Clutch Size in Chesapeake Bay Bald Eagles: An unexpected history
October 6, 2017
By Bryan Watts, Center for Conservation Biology
(Photo: Two-egg bald eagle clutch along James River. This clutch size is by far the most common for bald eagles throughout their breeding range. Photo by Catherine Markham)
There was a time during the late 1800s through the mid-1900s when bird eggs were collected and sold or traded like stamps or coins. During this period, bald eagle eggs were valuable and in demand. The price for a single bald eagle egg was listed as $15.00 in the 1922 American Oologist Exchange Price List of North American Eggs. Due to the general interest in eagles and the value of their eggs, eagle egg collectors were widespread throughout North America. Several major collectors including Harold Bailey, Fred Jones, Edward Court, Richard Harlow, and Willet Griffee were active in the Chesapeake Bay through the 1940s. Each had their own collecting area that seemed to be respected by gentlemanly agreement but all were highly secretive about the location of prized nests where they collected.
Although eagle egg collecting has now gone the way of the stagecoach, compilation of clutch sizes from egg collections provides exciting new insight into the ecology of eagles during a time period before the introduction of DDT into the estuary. Compared to all other accounts throughout the species range, clutch size within the Chesapeake Bay during this early time period was extraordinarily high, averaging 2.46 eggs. In his remarkable book, The Bald Eagle, Mark Stalmaster summarized 16 studies from throughout the breeding range that reported clutch size and indicated that 17% of clutches contained only one egg, and only 4% contained more than two eggs. In stark contrast to this finding, 3.3% of clutches collected in the Bay contained only one egg and 43.0% contained three or more eggs.
Amazingly, eagle egg collections from the Chesapeake included three four-egg clutches and two five-egg clutches. Although rare, four-egg clutches have been documented in recent times within the Chesapeake including three four-young broods. The five-egg clutches are an oddity and have not been reported previously within the bald eagle literature. The clutches were collected by E. J. Court, who was a prolific collector for more than 30 years along the upper Potomac River below Washington, D.C. Examination of the collection notes provides confidence that these clutches were real. The possibility that these clutches resulted from contributions of two females cannot be resolved. Incredibly, the two clutches were collected from the same territory just two years apart. The territory was on the Virginia side of the Potomac in a site known commonly as Crow’s Nest, which continues to be a center of eagle activity to the present day. The circumstances that lead to these clutches are unclear, and because they were collected we are left to wonder if the pair could have hatched all five eggs and raised the young to independence.
One of the most surprising discoveries from looking back through the historic clutch record is that bald eagle clutch sizes have changed dramatically over the past century. By the time the aerial survey was initiated in the early 1960s, the average clutch size had been reduced by nearly half compared to the period of egg collecting that closed merely 20 years before. During the 1960s and 1970s, 20 (66.7%) of 30 documented clutches were single eggs and only one (3.3%) contained three eggs. By the 1980s and 1990s, clutches were trending larger with only 26.8% (N = 56) single egg and 16.1% containing three eggs. Although recovery is not complete, after the year 2000 clutches have begun to resemble those in the early 1900s, with only 4.0% (N = 99) single eggs, 66.7% two-egg clutches, and 29.3% three-egg clutches.
The impact of legacy contaminants such as DDT on egg hatching rates, young survival, and even adult mortality from the 1950s through the 1970s has been reasonably well documented. A retrospective assessment of clutch size within the Chesapeake Bay suggests that contaminants likely caused a direct suppression of clutch size as well.