Week 44: Barn Owl Pellet Dissection & Analysis

With just 8 weeks left of this year-long Project, I thought it was about time I tackled the Barn Owl (Tyto alba) pellets I’d retrieved from Box Moor Trust land over the summer. These black, shiny regurgitated nuggets are the Rolls Royce of The-Stuff-That-Animals-Leave-Behind. Just one of these little gems can contain the tell-tale remains of a whole night’s feeding.

 

Photo: Barn Owl at a rescue centre in Yorkshire, 2009

Like a lot of birds, Barn Owls swallow the majority of their prey items whole: fur, bones, teeth and all. Unsuspecting Field Voles go down the hatch and, initially, into the glandular stomach (proventriculus) where digestive juices get to work. After this, it’s the muscular stomach or gizzard (ventriculus) which acts as a kind of sieve or filter. All the insoluble stuff like fur, feathers, bones etc are retained in here whilst muscular contractions enable the soluble contents to pass through into the rest of the digestive system (small and large intestines and so on). Once the evening’s feed is done, and before further feeding can take place, the gizzard has to be cleared out. It could be up to 10 hours after feeding has finished that the gizzard-shaped pellet is pushed back up through the glandular stomach and coughed up out through the bill. Pellets can be found on the ground at favoured roost sites and, when nesting, the female uses these as her nesting material. There’s nothing like laying your precious eggs in the fetid remains of partially digested rodents! Having said that, I'm the one who systematically collected more than a dozen of these things and lovingly placed them in her freezer next to the sausages. I’ll press on...

Small mammals, most notably Field Voles (Microtus agrestis), make up a large proportion of a Barn Owl’s diet. Reading suggested that one pellet, following a night’s feeding, could hold 4 full skeletons. I was keen to see if this really was the case and what exactly the Barn Owls had found to eat on/around Box Moor Trust land. I got hold of a copy of D. W. Yalden’s “Analysis of Owl Pellets” and downloaded the Pellet Analysis leaflet from the Barn Owl Trust (the images left/right are linked to the info), and set about dissecting a couple of pellets. The results were really very interesting. A special thanks to Martin Parr, Conservation Manager at Maple Lodge nature reserve, for his patience and help in identifying the prey items from the bones. Neither of us had any experience but we gave it a good go (tweezers and magnifying glass essential!).

Barn Owl pellet, approx 5cm long

Ok, so, this (below) is what came out of the (above) thoroughly dissected Barn Owl pellet (I abandoned the second pellet when I realised what a time consuming and painstaking task it was going to be). The bone fragments I couldn't identify are still in the plastic container. Essentially, within a fur matrix and amongst the 500 bones and bone fragments, the pellet very obviously contained at least 4 similarly sized rodents, identified as 1 Bank Vole (Myodes glareolus) (typically 9-11cm long) and 3 Wood Mice (Apodemus sylvaticus) (typically 8-11cm long). Thankfully, this is consistent with published research. Wood Mice are generally one of the main prey items of Barn Owls whilst Bank Voles also feature but to a much lesser extent (see National Owl Pellet Survey Report 2009 for further details). The most notable skeleton bones were 1 upper jaw set/skull, 4 sets of: lower jaw bones, pelvises, shins, thighs, upper arms and skull fragments. There were 3 sets of shoulder blades and forearms and a good selection of ribs, vertebrae, teeth and other likely limb bones. Ultimately, just as the literature suggested, the pellet contained at least 4 small mammals and was likely the sum of a night’s feeding in the wilds of Hemel Hempstead. It's incredible what you can dig out of something created within the digestive system of an Owl!

Approx 500 bones & bone fragments from 1 dissected Barn Owl pellet

I was lucky that the second pellet I half-heartedly dissected contained a Field Vole (Microtus agrestis), the main prey species for Barn Owls. So, I have a set of upper and lower jaws for Field Vole and Bank Vole, and a set of lower jaws for Wood Mouse.

A few photos (scale in mm) and brief notes follow in case they are of interest or use to others. Hopefully, the IDs are correct but, if not, give me a shout.

Bank Vole Jaws

Field Vole Jaws

Comparing Bank and Field Vole jaws

Similarities

3 cheek teeth in each jaw

Zig-zag chewing surfaces

Differences 

(UJ = upper jaw; LJ = lower jaw)

Bank Vole Jaws UJ: no extra loop on second molar (M2) on inner side. LJ: M2 has loops opposite and M3 has minor 3rd loop (not present in Field Vole)

Field Vole Jaws UJ: M2 has extra loop on inner (tongue) side LJ: M2 has loops alternate and M3 only has 2 loops on outer edge

Wood Mouse (Apodemus sylvaticus) lower jaw

3 cheek teeth, each with rounded cusps

Lower molars have 6 roots (2 on each tooth) [House Mouse (Mus musculus) has 5 roots; Harvest Mouse (Micromys minutus) has 7 roots]

First molar (M1) has 3 cusps along the front edge

Length of lower jaw approx 15mm (incl. incisor tooth)

Wildlife Explorers (Junior RSPB)

Last Saturday, the children of Box Moor Trust’s Wildlife Explorers’ club dissected 5 of the Barn Owl pellets. It was wonderful to hear how enthusiastic the children had been and that their curiosity and fascination had kept them digging for skeleton treasures for more than an hour. I collected the sum of their findings but haven’t yet had time to examine them in any depth. A cursory glance at the upper and lower jaw bones suggest more Field Voles (Microtus agrestis) and Wood Mice (Apodemus sylvaticus). However, there is one lower jaw bone which is particularly interesting. It has all the features of a Harvest Mouse (Micromys minutus), an increasingly scarce and therefore significant species. It’s a similar length to the Wood Mouse jaw; has 3 rounded cusp molars; 7 roots on those molars (3 on M1), and 3 cusps on the leading edge of M1. My only reservation is that it’s actually 1mm longer than the Wood Mouse jaw bone, which is something of an anomaly. However, assuming the other features are diagnostic, I’ll stick with the initial ID of Harvest Mouse (Micromys minutus), until/unless anyone can persuade me otherwise.

Harvest Mouse (Micromys minutus) lower jaw

3 rounded cusp molars; 7 roots (3 on M1); 3 cusps on leading edge of M1

A = Wood Mouse B = Harvest Mouse

Harvest Mouse lower jaw Length = 16mm

This is an exciting record and great work from the Wildlife Explorers. If I uncover anything else of special interest, I’ll be sure to include it in a later post.

Finally, if anyone is planning to dissect an owl pellet, I did discover that although a basic magnifying glass was useful, it didn’t give me enough magnification to be able to pick out the defining tooth features. Photographing the jaw bones with my macro lens was the best way to really see the pertinent details. Also, when the number of roots is significant, I did find that, with care, it was possible to pull the relevant tooth out a short distance (not the whole way) and then photograph the gum line. This enabled me to count the roots accurately.

Box Moor This Week

Although I do seem to have spent most of the week hunched over minuscule bones, clutching tweezers, trying to decipher tooth anatomy, I did escape into the fresh air of Box Moor on Tuesday morning. As temperatures reached around 14 degrees Celsius I shouldn’t have been surprised that a Small Tortoiseshell butterfly was fooled into flight over Fishery Moor. The Oak’s leaves are turning a beautiful orangey, yellow now and, out on the river Bulbourne, the Kingfisher family continue to squabble over territory. The young male was systematically fishing the river on Station Moor, moving from perch to perch, before going back to start all over again. It was interesting to see that the tip of his malformed upper bill looks as though it is in the process of breaking off. A second Kingfisher put in a brief appearance, quarrelled and then flew off east along the canal. The young male resumed fishing, regularly hovering over the water like a humming bird before diving for prey. I never tire of watching Kingfishers…

P.S. If anyone is interested in seeing something of the pellet dissection process, I made a short video here