What Drives the Number of Nests in a Territory?
Friday, July 30 2010 @ 12:59 PM EDT
Contributed by: davidh
Alternative Bald Eagle Nests - a Plethora in Some Areas - Not in Others- Why?
Here is a good Thesis topic -- but it needs a Sponsor!
In most of nature there are usually multiply reasons why some behavior or action takes place.
Why do some eaqles make several nests? In other active territories only one nest is ever found. Obviously some make a new nest to replace one that has been destroyed by the weather. The nest used for several years very successfully, simply rotted out and fell, partially or wholly to the ground. In other cases the tree blew over. In many of the urban areas the availability of strong cedar, Sitka spruce or
Douglas firs are no longer present and only short living quickly decomposing alder or cottonwood trees are available for nesting. A fine old cedar that might support a nest for 100 or perhaps 500 years is replaced by a cottonwood that has trouble surviving 100 years in total.
Disturbance of the eagles' nest is another cause for changing a nest site. I have seen over 40 cases that when a nest was visited by a person, intentionally or accidentally and that visitation has been seen by the territorial eagles, that site is abandoned . I was the causative agent in a few cases where I climbed a nest to band the young. In those years working in more wilderness areas, the eagles always finished rearing the young in the year of banding, but in the following year they built another nest in a nearby tree. I know many of my biologist friends have annually banded eagles in the same nest and the eagles re-breed there the next year. However, when I have 100% personal evidence of the eagles abandoning a nest site and building another after a nest was visited, then I have to act on my records -- for our area.
But I suspect there is a logical explanation emerging. In wilderness or areas supporting a lot of trees, where most of my observations were made, optional trees for building another nest are everywhere. In the urban and suburban areas alternative sites are not so plentiful and, consequently, I am more apprehensive of being seen in an urban nest. Most landowners love their eagles. For me to risk them losing their beautiful nesting birds is not something I wish to risk. I always restrict our cam cleaning or new cam placement during the eagles' absence on migration.
In other cases, as we have seen in BC with someone else not respecting the incubating bird in the nest and climbing the nest to place a cam, the eagles abandoned not just the eggs but the entire nest. Indeed they have built another nest in the region, if not then, in the following year, but for some landowners the change to a different nest tree is a devastating loss.
The above reasons for eagles to build an alternative nest is really quite different from what I was really wishing to address here. In several areas of the Chilkat River valley of Alaska, an incredible example and where we have lots of good data over some years, I believe another phenomenon is taking place. First, this is the home of the American Bald Eagle Foundation Chilkat-Haines Alaska Bald Eagle Festival every November. This is where 3000 to 4000 or more eagles congregate every fall and winter along some 20 miles of the Chilkat River. This river is not only one of the world's greatest regions for people to watch predators, but it is where eagles learn to be eagles.
So here is the point. In good food-productive eagle habitat, it is reported that bald eagles will nest as close as one pair every lineal mile of coastline. With the perhaps millions of salmon spawning over a 5 month period along the Chilkat you might say this is prime eagle habitat. And indeed it is for wintering eagles.
On the other hand I am not so sure that during the critical months when supporting a couple of nestlings is the big demand on parents, those months of April through July, that the Chilkat produces very many easily caught fish. But at the critical time when eagles are setting up nest breeding territories, from October through March, the River is loaded with dead, easily scavenged fish. Unfortunately for those eagles planning to nest in the area, the region is the winter home of a few thousand eagle intruders, adults and juveniles, that could be considered territorial competitors. Does a nesting bird attempt to drive away hundreds of intruders every day? Or come up with another safer strategy that reduces "beak and claw" contact?
My hypothesis of the situation is this. The resident eagles, as a way of making a statement of territoriality in the presence of hundreds of other eagles passing their nest sites hourly, can't undergo territorial beak and claw battles, that would be disastrous. The battles would be continuous and the risks of harm I suspect too great. So the territorial holders in my opinion undertake a displacement behavior that says, " See, I live here. See all my nests and the new nest I am building -- so go away."
In the Chilkat Valley we have 119 nests along 20 miles of River at the ABEF's last count, spread over that area appear to be 20 active territories. That is a record 6 nests per active territory. Was the construction of all these alternative and non-productive nests simply a waste of time and effort? Probably not, nature does not usually evolve to produce such waste. Or was this a very effective way of diverting dangerous territory fighting into a more benign safer alternative behavior, that of nest building? I suggest the effort and cost of building a new nest is less than the cost of the beak and claw battles to defend the area. Perhaps we have another lesson to learn!
The above should be an interesting broadly applicable hypothesis to test for a graduate thesis. The student could not just take the high eagle density of the Chilkat and compare the time devoted to nest building vs. fighting, but compare the number of alternative nests in other areas where wintering or even legitimate competition for nesting space could be a spacing factor for a comparison. The alternative sites where urban eagles can build is often very restricted due to lack of trees, but I would suggest this hypothesis applies elsewhere as well. Nest building is a powerful statement of territory -- but then nobody had to tell the Jerusalemites that!
While the parasites or bacteria might be health issues in nest abandonment, falling nests and nest trees is obviously impactful, and certainly disturbance is a known factor, I think that the major factor driving nest building in high density eagle habitat is that the activity of building a nest tells other eagles that the territory in occupied and also keeps the builder busy and not attacking an intruder and risking harm. This seems the reasonable conclusion looking at the Chilkat. If you put 4 or 6 nests in a territory, wouldn't that tell a passerby that he would be best to look elsewhere?
What an interesting thesis question! What are the factors driving nesting density and the number of alternative nests per territory? In very food-rich nesting territories AND in areas of high winter and spring populations of visiting eagles, I think there is an extra effort put into new nest building to make a bigger statement to intruders that the territory is occupied -- and at the cost of saving effort and lives.
We have students wishing to test this for a Masters or Ph.D.! Is there a good soul out there to fund this student project?