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Bald Eagles of Alaska - Section: 13 Page: 98 - Perspectives on the Breeding Biology of Bald Eagles in Southeast Alaska

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populations elsewhere in the country (Stalmaster 1987).

Eggs are usually laid several days apart, although inter-egg intervals can be up to a week
(Wiemeyer 1981). Eagles begin incubating eggs immediately after laying. Although
males and females share incubation duties (Cain 1985), I have observed females doing
most of the incubation immediately after laying.

Once females complete their clutch, the eggs must be incubated almost 24 hours a day.
The embryos in eggs left unattended for more than a few minutes in Southeast will
probably die of exposure, or possibly predation by corvids. Reflecting this constraint,
eagles seldom leave an active nest during incubation. While one adult sits tediously on
the nest, the other adult perches nearby. The non-incubating adult forages and, once a fish
is caught, will fly back to a tree and eat it completely. On only two occasions have I
observed an incubating adult come off the eggs to try to capture a fish. In both instances,
the nest was eventually abandoned before the eggs hatched, suggesting that the
incubating birds may have been food stressed. Duration of incubation bouts depends on
the weather. Eggs are incubated for much longer periods during windy, rainy days,
usually by the female (Cain 1985). When the incubating bird wants to switch duties with
the foraging bird, it often calls to its mate and stands up on the rim of the nest before
flying off. Or the foraging bird will fly onto the rim of the nest before the two switch
incubation duties. For successful incubation, therefore, nesting pairs must have good
timing and communication.

Once chicks hatch, almost all prey caught by either adult is taken directly to the nest.
During supplemental feeding experiments, adults (mostly males) would, on occasion, eat
part of the fish before bringing it to the nest, although in most instances adults took fish
directly to the nest. During one climb to a nest with newly hatched young, I found over
18 uneaten adult herring and several partially eaten walleye pollock. The nest eventually
fledged both young. Second, the change in behavior of an incubating eagle is another
good indication of when hatching occurs. There are few activities in nature more boring
to observe than an incubating eagle. On average, eagles in Southeast will stand and turn
the eggs every 75 minutes or so, longer if the weather is cold or wet. During or after
hatching, however, eagles will stand and examine the contents of the nest much more
frequently, about every half hour just after hatching.

Nestlings develop for 10-12 weeks before fledging, which occurs, on average, in midAugust. From the time they hatch to just before fledging, eaglets will gain up to 6 kg (Bortolotti 1986). As eaglets get older, adults spend less time at the nest. Often, when chicks are 6 weeks old or older, adults will catch a fish and simply drop it in the nest, rather than stay in the nest and feed the chicks. Nesting birds will take only food small enough to carry immediately to a safe place to eat, avoiding any major confrontations with pirating eagles that would require the incubating adult to leave the nest (10).

98

OCR
Tag: bortolotti carry corvids eaglets eggs embryos fledged fledging nestlings stalmaster wiemeyer

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