The eyasses are slicing (all you need to know about hawk poop!)

Many viewers of the eyasses have commented on their..... excretory functions! John Blakeman describes those as "remarkable feats of defecation!" For such little birds they certainly achieve distance.

Stop reading if this is already more information than you need! If, however, you feel there should be no boundary to your quest for hawk knowledge, then read on as John Blakeman tells us everything we need to know about hawk excretion.


"Watching the eyasses for any length of time reveals their remarkable feats of defecation. They lift their little tails and in an instant squirt out a white blob of feces with remarkable velocity. And it will be noted that they (generally at least) take aim toward the edge of the nest so as not to soil its center.

Let’s get the terms of all of this correct. When a hawk (but not a falcon – later) defecates, she is said to have "sliced," whether an eyass or haggard. The slicing of the little eyasses seems pretty muscular, but doesn’t compare with the fecal ejections of the haggards, who can propel a stream of feces horizontally off the nest many feet.

For a hawk to defecate is to "slice." The expelled material is called "slicings." There will be a bit of that in a low rim around the nest on the window and building wall. Fortunately, the camera angle puts us above the slicings on the window, which would otherwise obscure the entire spectacle.

In the brief instant that the eyass’s slicings can be seen shooting through the air, they appear white, quite unlike the defecatory materials of mammals. We’ve all had the droppings of pigeons and other birds soil our car windshields or other surfaces, and have noted the preponderance of chalky white material. This is significant.

The bulk of the hawks’ slicings is uric acid, an insoluble nitrogenous residue from proteins in the diet. Hawks eat mostly proteins, the meaty flesh of their prey, so they excrete massive amounts of this acid after processing and using the proteins for growth and energy.

We mammals use proteins, too, but because we can (and must) so easily drink water, we excrete our processed proteins (nitrogenous material) in the form of urea, a water-soluble molecule in our urine. But this doesn’t work for a bird, hawk or otherwise. We can walk around at length with a belly and bladder full of water. But a bird must fly around, not walk, so the excess weight of urea-dissolving water weighs the bird down, and therefore it drinks much less.

So bird kidneys process and concentrate protein wastes into uric acid, which forms microscopic crystals that en masse make the slicings appear white. The white slicings are the result of a biochemical mechanism that allows efficient excretion of nitrogenous (protein) wastes without the weight burden of ingested or carried water.

But slicings aren’t all white. There is usually a smaller portion of dark black or brown material, which is the intestinal portion. The white stuff is uric acid, from the bird’s kidneys. Buried in the middle of that, when excreted ("sliced") is also a much smaller intestinal component, the digested, non-protein remains of recent meals.

Again, defecation of hawks and their eyasses is properly called slicing. Not so with true falcons, however. Falcons, too, excrete large amounts of uric acid, the white material. But falcons haven’t the musculature to expel their wastes with any velocity. Instead, a falcon stands on the edge of a perch, lifts her tail, and merely drops her feces below her. She can’t expel them with any velocity.

Consequently, we don’t say that a falcon has sliced. Instead, a falcon is said to have "muted." And her droppings are called "mutes."

Because falcons can only mute, not slice, their nest sites, aeries, become very whitened. We falconers call this "hawk chalk," in reference to the white material beneath falcon cliff-side nests. I’ve studied some prairie falcon nests in Nevada, and on these cliff-side aeries the mutes can be many inches deep. Falcon aeries can be used continuously for centuries, with a few millimeters of mutes building up each year. In the arid West, this hawk chalk can be an impressive deposit. Here in the East, the mutes are more easily washed away by our ample rain.

Later, I’ll explain the other expelled material from hawks and falcons, the pellets or "castings." We haven’t seen those yet from the eyasses, but they will show up soon."

–John Blakeman