News Advisory

FROM:           Center for Conservation Biology, William & Mary

FOR IMMEDIATE RELEASE:  13 September 2024

MEDIA CONTACTS:         Dr. Bryan D. Watts, Director
                                          Center for Conservation Biology
                                          William & Mary
                                          bdwatt@wm.edu
                                          (757) 221-2247

BRIEF

          (Williamsburg, VA)— The Center for Conservation Biology has compiled 2024 breeding performance results for osprey in the Chesapeake Bay. The monitoring effort included 571 osprey pairs distributed among twelve study areas. Ten study areas were within the main stem of the Bay where salinity exceeded 10 parts per thousand (ppt) and two study areas (used as reference sites for comparison) were positioned on upper tributaries within tidal fresh reaches where salinity was less than 1 ppt. Osprey pairs nesting within waters above 10 ppt salinity are believed to be highly dependent on Atlantic menhaden to raise broods. Osprey pairs nesting within tidal fresh waters feed primarily on catfish and gizzard shad. Breeding pairs were monitored throughout the nesting season (March-August) to determine nesting success and productivity. Cameras were mounted on a subsample of nests within all study areas to quantify diet and brood provisioning and to determine the cause of nest failure. Compilation of camera data has not been completed.

Mean breeding performance for osprey pairs nesting within the main stem of the Bay did not meet levels believed to be required for population maintenance (1.15 young/pair). Collectively, reproductive rate was 0.51 young/pair. However, breeding performance did vary between study areas with two areas falling in the range of “minor deficit” (0.8-0.9 young/pair), two sites falling in the range of “moderate deficit” (0.6-0.8 young/pair) and six sites falling in the range of “major deficit” (less than 0.6 young/pair). By comparison, reproductive rate within reference sites was 1.36 young/pair that is above the maintenance target.

Based upon direct observations during nest visits, the largest contributing factor to poor breeding performance was seemingly the loss of young due to starvation. Low food availability leads to a sequential loss of young and results in smaller brood size or nest failure. One of the best indicators of food stress in Chesapeake Bay ospreys is the frequency of single-chick broods in the population. Of all broods successfully produced within main stem study areas (N = 163) 54.6% were single-chick broods. In contrast, only 18.2% of the 55 broods within reference study areas were single-chick broods. On average, main stem pairs lost 1.1 young between hatching and fledging compared to only 0.3 for pairs in reference sites.

The osprey breeding performance in the main stem of the Bay that was documented in 2024 (and for the past several years) is not sustainable. In the absence of immigration from other parts of the Bay or outside of the Bay the population would be predicted to decline. To date we have not conducted surveys throughout the entire main stem to evaluate trends in the breeding population.

ADDITIONAL DETAILS

2024 Objectives

In recent years we have published papers on the historic decline of osprey breeding performance in Mobjack Bay (a subestuary of the lower Chesapeake) and the role of menhaden in driving the decline.  One of the criticisms of this work is that “Mobjack Bay only reflects conditions within a small area of the larger Bay” and is not representative.  The primary objective of fieldwork in 2024 was to expand the geographic scope of osprey monitoring to better understand their reproduction throughout the portion of the Bay where the species is believed to be menhaden-dependent.  Additional objectives include 1) achieving a better understanding of the spatial variation in osprey reproductive performance, 2) quantify osprey diet throughout the main stem of the Bay and 3) work to develop a field metric that is a reliable indicator of food stress.   

Study Areas

The main study area was delineated based on the 10 ppt contour throughout the Chesapeake Bay.  Ten study areas were delineated throughout the main study area in early 2024 based on logistics and the known density of osprey to facilitate efficiency.  Specific study areas include VA – Lynnhaven River, Elizabeth River, Poquoson River, York River, Mobjack Bay, Piankatank River, Fleets Bay and Eastern Shore bayside and MD – Patuxent River and Harris Creek at mouth of Choptank River (monitored by USGS).  Reference study areas were selected within tidal fresh reaches of upper tributaries based on the same criteria.  Reference study areas include the upper James River and upper Rappahannock River in Virginia.

Breeding Performance

Poor breeding performance was widespread throughout the main stem of the Bay and none of the study areas reached demographic targets.  Although spatial variation in performance was evident throughout the season, most of the study areas were considered in the range of “major reproductive deficit”.  The overall reproductive rate for pairs in the main stem of the Bay was approximately 50% of that believed to be required for population maintenance.  By comparison, reproductive rate for pairs breeding within reference sites was well above maintenance levels.  Both clutch sizes and hatching rates were generally consistent between the main stem and reference areas and throughout the main stem sites.

A large number of osprey pairs did not lay clutches during the 2024 nesting season.  These pairs arrived from wintering grounds in a timely manner (late February – early March).  Most of these non-breeding pairs remained resident throughout the nesting season and defended territories but were never documented to lay eggs.  This is the first time this behavior has been documented on a large scale within the Chesapeake.  A likely explanation for the behavior is that females were not able to reach the adequate physiological body condition required to lay eggs.    

Causes of Nesting Failures

Osprey pairs are subjected to a wide range of forces that may lead to nesting failure.  These range from contaminants to weather events to nest competitors to predators and many others. Based on other observations and published studies, disease, competition for prey, depredation, and pollution do not currently appear to be significant causes of reproductive failure.  Poor breeding performance throughout the main stem of the Bay in 2024 was driven by the loss of young after hatching.  A clear indicator of food deficit (stress) within an osprey nest is the development of asymmetric broods where the young differ in size and developmental stage.  Asymmetric broods develop when not enough food is delivered to provision all young equally and leads to the formation of a dominance hierarchy within the brood and monopolization of food by dominant young.  The appearance of asymmetric broods is a precursor to brood reduction by the sequential loss of subordinate young to starvation. 

One example of food stress and brood reduction leading to nest failure was captured on a nest camera within the Eastern Shore study area.  The female laid and hatched three eggs.  The signs of food stress appeared early in the brood dynamics.  Over a period of three days the two smallest young died.  The third nestling survived another four days but after 38 hours without food died during the night.  The next morning the male delivered a fish and the female attempted to feed the dead young.  The female continued to shade the young for the rest of the day.  This is a typical starving sequence where an extended period without food pushes the young past the breaking point.

Asymmetric broods were common and widespread throughout the main stem of the Bay.  On average, pairs in the main stem lost 1.1 young between hatching and fledging.  Both the high failure rate of nests and the high frequency of one-young broods for successful nests were driven by brood reduction caused by food stress.  In contrast, asymmetric broods were uncommon within reference sites; on average pairs lost only 0.3 young and success rate was relatively high.

Implications

Overall, poor reproduction in ospreys is not restricted to the historic study area of Mobjack Bay but is widespread throughout the main stem and likely involves thousands of nesting pairs.  Whether or not we will see a broad-scale decline in the osprey breeding population ultimately depends on the relationship between areas (such as the main stem) that are in reproductive deficit and areas (such as the reference sites) that are producing a reproductive surplus.  A determination of whether or not the Bay population as a whole is sustainable given the current prey situation is a topic of ongoing investigation.

Project Partners

Center for Conservation Biology, William & Mary
Virginia Aquarium
Maryland-National Capital Park
Elizabeth River Project