Friday, November 21, 2008

No bones in the pellets?

After posting John Blakeman's essay about hawk castings yesterday, I took another look at Lincoln's photo of the "pellets" and promptly sent the following note to the Ohio hawk expert:

John: What are all those sticking-out things in Lincoln's pellet photos if not bones?

He answered within an hour:

The white strips are actually the bases of small feather shafts. Most of them have had the thin feather filaments separated by the digestive enzymes. The feathers aren't totally digested, but the thinner filaments do get weakened and break off. The thicker feather bases come out as shown. Yes, they can look like little bone fragments, but they are really feather shaft fragments.
Again, pigeons have lots of body feathers, and those made most of the white casting fragments.
But what about that big white thing on the bottom of the casting on the right? Sure looks like a bone.
It's not. It's almost surely a small pebble, a bit smaller than a pea. It was probably in the stomach of the pigeon, deliberately ingested by the bird to assist physical digestion. Many larger seed-eating birds actually require these "gastroliths," "stomach-stones."
The hawk has no revulsion in swallowing stones like this. We aren't really sure if wild Red-tails ever deliberately swallow small stones. They might. Falconers for ages have known that trained hawks and falcons will often swallow provided gravel. In granivorous (seed-eating) birds, the stomach stones act as teeth, grinding ingested seeds in the gizzard. Just what ingested stones do for raptors is not really known. But falconers have known that their trained hawks often fly better, hunt better, and just act better after swallowing these stones.
Falconers call these swallowed stones "rangle." Personally, I've never offered rangle to my falconry Red-tail, Savanna, so I have no personal experience with this. But there is a good deal of this in the falconry literature.
One last note on raptor castings. As repulsive as these might appear, they are innately clean, almost sterile. Powerful digestive enzymes have chemically degraded stomach and crop bacteria. We raptor biologists delight in finding these. They accurately reveal the hawk's meals of the previous day, down to exact species. By carefully pulling these apart and comparing the included feathers and fur with known prey species, we can know exactly what the hawk ate the previous day. This is how we can be sure that Red-tails don't spend much time hunting or taking "desirable" species. In Central Park, it's mostly pigeons, rats, and squirrels, as shown in these castings. Out here in the Ohio countryside, it's almost exclusively field voles, small lemming-like rodents.
However, sometimes we encounter a hawk that's eating something else, revealed by its castings. An example is my falconry hawk herself, Savanna. When I trapped her eight years ago, in autumn, in her first year, I immediately noted that she was pretty thin. The next day, her first in captivity, she put up a casting made entirely of grasshopper exoskeletons. The poor bird just wasn't finding any field voles and was trying to subsist on grasshoppers. Had I not taken her into captivity, she would have surely died a few days or weeks later.
--John Blakeman