Thursday, November 20, 2008

Blakeman on hawk pellets, slices and mutes

Photos courtesy of

Bill Trankle, of Indianapolis, IN writes:

Marie, I saw on Lincoln's site that he's got pics of PM casting pellets, and a picture of the pellets as well. I didn't think that hawks cast pellets since they don't eat their prey whole like owls do. From the look of the pellets, PM doesn't do the best job of plucking or bone removal! In fact it almost looked like one of them contained a pebble. Is this fairly common behavior in red-tails and/or hawks in general, or is PM just a bit lazy when he eats?

Similarly, Nan Holmes of Central Massachusetts writes:

How did I not know this? Do red tails and other raptors, besides owls, eject pellets? I must live in a cave. I thought only owls did so. See today for photos ...

Blakeman to the rescue!

The pellets -- called "castings" by falconers (a term I shall use) -- are quite typical. Here's a summary of the matter.
Yes, both hawks and owls create these, with a difference. Curiously, owls are unable to digest the bones of their prey, so they are expelled with the rest of the undigested material. Most of both owl and hawk castings are undigested fur or feathers. Hawk (and all other diurnal raptor) casting material is only fur/feather, as shown in the photo.
Generally, hawks and owls will regurgitate a single casting each day, although if the bird ate at various times on the previous day, castings can be produced at various times. If two are produced at the same time, they originated from the same meal, a large prey item, which in Central Park was almost surely a pigeon.
Pigeons have a lot of feathers. Red-tails eating one will take some time in trying to pluck off as many feathers as possible before pulling off chunks of flesh. Red-tails generally try to avoid swallowing feathers, but they gulp down a good number anyway. Without a knife and fork (and hands and fingers), it's impossible for a hawk to extract tidbits of feather-free flesh.
What happens to both the feathers and flesh after they are consumed? As many photos of Pale Male have shown, after eating a pigeon or rat or squirrel (which are about the only Red-tail prey animals in Central Park), the hawk's crop is seen to protrude. When watching a hawk feed on a large prey item such as a pigeon or squirrel, the crop is seen to fill up. The food does not go into the hawk's stomach. It accumulates in the proventriculus, the crop, just below the chin.
When eating a big meal, the hawk will often stop gulping the chunks of flesh, hold its head erect, and then rock the head and neck back and forth. This strange rocking motion is called "putting over" by falconers. Putting-over rearranges and packs the food chunks down into the crop.
I commonly feed Savanna, my falconry Red-tail, turkey necks. They have a lot of bone, which I slice up with a strong scissors. When Savanna eats pure muscle, she does little putting over. But when she gulps meat chunks that have porous bone fragments, she is diligent in packing these into the crop. She "puts them over."
Only small portions of crop contents are dropped into the stomach for digestion at one time. The crop is slowly emptied over several hours. Digestion in Pale Male takes apart both the proteins of the consumed flesh and bone material is rendered. Not so with the keratin proteins of hair and fur. Those remain in the stomach and are eventually pushed back out undigested, into the compact, consolidated casting.
Marie, you mentioned that you have seen looser, less consolidated castings. These happen when the hawk eats food that has very little consumed fur or feathers. This happens when the bird has eaten a squirrel. Squirrel skin is very thick and difficult to tear. Yes, a Red-tail can to this, but because the skin does not easily rip into consumable pieces, the hawk generally leaves the squirrel carcass with the large bones and skin intact. The loose casting are the expelling of the small, incidental amounts of consumed fur.
The castings in the photo clearly show some larger feathers. These castings came from the previous day's pigeon meal. Most of the consumed feathers in the pellets are small downy feathers, of which pigeons have large amounts. So only a few incidental, large flight feather are seen.
While on the subject of hawk digestion, let me comment on the other end of the process. "Roughage," undigested material, simply isn't allowed to pass through the raptor gastrointestinal tract, only liquid materials. Most of what the hawk wants to eat is pure protein, muscles and organs. Digestion quickly renders this material into constituent proteins, which are absorbed and used to power the bird. Unlike us, hawks are not powered by carbohydrates. They run on proteins and a bit on lipids, prey fats.
The digested and physiologically-processed proteins end up as uric acid, the white component in bird droppings. Mammals excrete protein wastes as urea, which is highly water soluble. We excrete the remnants of yesterday's hamburger (the meat portion, anyway) in our urine, which has lots of dissolved urea. Birds, however, can't be flying around with large volumes of water in their blood and bladders. Too heavy.
Instead, the protein wastes are concentrated into water-insoluble crystals of uric acid. Those microscopic crystals make the white portion of bird poop splats.
But hawk feces also have some streaks of black or green. The green are bile fluids used to digest the fats. The black blotches are other non-protein remnants of previous meals.
All of this is excreted in a markedly typical hawk defecation motion. Once seen, it's not forgotten. The hawk, sitting on a perch, bends over, lifts the feathers of its tail, and then in a compulsive squirt expels a long stand of liquid excrement out behind. It then flicks its tail feathers, assumes an erect posture, and life continues.
New York hawk watchers should begin to use the proper terms regarding all of this. When Pale Male, Lola, or any other hawk (not falcon) is seen to defecate, it has, properly, "sliced." Moreover, the loose strand of projected excrement is properly called a "slicing." In defecating, the hawk has "sliced" his "slicing."
It may have been the threat of descending slicings that prompted the removal of Pale Male's natural nest several years ago.
For falcons, different terminology applies. Falcons, whether little kestrels, or full-sized peregrines, don't have the rectal musculature to project their fecal matter any distance. Falcon defecation appears similar to that of hawks, but with a major difference. Yes, the bird lifts its feathers and bends over. The droppings, which for falcons are called are called "mutes," (not slicings), are merely dropped below the anus. The process is called "muting." A falcon "mutes" its "mutes."
And because they don't get projected away from the nest or roosting perch, the mutes accumulate. In fact, peregrine aeries, cliff-side nests, are often hundreds of years old, with layers upon layers of accumulated uric acid crystals. Falcon nests have been analyzed and determined to be very old. Once a peregrine nest is established, peregrines will be attracted to the white pile of mutes on the ledge.
One last term. The white uric acid, whether the mutes expelled by a falcon, or the slicings of a hawk or eagle, after it falls to the ground or gets splatted against a surface, has a specific name. It's called "hawk chalk." The white hawk chalk of Pale Male and all of his raptor compatriots can be seen all over Central Park. I hope Central Park and other parts of NYC continue to be sliced upon in coming seasons. May the hawk Chalk continue
--John Blakeman