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).