Biology

Bald eagles are highly social outside of the nesting season, but are extremely territorial when nesting.  They are capable of breeding in their fourth year, while still in subadult plumage, or the coat of feathers worn by young eagles not quite fully developed. Eagles may not breed until their sixth or seventh year where breeding competition is intense (Buehler 2000).  Bald eagles are thought to be monogamous, with a pair of eagles bonding for several years, but this is largely unproven.  Eagles are typically single-brooded during each nesting season. Although, pairs may renest if the first clutch is lost, meaning a pair of eagles generally only have one set of young each nesting season unless the first set of young is somehow lost.

Bald eagles in Florida begin building a nest or start gathering materials for a nest in late September or early October.  The nesting season is prolonged. Eagles begin laying eggs as early as October or as late as April (nests that are built later in the season are mostly renesting attempts or nest built after the first attempt fails, Millsap et al. 2004).  For purposes of the FWC Bald Eagle Management Plan, the bald eagle nesting season is defined as the period from October 1 through May 15. Nest sites tend to be built near the edges of eagle habitats (McEwan and Hirth 1980) such as in a living tree that offers a view of the surrounding area and that can support the eagle's often sizeable nest.  Substrates, or the base of a structure where eagles build their nests in Florida vary according to local conditions, and include pine trees (Pinus palustris and P. elliottii), cypress trees (Taxodium spp.), mangroves (Avicennia germinans and Rhizophora mangle), great blue heron nests (Ardea herodia), artificial structures such as communication towers, transmission towers, and raptor nesting platforms, and even though very rarely on the ground (Broley 1947, Shea et al. 1979, Curnutt and Robertson 1994, Curnutt 1996, Millsap et al. 2004).  However, bald eagles in Florida strongly prefer living native pines to all other substrates; 75 percent of all eagle nests surveyed during 2006 were built in living native pines. This is based on Florida Fish and Wildlife Conservation Commission (FWC) unpublished data.

Nearly all bald eagle nests in Florida are built within 1.8 miles of water (Wood et al. 1989). Territory size varies depending on habitat and prey density but is thought to encompass 0.6-1.2 square miles (Buehler 2000).  Bald eagle nests are spaced apart to ensure sufficient food resources for nestlings and to raise young with minimal disturbance from other eagles.  Eagle pairs often build more than one nest, which allows them to move to an alternate nest while remaining in their territory.  Throughout their range, eagles maintain an average of 1.5 nests per territory, ranging from one nest to five nests (Stalmaster 1987, Buehler 2000).

Most clutches of eggs in Florida are laid between December and early January.  Average clutch size throughout the bald eagle's range is 1.87 eggs, with most nests containing two eggs.  Incubation lasts about 35 days. Average brood size in Florida is 1.56 nestlings per nest (FWC unpublished data).  Nestlings in Florida fledge, or become able to fly from the nest, at around 11 weeks of age and remain with their parents near the nest for an additional 4-11 weeks (Wood 1992, Wood et al. 1998).  Fledglings begin to fly regularly in the vicinity of the nest before initial dispersal, which occurs from April to July (Millsap et al. 2004).  Based on a sample of 18,838 nests in Florida during 1973-2004, average annual breeding productivity was 70.6 percent, ranging from 52.2 percent in 1974 to 82.7 percent in 1996 (Nesbitt 2005).  Average reproductive success during 1973-2004 was 1.16 fledglings for all nests and 1.54 fledglings per successful nest.

Most of Florida's breeding bald eagles, especially those nesting in the extreme southern peninsula, remain in the state year-round, but most subadults, or birds not quite fully grown, and non-breeding adults migrate out of Florida (Stevenson and Anderson 1994, Curnutt 1996, Mojica 2006).  Eagles migrate northward between April and August and return southward from late July through late December.  Juveniles migrate northward later than older subadults (Broley 1947, Wood and Collopy 1995, Mojica 2006).  Most juveniles disperse at about 128 days of age and spend their first summer as far north as Newfoundland, with peak numbers summering around Chesapeake Bay and the coastal plain of North Carolina (Broley 1947, Millsap et al. 2004, Mojica 2006).  Florida's bald eagles use three migration flyways - the Atlantic coast, Appalachian Mountains, and the Mississippi River valley - with equal frequency, and they use stopover sites for resting or foraging (Mojica 2006).  Eagles also exhibit nomadic wandering, mostly by subadults.  Northern-breeding alascanus bald eagles winter in Florida at least occasionally (Stevenson and Anderson 1994).

Bald eagles are opportunistic foragers, feeding or scavenging on a wide variety of prey.  Primary prey of eagles in Florida includes various fish and waterfowl species.  Prey from one study in north-central Florida was composed of 78 percent fish (mostly catfish, especially brown bullhead; Ictalurus nebulosus), 17 percent birds (mainly American coot; Fulica americana), three percent mammals, and one percent amphibians and reptiles combined (McEwan and Hirth 1980).  Most prey is captured from the surface of the water, but bald eagles often harass ospreys (Pandion haliaetus) in flight to drop fish that they have captured.  Bald eagles in Florida often scavenge carcasses along roadways or garbage at landfills (Millsap et al. 2004).

The record lifespan for a bald eagle in the wild is 28 years.  Eagles follow a pattern typical of raptors, with lower juvenile survival followed by increasing survival to adulthood (Buehler 2000, Millsap et al. 2004).

Throughout their range, bald eagles use forested habitats for nesting and roosting, and expanses of shallow fresh or salt water for foraging.  Nesting habitat generally consists of densely forested areas of mature trees that are isolated from human disturbance (Buehler 2000).  Daytime roosts are generally in "super canopy" trees which are very large trees which will poke above most trees in the forest and are adjacent to shorelines, and are typically located away from human disturbance (Buehler 2000).  Communal roosts, which are rare in Florida, are located within three miles of water (Mojica 2006).  The quality of foraging habitat is characterized by the diversity, abundance, and vulnerability of eagle prey, the structure of the aquatic habitat (such as the presence of shallow water), and the extent of human disturbance (Buehler 2000).  Bald eagle nesting habitats are protected by law, but little or no emphasis has yet been placed on the preservation of roosting or foraging habitats (Mojica 2006).  The greatest numbers of bald eagle nesting territories in Florida are found along the Gulf coast and around some of the larger inland lakes and river systems in the Florida peninsula.