Owl ecology is one my most popular studies. I always find it very interesting. From the feedback that I get I know the students that I share this with find it as interesting as I do.
I don’t know how many pellets I have dissected with students over the years. Pellets Inc., the company that I buy pellets from (see link to the right), says I’m one of their best customers. Each time I take apart pellets with elementary school ornithologists it is like a treasure hunt – even though I have an idea of what I might find, there are sometimes surprises.
One of the surprises this year with the pellets that I dissected with Circleville Elementary School 4th grade ornithologists was the number of bird skeletons that we found. Out of the 50 pellets that we dissected there were 4 bird skeletons. That might not seem like a lot but if we look at some other data that I have collected over the last few years we do find something unusual.
What’s unusual? Let’s look at the data. Not including what we dissected at Circleville, I have helped students take apart 251 pellets. In all of those pellets we found a total of 7 birds. How does that compare to what we found in our latest investigation? In order to compare we need to do some math. By doing an average we find that for every 36 pellets we dissected we discover 1 bird skeleton. In this recent investigation we found 1 bird skeleton for every 13 pellets. That’s a rate of twice as many birds eaten by owls in our sample compared to all of the pellets in my studies in the last few years.
Why? I don’t really know, but I do have several hypothesis. I do find that there are at times patterns in how and what the owls are eating. For example in the spring I find that many of the pellets have extremely crushed bones. In those cases, you will not find any whole skulls in the pellets. They will all have been crushed into small pieces. I suspect that these pellets come from young owls. The parents may crush the mice up with their beaks to make it easier for the young to eat. Or maybe the young, being smaller in size or not being very experienced eaters end up crushing the prey as they eat it, more so than older adult owls would. This may be showing us an eating behavior demonstrated by owls. Maybe there are patterns that help to explain what they eat.
One pattern that I have noticed is that if you do get some of the secondary prey (i.e. shrews, moles or birds) you will likely get several pellets in a batch with that kind of prey. Why? Well, there might be a couple of reasons for this. One is that individual owls, just like any other animal (including us), can sometimes have certain foods that they particularly like (for you or me, we might like to eat one particular thing, for example lima beans). If we ended up with pellets from an owl that has an unusual food favorite, then we might see a greater number of that kind of prey. Maybe a particular owl is really good at catching an unusual kind of prey. Again, if we got pellets from that owl we'd have a greater number of that unusual prey. When I purchase the pellets I buy them in bulk. It's likely that several of the pellets come from the same owl. In that case we might get a higher number than expected of the secondary prey. That might be the reason for why we found twice the usual rate for bird predation with our owl pellets - we happened to get pellets from an owl that was particularly fond of or particularly good at catching birds for food.
Another reason is that the unusual prey may be more abundant or more easily caught at certain times of the year, for example there may be more shrew nests in the spring providing for more chances for owls to find them. Often you will find four or five shrews skulls in one pellet, in that case I’m quite sure that the owl heard the shrews in the nest, dove down and grabbed that spot in the leaves and in one swoop catch a whole family of shrews for dinner. In the fall young birds begin to disperse from where they hatched and are no longer being cared for by their parents. These young, inexperienced birds might be more easily preyed upon by owls. This might be the reason why we had such a high number of bird prey in our investigation.
As you can see the web of life is very complex. There are many interconnections between predator and prey. Seasonal changes can affect the number and availability of food. The behavior of prey at different times of the year can make them more available for the owls. All of these things can affect what the owls eat. When we dissect their pellets we can get a glimpse of their world.
Here is the data from the pellets we dissected with 4th graders at Circleville Elementary School:
Mrs. Urmston’s class – 10 pellets, 23 mice, 0 shrew, 0 mole, 1 bird
Mrs. Przybocki’s class – 10 pellets, 24 mice, 4 shrews, 0 mole, 0 bird
Mrs. Conklin’s class – 10 pellets, 21 mice, 0 shrew, 0 mole, 0 bird
Mrs. Herb’s class – 10 pellets, 21 mice, 1 shrew, 0 mole, 2 bird
Mrs. Negron’s class – 10 pellets, 30 mice, 0 shrew, 0 mole, 1 bird
Here’s a question. Were the number of shrews eaten by the owls in our study sample typical or average for owls based on what my previous investigations have shown (hint – look at the number of shrews eaten in all of the dissections I have done with students and compare to what we found yesterday)? E-mail me your answer. Good luck.
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