From Start to Finish
It is normal for a pair of White-tailed Eagles to lay two large uniform white eggs. Clutches of one and three eggs are less usual, although a pair on the Isle of Canna, North-west of Mull, successfully rearing three chicks from a nest in 2011. Four eggs have on more than one occasion been reported from within a White-tailed Eagle’s nest, but it is unknown whether this was the result of two separate females laying in the same nest.
Eggs are laid asynchronously, at intervals of between two and five days, from early March onwards. Incubation takes place after the first egg is laid and lasts for an average of 38 days. The first chick to hatch is always the largest and strongest and, during periods of food scarcity, often due to inclement weather, the older bird may be forced to eat its sibling in order to survive. However, despite its larger size, the oldest chick rarely shows any aggression towards its younger sibling(s), unlike the often deadly violence associated with Golden Eagle nests*. Regardless, it is still remarkable should a pair successfully fledge three healthy chicks in one season. Both sexes share responsibility for sitting on the eggs, although the bulk of the incubation duty is carried out by the female.
* This behaviour of young Golden Eaglets is referred to as Cainism or obligate siblicide, when the older sibling will kill its younger brother or sister regardless of whether there is any shortage of food or not.
Patience and Luck
The near-six week period when birds are incubating can be a frustrating time for would-be White-tailed Eagle watchers on the Isle of Mull and a greater degree of patience and luck can be required to secure good sightings. Change-overs may be expected to occur approximately every two hours, although incubating birds may sit tight for hours at a time, while the off-duty partner forages away from the nest or roosts in a favoured location nearby. Off-duty birds may provide impressively close views at such times, but loafing birds may simply appear to sit around doing very little until it is time for a change-over to take place at the nest. During incubation, the luckiest eagle watchers may be fortunate to witness one of these changeovers, when both adults may be present at the nest for a short time. However, despite their enormous size (especially when in flight) it is possible for the birds to outsmart their watchers, allowing changeovers to take place unbeknown to the would-be observer. When watching White-tailed Eagles at a known nest location, it always pays to be especially vigilant, as well as patient. Take your eye off the ball at your peril!
For the first two – four weeks after hatching, the female remains in close proximity of the nest and may be seen actively brooding the young eaglets at this critical stage in their early development. White-tailed Eagle hatchlings are unable to regulate their own body temperature at this time and require to be sheltered from the vagaries of the Isle of Mull climate, in order to prevent excessive heat loss and potential mortality. The nest of a White-tailed Eagle can become saturated or blown out of its location during wind and rain in Spring. ( Several pairs lost their nests to the violent storms during mid-late May 2011.) The male provides all the food required by its partner and the chicks at this time, with the female not usually commencing hunting duties until after the third week. The eaglets may now be left unattended at the nest for long periods as both parents extend their hunting forays in search of food to satisfy the appetites of themselves, as well as their chicks.
Sea (Eagle) Cadets
Young White-tailed Eagle chicks start to feed themselves in the nest when they are about 6 weeks of age and begin to explore their nest surroundings during the following weeks. Depending on the nest location, and as fledging approaches, the adults will encourage the eaglets to exercise and will attempt to get the chicks to fly short distances from the safety of the nest in order to receive food. At 10 – 12 weeks, the young eagles are ready to leave the sanctuary of the nest, but will often stay in the vicinity for several months while they remain dependent on their parents for food and learn to fend for themselves.
From the time spent re-building nests in January until that year’s fledglings are successfully weaned from their parents in Autumn, the breeding season for White-tailed Eagles on the Isle of Mull may last in the region of nine months.