Bald Eagle
Breeding & Nesting
Breeding and Nesting
Bald eagles mature at 5 or 6 years of age. At
this time they pair up (usually for life), establish and start to
defend a territory, and engage in breeding activities. Nest
building, nest repair and egg laying can occur from late February to
early June in British Columbia, but 50% occurs between early March
and mid-April. One to three eggs (usually two) are laid by the
female and incubated by both parents for approximately 35 days.
Chicks are raised by both parents, remain in the nest for 11 to 12
weeks, and stay in the nest vicinity for up to another month. Only
about 10% of the young reach maturity, but adult eagles have a low
mortality rate (can live 20+ years) and have a breeding span of 10+
years. Only a portion of the adult population breeds every year (60%
is typical for Vancouver Island). Some pairs may breed every year,
but others may miss years or not breed at all. Reasons for this are
not known but may relate to all available territories being
occupied, food supply, or some form of disturbance at the nest site
causing abandonment.
The nest is the focal point of the eagle's
territory. Fidelity to nesting territory is very high, with most
pairs using the same area for all their breeding life. Within their
territory, the eagles build at least one nest, and if other trees of
an appropriate structure exist nearby, they will usually construct
an alternate or second nest. They may show a preference for one
nest, or regularly alternate between nests within their territory in
different breeding seasons. New pairs will establish in vacant
territories and may build new nests or repair and use old nests.
Eagles repair and add to their nests each breeding year and
throughout the breeding season. Causes of abandonment of a
particular nest within a territory, both permanent and temporary,
include tree damage or removal, severe nest damage, or human
disturbance at or around the nest during the critical nesting period
(February 1 to June 30). Nests may be damaged by storms and
occasionally fall because of their own weight.
Nest Trees
Eagle nest trees are typically large, very old
trees located near water, at the forest edge, or in trees protruding
above the rest of the canopy. A large open crowned tree with broad
upper branches is required to support the heavy, bulky nests. Trees
selected are almost always the dominant or co-dominant trees in a
stand occupying a prominent location with an unobstructed view of a
nearby aquatic environment. The prominent height and position are
obviously advantageous for observing prey or food, advertising that
the site is occupied, and defending the site from other eagles. Due
to their large size, eagles require unobstructed flying access to
the nest from several directions. Eagle nest tree requirements can
therefore be considered to be somewhat specialized, and hence are
not common or easily replaceable.
Most second-growth trees don't have the size and
structure necessary to support eagles nests. Most eagle nest trees
on the east coast of Vancouver Island (81%+) are veteran
Douglas-firs over 150 years of age (T Martin, pers. com.), usually
found within a kilometer of the shoreline.
In the past, most old
growth Douglas-fir forests on southeast Vancouver Island have been
logged and have given way to settlements but, until recently, enough
small stands of old growth, or lone veteran trees amongst smaller
second growth, have remained to support a sizeable nesting eagle
population. However, accelerating land development (e.g., waterfront
subdivisions) and logging of lands fringing Georgia Strait have
caused the loss of nest trees, threatening the long-term maintenance
of a breeding population of eagles. Bald eagles are territorial and
on Vancouver Island appear to generally require 1000m (some sites
near exceptional feeding ground can can tolerate 500m) between
different pairs nesting sites. It therefore cannot be expected that
removal of nesting habitat in one area will simply result in
displaced eagles crowding into remaining available habitat. Loss of
nest trees in this area can therefore result in a permanent
reduction in the nesting population. Gradual loss of nesting habitat
is considered to be the most significant factor affecting bald eagle
abundance in the Strait of Georgia. Long-term prospects for the
species in Georgia Strait approximate those in adjacent Washington
State where the bald eagle is designated as threatened. In that
state, both the eagle and the nests are protected by laws similar to
that in British Columbia but, in addition, landowners having eagle
nests on their property are required by law to develop a management
plan showing how the nest tree is to be preserved.
References:
Blood, D.A, 1989. Conservation Plan for Bald
Eagle Nesting Trees in the Nanaimo Area. Prepared for the BC.
Ministry of Environment, BC. Conservation Fund.
Blood, D.A., and G.G. Anweiler, 1991. Status of
the Bald Eagles in British Columbia. Draft Report prepared for BC.
Ministry of Environment, Lands and Parks.
Davies, Rick. Internal fact sheet, Bald Eagle
nesting requirements.
Martin, T. Strait of Georgia Bald Eagle Nest Tree
Inventories. Fall, 2000. BC Naturalist.
Vermeer, K. and K.H. Morgan et al, 1989. Populations, nesting
habitat and food of Bald Eagles in the Gulf Islands (in “The Ecology
and Status of Marine and Shoreline Birds in the Strait of Georgia,
BC”. Special Publication).
|