinity sites in tidal fresh reaches of upper tributaries.

The project included partnerships with the Virginia Aquarium, Maryland-National Capital Park and Elizabeth River Project. Biologists from the United States Geological Survey also assisted in gathering data.

In order for the osprey population to remain at its current level, an average of 1.15 chicks need to fledge from each nest. Among the entire 571 osprey pairs monitored in this study, the reproductive rate was 0.51 young per pair, but there was a marked difference between the higher salinity sites with a fledging rate of 0.6 to 0.9 young per pair, and lower salinity areas, which averaged 1.36 young per pair.

Evidence of food scarcity

The study found multiple indications of food scarcity as the driving force behind the poor breeding outcomes in the high-salinity areas. For example, a high proportion of osprey pairs in the main stem of the bay did not lay eggs at all.

“Most of these non-breeding pairs remained resident throughout the nesting season and defended territories but were never documented to lay eggs,” said Watts. “This is the first time this behavior has been documented on a large scale within the Chesapeake.”

Watts explained that a likely explanation for this behavior is that females did not have adequate nutrition levels to lay eggs.

Single chick broods are another indication of inadequate food resources and accounted for 56% of nests in the main stem of the bay. By contrast, 18.2 % of ospreys nesting in lower salinity sites raised single chick broods.

When adequate food is available, osprey young in nests with multiple chicks develop at the same rate. In an environment where food is scarce, however, chicks tend to develop at different rates. A food hierarchy develops in which the most dominant chick receives most of the food, and the young die in a sequential order from least to most dominant.

These asymmetric broods were common and widespread throughout the main stem of the bay and lost an average of 1.1 chicks per nest between hatching and fledging. By contrast, asymmetric broods were rare in the lower salinity areas where the average loss of chicks per nest was 0.3.

By comparing high salinity sites and low salinity sites within the same region, this study helps to rule out other possibilities that could be driving the low reproductive success of ospreys in the Chesapeake Bay such as weather, diseases like avian flu or conditions at wintering sites in South America. Any of these factors would affect sites in both the main stem of the bay and in the lower salinity reference sites.

High breeding density can be a factor in low brood success, but breeding density was higher in the successful low salinity sites. Additionally, eagle density is approximately twenty times higher in the low salinity sites than in the main stem of the bay, so eagle predation is unlikely to have had such a marked effect on ospreys in high salinity areas.

In short, Watts explained, osprey chicks in high-salinity areas are starving.

Ospreys are piscivorous species that depend heavily on specific fish species to feed their young. In order to understand food availability challenges for Chesapeake Bay osprey, Watts noted, research is needed to determine levels of prey species within the main stem of the bay. Without the data provided from such a study, scientists and policy makers are unable to effectively address the current prey availability issue.

Undergraduate osprey researchers

Slone and Arthur spent the summer collecting and processing data for this study, dividing their weeks between boat surveys and office days. Their work involved surveying osprey demography, observing male provisioning behavior and monitoring and sorting photographs from trail cameras.

“For the most part, the boat days were the highlight of my week,” said Slone. “We got to see dolphins, rays and numerous other species I had never interacted with before.”

Arthur explained that due to surveying the same nests for months, she felt that she developed a strong connection with the ospreys that she monitored.

“I was rooting for them and their nest success,” she said. “I was always deeply saddened when a nest that had chicks one day had no chicks the next.”

On office days, Arthur and Slone focused on data management and organization. They sorted through trail camera photos for up to eight hours at a time to document male provisioning behavior as well as how frequently and what types of prey the birds ate.

Although they preferred field days to office days, both students remarked on the necessity of sorting and processing data as well as the importance of a strong and consistent research protocol. After all, they explained, such steps are crucial to gain accurate results from hard-won field data.

Both students remarked that they would eagerly participate in another project like this one.

“I loved being able to see the other side of biology apart from books and tests,” said Slone. “Real world application is a whole other beast, but it is a worthwhile beast to study, stalk and stare at starry-eyed.”