The early seasonal breeding schedule for Bald Eagles of southern British Columbia.
The nesting season of the individual bald eagle, in the southwest of British Columbia and north West Washington State, lasts from late September through early August. Each pair can vary 30 or more days from its next-door neighbor — in some years as much as 60 days separates the starting date of egg laying.
Seasonal Cycle:
- Establishing the Territory & Nest Building: Arrival on the breeding grounds is a very key time for determining who settles where. Today local bald eagles initiate their breeding cycle immediately upon return from migration in late September or early October. Fifty years ago, the season started in later October or early November. This is an incredibly busy period re-bonding and focusing on re-building the nest, copulating and a lot of territorial defense calls to passing eagles. The northern eagles that winter in the lower Fraser Valley but breed across the northern boreal forest and into the tundra are still present in southern BC and Washington State and don’t even depart our area until late March or even early April. However, the breeding season for our nesting eagles is underway in late September and not later than early October.
- Screaming Territory Defense: November and December are equally important times for noisily defending the territory, particularly sitting on territory borders proclaiming ownership but usually with less nest building – as this was done earlier. One of the pair is usually on territory while the other may depart to nearby rivers that flow into the Fraser along the lower coast for easy feeding on spawned out salmon.
- The Finishing Touches – Nest Cup Building: January, and particularly mid January through February is final nest touch-up. This involves a lot of nest cup infill with lichens and mosses — even a lot of sitting in the nest and shaping it — sometimes for a week or more before the first egg is laid. Nest building, bringing in fresh greens, copulations, incubation, feeding young and territory proclamations carry on from late September right through departure on migration in August.
- Incubation: February is the start of egg laying though in some adjacent pairs it doesn’t start for another month. Egg laying is characterized by a lot of time of the hen sitting in the nest, often a few days to a week constantly before the first egg is laid. Perhaps she is getting into the mood of incubation. The eggs are laid 3 days apart — usually two in number but about 15% of the time there are 3 eggs — only rarely is one laid. Most females lay the same number each year on very similar dates. Consistency is a thing about different BAEAs. Incubation takes 36 to 37 days. Most often the eagles start incubation with the first egg laid — and stay incubating alternatively with the male exchanging every 3 to 5 hours through the incubation period. The females largely sit through the night. Egg rolling is indicative that the pair have eggs. On occasion the female does not sit tight but sits a little above the egg for the first day or two. This results in the egg temp not getting up to the required embryonic germination temperature on the first day or two and the first and last eggs may hatch closer than 3 days between each. We have records of all hatching the same day – though they were laid 3 days apart.
- Raising Young: This is characterized by the arrival of food into the nest. The parents start offering food, sometimes a day or two before the chicks are willing to eat as they have to first digest the last absorbed egg yolk before new food enters the gut via the mouth. By 10 days of age the white down is replaced by a grey down that offers much more warmth and permits both parents to leave the chicks a few minutes to a few hours without brooding. By 5 weeks of age a few brown body feathers are emerging as speckles on the back. At 10 weeks the chicks are full size and the flight feathers become hard-pinned — the blood is withdrawn. This nourishment is quickly absorbed to nourish the pantaloons and feed the eaglets when they get deserted by the parents to encourage that first flight.
- Branching: is the movement around the nest tree characterized by hopping from branch to branch and usually takes place from when the flight feathers become hard pinned on the 10th week. By 11 weeks the eaglets are fully feathered, fully branched if the nest permits jumping around getting eye – foot coordination, so important for the first landing. This is the season of lots of nestling activity – including jumping vertical helicopter flights. By the time the eaglets are fully grown they are heavier than that bird will be at maturity. Its wings will be longer when it leaves the nest than when it is an adult. So much for little eagles flying around!
- Fledging: the chicks usually fledge on the 83 or 84th day. Fledging means leaving the nest tree for another tree. During this period the eaglets fly around a lot, may get some food but often do not. This is the period when the eaglets lose a pound or two (500-1000 grams) in preparation for migration — perhaps five hundred to 1000 km north. An eaglet glides further on its big wings if it weighs a little less. In some instances, the parents bring food to city nests but generally the eaglets try to steal from the adults. This lasts about 5 to 10 days before the eaglets are deserted by the parents — who depart north on migration alone.
- Desertion & Migration: after about a week of overseeing and defending the flying eaglets, usually after an exuberant attack for food by the eaglet, the parents both leave on migration abandoning the eaglets. Then the eaglets leave the home territory in about another 3 to 6 days — presumably also headed north. Our only gps tracked juvenile went from the Canadian border to the mouth of the Stikine River on the Alaskan panhandle, some 1100 km, in two days (about 680 miles). Hopefully they return after 5 years of learning to be eagles.
David Hancock www.hancockwildlife.org
David@hancockwildlife.org 604 761-1025