Bald eagles have made an amazing comeback since the days of DDT. From less than 900 birds nationwide in the early 1960s to more than 20,000 in the U.S. in 2007, their population more than doubled in Pennsylvania in the last 10 years.
At some point the number of nesting bald eagles will reach Pennsylvania’s carrying capacity. What happens then? How do bald eagles respond to match available food and nest sites? We can look to Virginia for the answer.
Since 1964 the Center for Conservation Biology in Williamsburg has monitored and mapped bald eagle nests in the James River watershed. Every year they do a flyover of the entire area to count both nests and chicks. Their eagle population grew from none in 1976 to a record 289 nests in 2018. Meanwhile the number of chicks per nest — called “productivity” — rose sharply in the early years of recovery and now is dropping. CCB announced this trend in two articles:
- Eagle Productivity Continues to Slide, July 2018 –and–
- James River Eagle Recovery Enters Final Chapter, July 2017
To see how this works, I have made a slideshow of two graphs from the Center for Conservation Biology’s article. The bar chart is the count of nesting pairs, 0 to 289. The line chart, the arc, is the average number of chicks per nest, 0 to 1.6.
As you can see, the number of nesting pairs continues to increase while the number of chicks per nest moves down.
CCB reports that in 2018 the number of chicks reached 1.09 and adds, “The two opposing trends appear to continue the population’s path to stability.”
“Productivity decline” sounds bad but it’s actually good news. Breeding eagles respond naturally to accommodate lots of adults in the habitat.
So what does this mean for Pennsylvania’s bald eagles?
If the James River experience applies here, we’ll still see an increasing number of bald eagle nests that will eventually average one eaglet per nest. We know bald eagles can produce more if they need to. The good news is, they don’t need to.
p.s. See how rapidly the James River nest count grew! Click here for CCB’s maps of the James River bald eagle population, 1990-2017.