Here in Minnesota we do things a little different sometimes. Let me splain. First, a little background.
Bird parental investment is intense, or at least it can be. You all know the stories. A bunch of carp are regularly fed in a pond, so they learn to come to the edge of the water when they detect a presence there, and stick their big round mouths out of the water to beg for bits of bread. A mother or father bird has just started to feed the little hatchlings, who beg for food with their gaping maws. A windstorm. A weakened branch. Some bad luck. The bough breaks and the nest, with baby and all, comes down. The rats and cats feast.
The next day the disoriented parent bird happens by the pond and triggers the mouth-gaping begging of the carp. The bird is cued into action and finds a morsel of food for the fish, and the fish respond with more gaping, and the parent bird responds with more feeding. This goes on for a while until the hormones wear off and the bird goes on to other things.
A pair of bald eagles are nesting with a web cam in Minnesota. One of them killed a duck and brought it back to the nest. In case you were wondering, raptor style birds eat a lot of other birds, so that is not especially odd. Anyway, they ate the duck but the duck was about to lay an egg, which was inside her. The egg fell out. Normally, a hungry eagle would have just eaten the egg along with the rest of the duck, but in this case, the nesting, would be parental eagles, chose to nurture it instead. They moved the egg into the center of the next and are trying to hatch it.
What could possibly go wrong?
Hat tip: Jaf.
They're just stocking up their larder for later :)
Does it mean that any type of bird can incubate and hatch the eggs of another type of bird? What would happen once the egg is hatched, won't the fledgling die because it won't be raised by the same species?
It depends. Duck-like birds and chicken-like birds are highly precocial, meaning that the young can walk very early, can forage with help very early, etc. Eagles generally are highly altricial, meaning that the young are very helpless for a long time. There may be only a handful of the 10,000+ bird species on the planet Earth that are more altricial than the Bald Eagle and its closely related siblings species of Ern, Fish Eagle, etc. In order for this particular arrangement to work there would have to be a lot of compromise.
There are two reasons this could possibly work, and several it may not.
In favor of it working:
1) Eagles are very caring of their young and for a long time, and these eagles have other chicks. So all the cues that may cause the eagles to try and try to care for their young may be maintained, overriding the instinct to eat a duck.
2) The duck will not need much care anyway, other than keeping predators away, so the Eagles can be bing scary but otherwise incompetant caretakers and that just could work.
Reasons against it working, short version:
1) The duck will not stay in the nest and may wander off, and will be hard to care for at night.
2) The cues for a parent bird to bond to the young are narrowly defined, sometimes; while the chick will bond easily to "mom and dad" the parent eagles won't bond as easily to a young that does not give certain clues. Having said that, this instinct in an eagle will be less developed than in many song birds because the cues evolve in cases where birds parasitize each other and nobody parasitizes an eagle.
3) The egg may not even incubate because there is proper way to incubate each kind of egg and the Eagles probably don't know how to do this.
4) Eagle eggs probably take a very short time to hatch compared to duck eggs; there is a good chance the eagles will eventually figure that the duck egg is not going to hatch and they'll chuck it or eat it.
5) The duck's siblings, either while the duck is still an egg or afterwards, will want to kill it or dump it from the nest... they will want to do this to each other, in fact. The duck will have no anti-siblicide mechanisms in place because ducks don't do this.
According to wikipedia, the incubation time for eagle eggs is 35-36 days while that for ducks is 27-28 days.
Presumably then this egg will take 27-28 days to hatch, which is likely too long for the eagles to put up with. They will likely presume the egg to be non-viable and chuck it.
Interesting, I had guessed the reverse. Well, relative to some allometric scaling coefficient I have high confidence that I am correct.
Right, since the eaglets are already hatched, this egg is probably toast.
is it safe to say that the motherly, nurturing instinct took over the hunting instinct?
Is there a gene for patience in humans?
Thank You for the response Greg. I found this post quite fascinating.