Since that post I have observed this kestrel behavior of discarding the intestines several times, including these two shots from this morning. I'm beginning to form, or at least modify, a theory as to why these birds discard the intestines even though they would include some protein.
I'm doubting that it's because of the "taste" of the intestinal contents and beginning to wonder if it's because of the sheer volume of the intestines, large intestine in particular. We watched this bird voraciously consume about half of the mouse and then just stop eating, like it was full (perhaps it had eaten another rodent very recently). Rodent intestines would typically be full of "low quality" food for a kestrel. Particularly this time of year when it's so cold kestrels would need protein, lots of protein and almost nothing but protein, seems to me.
So, I'm wondering if that's why they reject the intestines - if they didn't they'd get too full before they'd had sufficient protein. Maybe it's just a winter time thing as I seldom get to watch them feed when it's warmer.
Any more thoughts on the subject?
Posted by Ron Dudley on December 19, 2009 at 5:24 PM
Rejecting the intestines
Posted by Luke Ormand on December 19, 2009 at 6:47 PM
Only 2 thoughts.
1. That is disgusting!
2. I can't believe how often you get to see/photograph kestrels.
Ok, but seriously, that sounds rather logical - and it's interesting how full these intestines were, which indicates the mouse was quite full, and as such, probably rather healthy and full of good solid protein. However, do have you noticed this behavior in all seasons? I have seen an Osprey discard the innards of a fish it ate in spring (though that was shortly after a long migration, so perhaps the theory could apply?).
- Luke
Posted by Ron Bielefeld on December 20, 2009 at 4:12 PM
Ron, since your first post on this subject I have tried to pay attention to raptors when they are eating and look for or file (in my brain) other images of such behavior. It seems to me that I remember seeing this behavior quite a few times and among a variety of species (red-tailed hawks, red-shouldered hawks, and ospreys come to mine). After thinking about this behavior it seems logical that raptors, who generally are after protein and fat from their kills, would discard the intestines for just the reasons you stated. They do not contain much nutritional value and thus, are just bulk that the bird's system needs to digest before it can eat again. It would make sense that raptors would do this during all seasons (but I do not know if they actually do) given that when they kill they do so because their bodies need nourishment, and in nature, no matter what time of year it is, a bird is udoubtably best served by getting the most out of every kill. If intestines always are just bulk, then why ever eat them?
Ron, I use to have ducks on my pond when I lived in Colorado. The Raccoons would often eat the ducks and leave behind nothing but the intestines, beaks, and legs/feet. I always wondered why they rejected the intestines. I know this doesn't answer your question, but wanted to share it with you. Beautiful bird!
Posted by Nick Saunders on December 21, 2009 at 10:49 AM
I'm just amazed all those innards fit inside that mouse !!!!
Posted by Peter Conway on December 21, 2009 at 12:10 PM
Ron,
First, cool photos.
I think your theory has merit but I wonder if there is another possibility.
Intestines are filled with bacteria needed to break down whatever is being digested. I wonder if the ingestion of all that bacteria for breaking down the grasses and grains the mouse feeds on would cause an imbalance in the digestive tract of the raptor as it would compete with the bacteria needed to break down the proteins. Most mammals (as mentioned about the raccoon) seem to have an aversion to intenstinal content if other food is plentiful. I have also noticed that raptors and mammals that are feeding on a large carcass will eat the intestine but none of the contents of it. I am going for a "universal theory".
When I volunteered at the Rocky Mountain Raptor program one of our main duties was chopping up food for the raptors (everything from small owls, to the falcons, hawks, larger owls, and eagles). We had specific instructions to follow on what to and not to feed the critters, and uniformly, except for the vultures who would eat ANYTHING, we had to remove and discard the guts. I'm not entirely sure why, but we were told that the vital ingredients the raptors required were the blood and moisture for liquid intake, bones for calcium and other minerals, skin and hair for fat and "roughage" to assist digestion, but never the gut contents (which generally stank as we removed them from the whole rodents, etc that we chopped up). If you think about it, who or what would want to eat a bunch of crap??? (Maybe I'm ignorant and there are some creatures who do, besides children with pica which is pathologic, but I don't know of any. Our feathered friends must have discriminating tastes.)
Discussion
7 replieshttp://www.naturephotographers.net/imagecritique/bbs.cgi?a=vm&mr=44304&CGISESSID=d2523a7aef5fa2b124affe13c62e8766&u=31066
Since that post I have observed this kestrel behavior of discarding the intestines several times, including these two shots from this morning. I'm beginning to form, or at least modify, a theory as to why these birds discard the intestines even though they would include some protein.
I'm doubting that it's because of the "taste" of the intestinal contents and beginning to wonder if it's because of the sheer volume of the intestines, large intestine in particular. We watched this bird voraciously consume about half of the mouse and then just stop eating, like it was full (perhaps it had eaten another rodent very recently). Rodent intestines would typically be full of "low quality" food for a kestrel. Particularly this time of year when it's so cold kestrels would need protein, lots of protein and almost nothing but protein, seems to me.
So, I'm wondering if that's why they reject the intestines - if they didn't they'd get too full before they'd had sufficient protein. Maybe it's just a winter time thing as I seldom get to watch them feed when it's warmer.
Any more thoughts on the subject?
1. That is disgusting!
2. I can't believe how often you get to see/photograph kestrels.
Ok, but seriously, that sounds rather logical - and it's interesting how full these intestines were, which indicates the mouse was quite full, and as such, probably rather healthy and full of good solid protein. However, do have you noticed this behavior in all seasons? I have seen an Osprey discard the innards of a fish it ate in spring (though that was shortly after a long migration, so perhaps the theory could apply?).
- Luke
First, cool photos.
I think your theory has merit but I wonder if there is another possibility.
Intestines are filled with bacteria needed to break down whatever is being digested. I wonder if the ingestion of all that bacteria for breaking down the grasses and grains the mouse feeds on would cause an imbalance in the digestive tract of the raptor as it would compete with the bacteria needed to break down the proteins. Most mammals (as mentioned about the raccoon) seem to have an aversion to intenstinal content if other food is plentiful. I have also noticed that raptors and mammals that are feeding on a large carcass will eat the intestine but none of the contents of it. I am going for a "universal theory".
Peter
Rebecca Saltonstall