Sibling Rivalry - the Great Survivial Technique

When owls, falcons, hawks or eagles lay their eggs in late winter or early spring they cannot be sure what the food supply will be to support the young some one to four months later as the chick's food demands become greatest. For millions of years nature has been experimenting with how best to resolve this issue.

The challenge for a species is quite simple. Nature wants to produce the most young that the habitat will effectively support. The system our raptors have evolved is quite ingenious. The balance is between developing a system in which the parents rear successfully what the habitat will produce food for versus a system in which all the chicks, if they did not get enough food, get collectively weaker and all starve. Sibling rivalry is where the bully takes all or most of the food until that bully is full, and only then does it sit back and let the next chick get fed. This then happens to the third chick. This means if lots of food is available then all three chicks will get fed and survive -- but not always will all chicks survive. This is nature's successful plan but not a plan that is easy to observe.

These species have worked out a fine scheme to insure that the most chicks survive under different levels of food productivity that might prevail at the different seasons. Now this system may seem harsh to us who get emotionally charged but it is a system that has worked well for the species over many millennia.

The northern owls I believe show the biggest disparity between the number of eggs laid and the number that the food supply might support. Some owls can lay 10 eggs and in years when the local food supply is very high they might all be raised. In very poor years of food supply only one or two chicks, or even none, can survive.

Our bald eagles show less variability. Most eagles lay 2 eggs and in wilderness areas, tend to average just over 1 young reared. A few eagles lay only 1 egg and a few more lay 3 eggs. There are a few instances where we believe bald eagles have laid 4 eggs -- by the single female.

Even in wilderness areas where eagles live and die as nature intended -- without being shot, electrocuted or run over -- the average number of young fledged per nest is just over 1 young -- meaning that nearly 1 young is lost per nest on the average. This loss can be from one egg simply not hatching (infertile or perhaps it died as an embryo), a chick dying at or just after hatching (got cold, deserted or was not strong enough at birth) or the growing chick succumbed to a host of challenges -- including competition from its siblings.

As we have seen at various other nests with CAMS, the whole range of selective pressures is constantly operating to reduce the number of young. Maine lost baby chicks, other sites have lost young at various stages, and the loss of two young by juvenile eagle predation on the Channel Islands last year was an incredible lesson to us all.

The 2006 Sidney nest production of Victoria and Sidney was "almost" as dramatic as they get -- but Sidney survived. There appeared to be a late season further abundance of food -- road kills etc. that allowed his big sister, once full, to allow Sidney access to the extra food and his little body just burst forward. He fledged to all our delight. Well do I remember the hundreds of emails & phone calls -- even threats -- begging and demanding that we "rescue" him from his horrible sister.

Our site is about nature and certainly about how we humans need to develop more compassion about what we destroy of our wilderness. At the same time there is a limit to what and how we can interfere. Interference I suspect can be of the wrong kind. And here is our dilemma -- both the legal, moral and compassionate elements.

From the long term protection of the bald eagle nesting territory I will not go into nests when adult eagles are around. I have 5 recent examples where local nests were climbed to and in all 5 cases the eagles did not re-nest in that tree the nest year. We all remember the devastation that Ms. Charlie created by entering 3 nests when the eggs were present. All eggs were abandoned. All nest sites were also abandoned. In all cases the eagles have built alternative nests in that or in the following year. I am not prepared to risk one of the landowners losing their nesting pair for an attempted rescue -- when of course the chick may survive as Sidney did.

The government also has regulations about disturbing and entering active nests that preclude such a rescue. Should a chick fall out of a nest we or someone will be there as soon as possible to assist it on the ground -- but not up the nest tree.

Please see my Hancock From The Field area in the discussion forum for discussion of this article.

Our CAMS are nature in the raw -- not always easy to watch in detail.

David Hancock
eagle biologist

Last Updated Saturday, March 09 2013 @ 05:01 PM EST| View Printable Version

3 comments

The following comments are owned by whomever posted them. This site is not responsible for what they say.
Authored by: Anonymous on Thursday, April 17 2014 @ 06:23 AM EDT Sibling Rivalry - the Great Survivial Technique
Dear Mr. Hancock Thank you so much for the article, it helps. GAYIn
[ # ]
Authored by: Annemie on Friday, June 07 2013 @ 08:41 PM EDT Sibling Rivalry - the Great Survivial Technique
Dear Mr. Hancock
Thank you so much for the article, it helps.
The best to you
sincerely Annemie Meyer
[ # ]
?

Please Donate

Please Donate!

Current & Ongoing Promotions

 

 

 

 

My Account





Sign up as a New User
Lost your password?