Castings - a more palatable body function product?

Just when you thought it was safe to read the blog again after John Blakeman's memorable comments on slicing, here is Part 2 from John of what could land on your head if you are in the vicinity of a hawk!

John Blakeman writes:

"In a previous posting, on a somewhat unpleasant subject, I outlined some of the biology and - can you believe this? - the proper nomenclature of hawk defecation. Here, I'll deal with some of the other not so detestable expelled materials that hawks excrete - in this case from the opposite end of the body.

First, it must be understood that hawks and falcons don't "chew" or otherwise process the raw flesh they consume as food. We humans carefully process the food we eat, especially meat products. I needn't outline what happens to a chicken, pig, or steer before these animals are rendered into table fare. Hawks have no need or method for any of this processing. They merely pull the flesh apart with the beak into swallowable-sized pieces, which are entirely gulped. No chewing. No slicing (oops, cutting) or dicing. Just rip, gulp, and swallow.


In the case of a captured house mouse, or other similarly sized prey, at least with adults, the animal is commonly swallowed whole. This can be a rather comic spectacle. The bird gulps the torso into the mouth, and then with some rather contorted exertions of the head and neck, the mouse is forced down into the crop, a food-holding chamber above the stomach. (Hawks with a full crop can be easily spotted, with a noticeable bulge at the top of the chest.)

But when swallowing a small rodent with a long tail, as in the case of house mice (Mus musculus) or deer mice (Peromyscus spp.), the tail often hangs behind, dangling from the mouth. It can take one or two more swallows to get the tail down into the crop, where digestion can begin. It's inelegant, to say the least. But it works.

Below is a red-tail hawk attempting to swallow a mouse or rat.


Digestion of either entire swallowed prey, or just pulled-apart pieces of raw flesh (as in the case of the food offered to the eyasses), proceeds rather quickly and completely. In just a few hours, the food is chemically rendered and absorbed for metabolism and tissue synthesis.

But hawk and falcon digestion does not affect consumed fur or feathers. The proteins of these are virtually unaffected by any digestive enzyme in the hawk's gastrointestinal repertoire.

It does not serve any bird to hold food for lengthy digestion. Whatever is swallowed must be flown around. Slow or weighty flight is a survival disadvantage, so all birds, especially hawks, digest their food quickly, in only a few hours or so (yielding all of the materials in my previous posting).

But with this rapid digestion, the indigestible fur and feathers would only get in the way. Instead, hawk stomachs and crops knead the fur and feathers into a compact mass, which is then expelled by vomiting.

This object has two names. Among ornithologists, it is called a "pellet." But among falconers and raptor biologists (most of them, anyway), it's properly called a "casting."


Raptor biologists like to find fresh castings. They accurately reveal what the hawk had for lunch the previous day. Castings are almost always expelled in the morning, before the hawk heads out to begin a day's hunting. When discovered, a casting can be carefully pulled apart, revealing its contents.

To do this, one must be able to identify the fur and feathers of dozens of common prey species. A careful segmentation of the various fur and feathers of a casting can identify all what was captured and consumed. It's sort of like a hawk restaurant credit card bill for all of the hawk's food of the previous day. There is no hiding what the hawk had to eat. Everything they eat has fur, feathers, or scales, which aren't digested; merely rolled into to a compact casting and orally expelled each morning.

Interestingly, hawk castings are not unhygienic. Digestive enzymes chemically pull apart almost all bacteria. Hawk castings are not fecal in any matter.

[Note from Della: This is a casting that came from one of the Franklin eyasses last summer. We saw the young hawk expel it, and then we peered down at it on the sidewalk at our feet!]


Back to John:

"Owls, it must be noted, are less competent digesters. They can't chemically render bones. So owl castings contain all the bones of their prey, making it much easier to determine what an owl had for lunch. Skulls are much easier to identify than obscure, nondescript fibers of fur. (Owls, to me, are not at the same level of interest, complexity, or rank as the more noble hawks and falcons.)

In regard to our eyasses, they will be soon casting up small castings, as they begin to take much larger chunks of meat from their parents which will have attached fur or feathers. At the start, the haggards attempt to offer tidbits of flesh without attached fur or feathers. They aren't entirely successful with this, but the little hawks may take longer than a day to accumulate enough material to form a casting. And when expelled, it may be tiny, pea-sized or so, and easily missed by human viewers.

But the eyasses are getting bigger now. Feathers will soon be seen emerging darkly from patches of follicles on the wings, body, and tail. And larger castings may be seen, too. Someone may be able to watch an eyass try to expel a larger casting. The process can appear to be very distressing, as though the hawk had some object immovably lodged in its throat as it bobs back and forth trying to forcefully expel the object. As straining as this might appear, take no concern. Eyasses and haggards ALWAYS get their castings expelled. Regardless of its awkwardness, it's a very natural process."

–John Blakeman