Breeding Behavior
Movements
Food
Longevity
Habitat
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Florida Fish and Wildlife Conservation Commission (FWC) unpublished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.
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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).
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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).
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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).
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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.
(1)
Taken from the Florida Fish and Wildlife Conservation Commission
Bald Eagle Management Plan (adopted April 9, 2008). View
the Management Plan.
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