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This is where I post various musings about wildlife and ecology, observations of interesting species (often invertebrates)
and bits of research that grab my attention. As well as blogging, I undertake professional ecological & wildlife surveys
covering invertebrates, plants, birds, reptiles, amphibians and some mammals, plus habitat assessment and management
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Tuesday, 12 April 2011

Bird pellets - they're not all about owls

Any book about the signs of wildlife will mention owl pellets - there's even an excellent little book about analysing just these (Yalden 2009), plus a fold-out laminated guide (Thomas & Shields 2008). However, many other carnivorous and omnivorous birds also produce pellets and so regurgitate indigestible material - raptors, gulls, corvids; if I remember rightly, Chris Packham even showed a kingfisher pellet on Springwatch (or was it Autumnwatch?). So, when I recently found a Tawny Owl (Strix aluco) pellet the same day as a Blackbird (Turdus merula) pellet, what could I do but dissect and compare them...

With owl pellets so well documented, I started with this one - approximately 55mm long and 30mm wide in the middle and consisting of greyish fur with some emergent bones. The habitat (under a couple of tall trees in a suburban location where I have heard tawny owls) plus the overall size, form and structure/content, lead me to ID it as a tawny owl pellet.

The owl pellet
The method is straightforward enough (if a little time-consuming and fiddly) - put the pellet in a white tray and dissect with mounted needles and fine tweezers. Pellets can be wetted, but I decided to leave it dry (either is fine). I started with the larger bones and found that, unsurprisingly, the owl had been eating voles - Field Vole (Microtus agrestis) and Bank Vole (Myodes glareolus).

Field vole lower jaw - note the toothless curved area (diastema) on the right

Field vole lower jaw - note the alternating loops of the teeth

Bank vole lower jaw - note the opposite loops of the teeth

A long bone of a small mammal, presumably one of the voles. The tiny yellowish spike near the upper left side of the bone is the chaeta (bristle) of an earthworm.

A tiny bone - assumed to be a vole 'toe' bone
Working through the pellet, another feature soon became obvious - the presence of a large amount of sand. This is a good indication that the owl had been feeding on earthworms and so ingesting sandy soil - something confirmed by the numerous chaetae present (there's one in the 'long bone' photo above). Overall, nothing too surprising - field voles are a favoured prey of many owls (though less so for tawny owls), and earthworms are comminly found. If I were undertaking a detailed study of owl diet or local mammal availability, I could weigh the different components to derive more information - in this case it's just a one-off pellet.

Now, onto the blackbird pellet. Firstly how do I know it's a blackbird pellet? Well, there the size (about 20mm long) and clear presence of invertebrate food (see below), but it was also found in my garden under one of the bird's favourite perches, and to make it even easier, I saw it deposit the pellet! So, what was in it?

Blackbird pellet
Well, the first thing that leaps out is the presence of complete millipedes - two were present and were clearly not easy to digest. I have yet to identify them, but they appear to be the same species - maybe Tachypodoiulus or Cylindroiulus - I'll need to check the telson (last 'tail' segment) to be sure. However, this is only about half the pellet - there are lots of smaller fragments, including a small green object bottom left...

A fragment of insect cuticle

The same fragment from the inside
This piece of cuticle is hard to ID, but my feeling is that it may be part of a shieldbug, though I can't be certain and I don't know of the depth of colour might change during its time inside the bird. There were many many other arthropod fragments - pieces of cuticle, antennae (at least some from small beetles, possibly Carabidae, ground beetles), bits of plant material, and various mystery structures. As with the tawny owl, there were also earthworm chaetae - given how often blackbirds are seen pulling at worms, this is unsurprising!

These are only single examples of pellets (do blackbird pellets often contain a large proportion of millipede remains?), so I can draw few conclusions beyond simply being interested in what could be found. However, I have kept the resulting bits (my wife is so proud...) and will look out for more to see if more data can be gathered. I've also found a nearby otter (Lutra lutra) latrine with numerous spraints, and have a copy of Conroy et al. (2005), so don't be too surprised if a future post invoves close-ups of the contents of otter poo...

References

Conroy, J.W.H., Watt, J., Webb, J.B. & Jones, A. (2005). A Guide to the Identification of Prey remains in Otter Spraint (3rd ed.). The Mammal Society, Southampton.
Thomas, L. & Shields, C. (2008). Guide to British Owls and Owl Pellets. FSC, Preston Montford.

Yalden, D.W. (2009). The Analysis of Owl Pellets (4th ed.). The Mammal Society, Southampton.

1 comment:

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