Week 46: Winter Sun & Ravens Over Dellfield

Unlike this morning, a number of days this week really have been the epitome of Dickensian, winter gloom. Headlight days, I call them, where, even at midday, cars run on dipped beams. On Monday, in the dusk-like milieu, I walked the length of the moors (from Bulbourne Moor, west, all the way to Dellfield). It was not a day for nice photographs unfortunately, but the bird life was cheerful and absorbing. One of the two regular Little Egrets was in the river on Harding’s Moor; 3 Jays were squabbling with Magpies; Crows fought over some bread one of them had found, and the two young Kingfishers were perched over the river on Bulbourne Meadow, eyes trained on the shallows. Close to the Two Waters Road bridge, a Little Grebe bobbed and dived in the company of a pair of Mallards; flocks of Blackbirds stripped berries from Hawthorns along the canal, and a charm of at least 30 Goldfinches was feeding in Alder trees. The two young Herons were doing their best to look like statues at odd locations on the moors; Robins flitted in and out of bushes, and a Grey Wagtail chirped and flew overhead.

On Wednesday, with clear skies forecast, I rose early, hoping to catch the Oak in the first light of day. With dawning realisation (pun intended), it became apparent that at this time of the year, the Oak doesn’t even glimpse sunlight until at least 11am and, in fact, stands cold, mostly in the shade, until, I’d guess, around 1pm. By 3pm, it is back in full shade. The sun skirts its way around, behind Hay Wood, and then dips behind Westbrook Hay school. So, this week’s Oak photographs are a little different. The first was taken at 10:30am, when the Oak was in full shade but the sun was breaking through the tree tops of Hay Wood, casting long shadows across Dellfield. With some tinkering in photoshop, I came up with this (the original is here)...

Photo-art. Into the Sun. The Oak and Scots Pine with the low Winter sun blazing through from behind Hay Wood

Up on Preston Hill, a Mistle Thrush was bathing in the pond and 2 pairs of Bullfinches and a handful of Redwing darted through the scrub. Into Hay Wood and I found some lovely fungi. New ones to add to the list were a coral-like growth. I think the genus is Ramaria, the species is likely Ramaria Stricta (Upright Coral)? The other was a tiny, delicate but beautiful sessile “cap” of the genus Crepidotus. Both species were growing on dead wood and likely saprobic.

Coral-like fungus (Ramaria Stricta) growing on rotting wood. Max height approx 11cm.

Tiny "caps" growing on dead wood. Approx diameter 7mm Genus: Crepidotus

As I walked onto Bovingdon Reach, a Red Kite flew up from the grass and lazily rose into the blue skies. By midday, I was walking down through Barnfield meadow and on into Ryders. As I was standing amongst the Limes and furtive bunnies, photographing the view across Dellfield, a pair of large birds caught my eye. I then heard the distinctive “kronk, kronk, kronk” of Ravens (Corvus corax). Through binoculars, it was easy to see the diamond-shaped tail and their sizeable wing-span (similar to a Buzzard’s). Unfortunately, by the time I’d changed lenses, both birds had flown behind the Limes and I failed to get even a decent record shot, much to my annoyance. Still, a great way to end the morning and my first sighting of this species in Hemel or over Trust land.

From Ryders, looking north-east across Dellfield at 12:10pm. Off to my right, a pair of Ravens kronk'ed and drifted west.

On Tuesday 18th November, I'd collected a fresh Barn Owl pellet from Trust land. It was a lot smaller than previous pellets, just 3cm long and about 1.5-2cm wide, which made it ideal for dissection one dark, dank afternoon this week. Interestingly, it contained the fur and bones of just one mammal. A Field Vole (typically 13cm long + tail of 4cm). It provided the perfect opportunity to get an idea of exactly what one of these little fellas consists of...

Frankenstein's Field Vole

From one small Barn Owl pellet (3cm long): the remains of a single Field Vole, crudely and likely inaccurately(!) reconstructed. Scale in mm 

I have no idea how many ribs or vertebrae a Field Vole (Microtus agrestis) is meant to have. Nor do I know which way around the various bones go (i.e. which is left and which is right) but above is my “artisitic impression” of a Field Vole skeleton. It’s not a good sign when you have bones left over (top right hand corner) but my excuse is that you can’t really place damaged and crushed bones very easily (especially when you barely know what you’re doing to begin with!). I'm missing a humerus and the bones of the feet/toes are “absent” or unidentifiable. Anyway, it was an interesting exercise and well worth the time and patience required. (For more detailed info on pellet dissection and bone identification see my previous post).

Field Vole (Microtus agrestis): Jaws & Teeth

If you’d like to see photographs of these little mammals before they get snaffled by Barn Owls, Biodiversity Gatwick were involved in a small mammal survey recently. The relevant post is here and I thoroughly enjoyed reading it and browsing their images.

I'll finish off with a photo from my Monday walk and Wednesday's Oak in the shade.

Black-headed Gull on Station Moor footbridge

The Oak in shade at 11:20am

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