Tuesday, 3 March 2015

What was for dinner?

On Sunday when I went on to my patch I found an interesting present left on the bird table.

This small lump is an owl pellet. I have found pellets before and know there are owls on the patch. Each previous pellet I have carefully dissected and did the same with this one. I thought I would share my findings.

The pellet was 35 mm long, 23 mm wide with a circumference of 75 mm. It was made mostly of fur and bones and its dark colour and shape indicates that it came from a Tawny Owl.

Next comes the weighing its full weight was 1.98g. After separating out the bones and fur the components were weighed again revealing that the fur composed of 0.9g and the bones 0.78g meaning that up to 0.3g was sand, gravel and other parts. 

As you can see from the photo the pellet is a mass of bones in a matrix of fur. Using tweezers you can carefully pull out each bone. Some sites suggest wetting the pellet to soak out the bones but I find this slows the process and is much easier when dry.

Once all the bones are extracted you can now begin the analysis of what was eaten.

You can usually work out the number of individuals eaten by counting jaws or femurs. In this particular case things were simple. The pellet contained a single individual, I could tell this as the following parts were present:
- Both clavicles
- Both femurs
- Both pelvises
- Both lower jaws
- An intact set of upper jaws and orbits (damaged brain case)

Now that I know that there was one individual I need to find out what individual. This is done by looking at the dentition. The teeth tell you everything. First of the incisors are long and curl under identifying it as a rodent and the size lead you towards Mice, Voles and Rats.

Next looking at the top of the teeth  I was able to see that the individual was a mouse. Voles have zigzag shaped teeth whilst Mice have lobed teeth. Balancing this with the length of the lower jaw I can assume that the prey item was most likely a Wood Mouse (Apodemus sp.)

The last piece of information I can glean amazingly is actually the sex of the mouse. On the extended part of the pelvis there is a kind of flange. If it is short and doesn't reach the end it is male and if it is longer and reaches the end it is female.

So all in all from this pellet I am able to establish the presence of a Tawny Owl on site that in the previous few days had eaten a mature female wood mouse. Confirming wood mice on the site.

If you want to have a go yourself when you find a pellet an invaluable guide is one provided by the Mammal Society written by Derek Yalden.

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