Bald Eagle Breeding Season

Bald Eagle Breeding Season
Bald eagle nest, photo courtesy of USFWS
The bald eagle has been the national symbol of the United States since 1782.  It is North America’s second largest bird of prey—only the California condor is larger.  With bald eagle breeding season well under way here in Southwest Florida, let’s take a closer look at these regal birds.

Mated pairs may begin selecting sites and nest building or reconstruction as early as late September.  A bald eagle nest can be quite massive.  Often returning to the same nest each year, the birds may add to it annually until the weight of the nest is more than the tree can support and the nest—or sometimes the entire tree—falls.  Typical nests average 5 feet in diameter and 3 feet tall.  The largest nest on record, in St. Petersburg, Florida was 9.5 feet in diameter and over 20 feet tall.

Constructed near, but not at, the top of the tallest tree in the vicinity, the nest affords the birds shade from the sun and a good vantage point to monitor activity in the area.  Nest shape depends on the tree in which it is located and can be flat, conical, or cylindrical.  The eagles often have an alternative nest sight within their territory of approximately one square mile.  They may switch sites if they have an unsuccessful season due to too much noise and commotion or too many predators.

While most bald eagles return to the same nest site and mate each year, if either is no longer available, the survivor will find a new mate and, if necessary, build a new nest or steal one from another large bird (typically, a great horned owl.)  In Florida’s subtropical climate, the eagle’s breeding season can range from October to April, but most clutches are laid during December and January.

The average clutch size is two eggs and incubation generally takes about 35 days.  Incubation begins as soon as the first egg is laid, so the eggs may hatch one or two days apart.  This is in contrast to other bird species that lay an egg a day over a period of time and do not begin incubation until the last one is laid, ensuring that most of the offspring hatch at approximately the same time.  Due to the rapid growth of the eaglets, the difference in size during the first few weeks can be a determining factor in the welfare of the chicks.

The parents will usually feed the oldest chick at every feeding.  If there is enough food after that baby is full, then they will feed the second.  So not only has the first-born started out larger, but has the advantage of better nutrition.  In this manner, the birds help to ensure they can successfully raise at least one chick even in times when food is scarce.

During the first two weeks, the male provides most of the food, while the female tends to the young.  At three to four weeks, the female brings as much food as the male, with the young pecking at the food, but relying on parents to tear the meat into edible portions.  By six weeks, the eaglets are able to self-feed when supplied fish and rodents by the parents.

As the hatchings grow, the competition for food and attention from the parents can get brutal.  The smaller sibling can be bumped from the nest by a larger more aggressive nestling.  While many people may find this practice unpleasant, it is vital for the natural selection process to ensure the long-term health of the species.  The strong survive.

Next week:  Bald eagles, part deux.

by- Robin Jenkins, DVM