Tuesday, May 22, 2012

Re-run: Why don't the eyasses fall off the edge of the nest?

This is a re-run from last year (May 20, 2011).  Though the images are of last year's eyasses, 
the information is completely relevant as we watch and worry about how close they are to 
the edge of the nest.

Why don't the eyasses fall off the edge of the nest?  As the eyasses become increasingly curious about the world out there... 

... nervous nest watchers worry as they see the eyasses move closer to the edge of the nest.

Each day, there is more wingercising...

... and scary losses of balance right at the edge of the nest, four floors up from the concrete sidewalk of Winter Street.

Experienced Franklin Institute nest watchers have lived through this twice already, and know that the eyasses manage to stay safe.  I asked John Blakeman why this is, and he shared this fascinating information:

"Actually, the little eyasses have a pretty developed and strong impulse to stay down in the bowl of the nest. They don’t rationally know that they sit precariously at height in a tree or on a ledge. But instinct and natural selection have left them with good instincts regarding the hazards of getting too close to the edge of the nest.

However, Red-tail eyasses do sometimes fall out of the nest, but almost always during defecation. It goes like this - unlike passerine birds (“song birds”), Red-tails and other similar raptors do not excrete fecal sacs. These little bags of feces are lifted out of the nest and carried away by the parents. This keeps the nest clean, and reduces the discovery of the nest by marauding predators such as raccoons who would prey upon the nestlings if discovered by olfactory hints.  But in Red-tails, the little eyasses have an instinct to, as falconers and raptor biologists say, “slice.” Watchers of the eyasses in The Franklin Institute nest have seen this. The little eyass stands and backs its tail out toward the perimeter of the nest. Then, in an instantaneous spurt, the eyass squirts the “slicings” out over the edge of the nest.

            Scott Kemper (2012)

But very infrequently, the little eyass backs up too close to the edge of the nest, and simply falls out backwards, all in the good attempt to direct its slicings away from the nest. Frankly, I’ve never encountered this, but it has been reported. I think that when an eyass falls from the nest during slicing, it may have some neuromuscular difficulties. It happens, but rather rarely. We needn’t concern ourselves with it. 
A word about the “slicings.” In defecation, hawks and eagles are said to “slice.” The feces are “slicings,” and the remnants in the FI case are seen as the white stains on the lower edge of the window. Our three eyasses have nicely sliced on that surface.

 They have also sliced on the wall alongside their nest.

But things are a bit different in true falcons. Falcons don’t slice, they “mute.” Falcons are unable to powerfully project their feces (called “mutes”). Instead, a defecating (“muting”) falcon merely drops her mutes beneath her. This has significant results at falcon aeries [nests], which are almost always on a ledge or cliff. 

Falcons are known to occupy aeries for literally centuries. There are Peregrine Falcon aeries in the UK that have been occupied since the Middle Ages. Actually, they have probably been occupied for millennia, since the ending of the Ice Age. 

How do we know this? Well, there is the historical record of falconry, where British and Scottish falconers have been taking eyasses from these nests as long as falconry has been in the British Isles, probably from the ninth century or earlier.  But these piles of falcon mutes tell their own story. The same phenomenon occurs in American Prairie Falcons, in the West. Out there, raptor biologists have bored down through many feet of ancient Prairie Falcon mutes and have counted the layers (a new layer each year). Some of these aeries are centuries old. 

A falcon can get away with this, as mammalian predators of eyasses such as raccoons, weasels, badgers, and the like, can’t climb around on steep, high cliffs. The eyasses are safe, even though their aeries can be easily spotted from afar. Falconers and raptor biologists scan the long streaks and thick deposits of falcon mutes, the “hawk chalk.” I’ve spent five summers in the West studying raptors out there, and finding a Prairie Falcon nest was not very hard. Just scan a long cliff face with a pair of binoculars and look for the white hawk chalk. Very obvious. (And obvious, too, to the falcons. New aeries have been created by merely painting white hawk chalk-like streaks on an empty cliff ledge. Within a year a pair of Prairie Falcons will often take up residence, subsequently creating their own hawk chalk.) 

About raptor feces. Hawk slicings and falcon mutes are predominantly white, with a central blob of darker, more consolidated material. The white, liquidy portion is primarily uric acid, a concentrated form of the liquid, dissolved urea found in mammalian urine, including us. We, like most mammals, both urinate and defecate. Not so with birds, especially our hawks and falcons. Instead of clearing waste proteins and other nitrogenous materials with water and dissolved urea (from the kidneys), birds can’t be carrying around ample amounts of water to make urine. Instead, they further process the urea their livers create into concentrated white crystals of insoluble uric acid. That’s the white part of the bird’s splat on a windshield. It’s concentrated, so as to use very little water. 

The dark part of a mute or slicing is actual intestinal feces. Because raptors very thoroughly digest their food, and there is no “fiber” content, there isn’t much left coming out of the intestines after digestion. Most of what raptors eat is bone and protein. Hawks and falcons (unlike owls) thoroughly digest all bone material, so there is no solid bone waste in the mutes or slicings. The proteins of the prey are completely digested and eventually end up as the white uric acid — except for feathers or fur, which is not digested but vomited up each day as a “casting,” or “pellet.” Castings are the balls of fur or feathers from the previous day’s meals, expelled through the mouth.

Well, I guess I’ve pretty much sliced up all of that. I’ll now go mute.

–John Blakeman

 While John is mute, here are some pictures from this week at our nest.

The eyasses still take long naps, sometimes spread out ....

... and sometimes in a cosy clump.

When they are awake, they are on full alert.  Here, the tiercel (dad) has just flown off the nest, and they are intently watching him as he flies across the Parkway.

John Blakeman tells us that "honing visual acuity is one of the very important maturation processes that the eyasses are learning while "just sitting around on the nest." This ability to follow distant objects moving in the landscapes these hawks live in is a crucial survival lesson. The FI eyasses are learning those ocular and neuromuscular skills now. They are learning to visually follow food, something that will allow them to follow rats and mice when they start to hunt."

The following are pictures I took through the Board Room window with a point-and-shoot camera, so the quality is nowhere near that of Kay's pictures, but it gives a sense of their growth, and also of the surrounding landscape.

Here's the formel looking out across Winter Street and the Parkway with the Vine Street Expressway running underneath.

She sees everything, and here she is keeping a very close watch on a passing dog walker.  You can see how huge the eyasses' feet are.  They have to keep them arranged out in front so as not to tangle in the nest sticks!

The formel is a very intimidating bird up close.  I was glad the window glass is thick.....

The eyasses' wing feathers are starting to grow through their fluff...

... and the characteristic chestnut and black banded tail feathers are apparent.  The red tail feathers do not appear for the first two years.

That bobblehead has a serious beak!  Their eyes are currently jet black, but by the time the eyasses fledge from the nest, their eyes will be a lovely gold in color.  The eyes turn back to black when they are mature hawks.  Golden eyes are a clear indication of an immature hawk.

The eyasses line up nicely while mom picks off tasty morsels for them...

... and there is no squabbling as she feeds each one.

After eating, they settle down for the afternoon, but it is not long before this eyass decides to play tug-o-war with a rat carcass, yanking on its tail.

They are intensely curious about what is going on behind the window, showing no fear whatsoever.

The tiercel flew in and joined the family for a quick visit.  He is noticeably smaller than his mate, and though he is a stone-cold killer, he has a gentler face than the formel.

He brought a nestling - looked like a young robin - pulled it apart, and enthusiastically fed the eyasses.

The nest now has three year's worth of sticks and towers above the wooden frame that the Franklin Institute installed after the haggards' first few attempts at a nest kept blowing off the ledge.

And so ends another great week for the eyasses and their parents.  Let's fasten our seat belts and have the valium on hand for when they start hopping and "catching air" and heading out to the ledge!


  1. I so look forward to each new post. This one was especially detailed and informative.
    Last Friday, driving past on 21st St. I looked up and saw one of the eyesses stretched up to full height, stubby wings visible surveying the scene below. Thrilling and very cute!

  2. I've lived in Colorado for 56 years, and have my own almost-resident pair of redtails...they nest across the road, but hunt in my pasture, have selected trees for watching for squirrels and prairie dogs, but carry the prey right back to the nest in summer instead of using the dedicated "eating tree." I'm learning so much from these posts, and also love seeing the wing of the art museum in photos to bring back memories of my childhood trips in from Delaware County.