Each spring, birders eagerly await the return of ospreys to the Chesapeake Bay—a welcome sight that signals the end of winter. An estimated 8,000 to 12,000 breeding pairs of ospreys live in the Chesapeake Bay, which is home to the largest breeding osprey population in the world. Since ospreys are sensitive to changes in their environment, they serve as a monitor of the health of the Bay.
So, when 2024 breeding performance results released by the College of William & Mary’s Center for Conservation Biology (CCB) showed that osprey chicks continue to starve in their nests in portions of the Bay where they typically depend on menhaden—a small, silvery, nutrient-packed fish—for food, the findings intensified ongoing, decades-long concerns.
According to a press release issued on September 13, 2024, by the CCB, osprey chicks aren’t surviving at rates that sustain the population. A survey monitored 571 osprey pairs in 12 sites in Virginia and Maryland during the nesting season (March through August). Ten study areas were located within the main stem of the Bay, where salinity exceeded 10 parts per thousand (ppt), and ospreys are highly dependent on menhaden. Two additional study areas (used for comparison) were located on upper tributaries within tidal fresh waters, where salinity was less than 1 ppt, and ospreys feed primarily on catfish and gizzard shad.
The generally accepted reproductive rate for population maintenance is 1.15 young/pair. Breeding performance varied between the higher-salinity sites—ranging from minor, moderate, and major deficit. However, the reproductive rate within the tidal freshwater sites was 1.36 young/pair, which is above the maintenance target (surplus).
“It’s clear to researchers that ospreys are struggling due to food stress, but the starvation issue is mostly confined to the main stem of the Bay, where waters are greater than 10 ppt salinity,” says Bryan Watts, Ph.D., director of the CCB. “We do not know why menhaden have become less available to ospreys. The osprey-menhaden issue is a moving target. Some have suggested that ongoing harvest, warming waters, water quality, or other factors are impacting menhaden in the Bay. However, we do know that osprey broods [groups of chicks] are starving in their nests and that there is not enough menhaden in their diet to support sustainable reproductive rates.”
 |
| Snakehead! It's what's for dinner! |
In addition to starvation, scientists and researchers are also observing uncharacteristic behaviors among ospreys. According to Watts, a large number of osprey pairs didn’t lay clutches during the 2024 nesting season. This is the first time that this behavior has been documented on a large scale within the Chesapeake Bay. Researchers believe that the most likely explanation is that the females didn’t reach the nutritional condition required to produce eggs.
Single-chick broods and asymmetric broods were also widespread throughout the Bay in 2024. Asymmetry develops when food coming into the nest is inadequate to feed all of the chicks—resulting in a dominance hierarchy, which allows the dominant chick to monopolize access to food. Depending on the available food supply, one or all of the chicks may starve.
For what it's worth, I'm beginning to give this hypothesis a little more credence. Ospreys have been noticeably in short supply in our neighborhood this year. I've seen them a couple of times, but usually, by this time of year, Ospreys would be hunting out in front of the beach almost constantly. Ospreys return to the same nest every year, which suggests that one or more pairs failed to return from their winter vacations. I do wonder about the role of Bald Eagles in this matter too. Ospreys bringing food back to the nest have to run the gauntlet of hungry eagles to get to the nest. From what I've observed, about one in ten catches results in a steal from an eagle. If fish are a limiting resource, that could definitely affect the Osprey nesting success.
No comments:
Post a Comment