Chris Watson

Owl Pellets: what are they and what can they teach us?

Raptors, ResearchChris Watson

Going through some old notebooks the other day, I found an owl pellet. I’d collected* it back in 2012 near the banks of the Finniss River in the Northern Territory during a fauna trapping survey. I decided to pull it apart over the weekend and go through the remains to see what I could identify. This is always an instructive exercise and this was long overdue. After posting a few pictures on social media I was surprised by the amount of interest they generated and how many questions I was fielding about the nature of owl pellets. So it seemed appropriate to put together this short post for those who haven’t encountered an owl pellet before.

Firstly, pellets are not faeces. Pellets are a regurgitated mass of the indigestible parts of a bird’s prey; they cough them up. Many different types of birds produce such pellets but they are probably best known from owls and diurnal birds of prey. Depending on the diet of the bird in question, the bulk of the pellet is usually made up of hair and feathers encasing varying quantities of bones, insect casings, beaks, teeth and claws.

A Grey Falcon Falco hypoleucos, straining to eject a pellet. I collected this pellet too and it was composed almost entirely of Budgerigar bones and feathers.

Despite not being faecal, pellets can contain infectious material so when dissecting them it is prudent to use surgical gloves and to thoroughly clean all instruments afterwards. Having said that, inspecting the contents of pellets is not as unpleasant as you might imagine. In fact it’s not unpleasant at all. Pulling pellets apart to identify the prey species of a bird is like Lego for biologists; I enjoy it immensely. The pellet is soaked in warm water to make it easier to separate and the only odour that might be detected is a bit of a ‘wet dog’ smell from all of the wet animal hair.

Mmmmm... wet dog smell. All that remains of the pellet after soaking in warm water; a lot of hair.

When I’m analysing pellet contents, I use paper kitchen towel as a blotter to dry the pieces as they come out of the water. Then I transfer them to a shallow dish for the painstaking work of separating all the hard pieces from the hair and feathers. For this I use a pair of surgical tweezers and a sharp penknife. Others prefer a pointed implement like a splinter probe or a safety pin.

Larger items like skulls and femurs are obvious and come out fairly easily but the smaller bones can be fiddly to extricate from the matted hair. I know some people who like to use a magnifying glass to see more clearly; I have a 200x USB microscope for dealing with the really small bits. Frog and smaller rodent bones, and teeth if they’re separated from the skull, can be tiny.

The bones are then arrayed on a second piece of paper towel for sorting through later. I lay them out in broad anatomical groups – arms, legs, vertebrae, teeth and miscellaneous. Once there are no more bones or other hard remains left in the pellet, the hair can be sent for further analysis. This requires a powerful magnifying glass or microscope and a fair deal of practice. A skilled observer can usually identify a good portion of individual hairs to species level. I don’t have this God-level skill yet so if I require hair analysis I usually send them away to be identified.

The identification of hard remains is more straightforward but still takes some practice and methodical work. There are a few references for this but probably the best and most widely-used is the trusty old Triggs. Tracks, scats and other traces: a field guide to Australian mammals by Barbara Triggs is a revered classic among Australian ecologists and an indispensable reference. As it says on the tin, it’s handy for identifying tracks and scat but it also has references for skulls and other skeletal remains that are usually sufficient to identify bones to at least genus level and sometimes species. Another handy reference is the Field Companion to the Mammals of Australia by Steve Van Dyck, Ian Gynther and Andrew Baker. This is a larger book which contains detailed keys to identification, but these don’t always include skeletal references. All the same, it can sometimes be useful for narrowing the number of candidates.

These references, combined with the knowledge of the collection locality can usually identify most bones satisfactorily.

This pellet had been collected at the base of a fairly large old stag (dead tree). It was about 8m tall and had a hollow opening upwards at the top. The pellet was large (~45mm in diameter) which narrowed the origin of the pellet to two likely species.

This pellet was found at the base of this burnt out tree. It's difficult to make out from this picture but it has a large hollow opening upwards at the top of the trunk. The beastie may well have been in there for all I know.

The fact that the pellet was found beneath a large hollow gave me further useful information. Rufous Owl Ninox rufa is found through this area but tends to stick to fairly dense areas of monsoonal vine thicket – the collection locality was open woodland. The pellet might have been from any of a few different diurnal raptors found throughout the area but it didn’t seem like an obvious roosting spot for any of those.

Another nocturnal raptor of wooded Top End areas: a Rufous Owl on the upper Daly River.

The Australian Masked Owl Tyto novaehollandiae kimberli, is uncommon to rare and not well-known across most of its range in the Northern Territory. The majority of NT records of Masked Owl seem to have come from the Cobourg Peninsula about 150km to the north-east of Darwin, the Tiwi Islands to the north of Darwin (which would be a different subspecies T. n. melvillensis) or Groote Eylandt in the Gulf of Carpentaria (T. n. kimberli again) a long way to the east. When I collected this pellet I was less than 100km almost due south of Darwin. Masked Owl are known to roost in tree hollows during the daytime whereas Rufous Owl roost by day on the branch of a large tree, usually in a well-vegetated area.

Putting all of this together (size, habitat preference, roost selection, distribution) made me tentatively confident at the time, that the pellet was from a Masked Owl. Sadly, the nature of the survey had me accommodated in a motel in a nearby town rather than camped out as would be more usual. This meant there was no opportunity for spotlighting or broadcast surveys which might have yielded a positive identification. Even so, equipped with knowledge of the prey species found within the pellet, I’m still fairly confident that it is from a Masked Owl.

 All the remains that I could identify were rodents including two complete skulls and the lower mandibles from a third animal. The humerus of most rodents has a distinctive crest on the shaft which distinguishes them from other mammals.

Skeletal remains of Pale Field Rat Rattus tunneyi, from within the pellet. The prominent crest on the humerus (labelled with an "H") is distinctive.

The larger skull is likely to have come from a Pale Field Rat Rattus tunneyi. This was the most common species we were getting in our nearby trap lines and matches both the skeletal remains and some of the lighter-coloured hair in the pellet.

What a Pale Field Rat looks like on the outside. Our nearby traps were full of this native species every morning. Not as bitey and more pleasant to handle than Long-haired Rat.

The smaller skull is from one of the native mouse species. The introduced House Mouse Mus musculus, can be ruled out as it has a prominent notch on the inside of the upper incisors; even in a live animal you can quickly check this by running your thumbnail up the inside of the animal's front teeth. The teeth in this skull didn’t have the tell-tale notch. It’s impossible to say with 100% certainty, but the pellet contained some hair with slight rufous/orange colouring so the skull might be from a Western Chestnut Mouse Pseudomys nanus. This was another species that was turning up in our trap survey most mornings so was certainly fairly common in the area.

If the pellet was from a Rufous Owl I’d expect there to be at least some sign of remains from Little Red Flying Fox Pteropus scapulatus, which was common in the area and a well-known prey item for Rufous Owl. Also with this species, I would expect there to be some remains of gliders or possums and probably also some feathers. These were all absent.

Dem bones dem bones dem...

Masked Owl is fairly well-documented as preying primarily on terrestrial animals and all the remains that I could identify were terrestrial mammals. For now at least, I'm pretty sure that there was a Masked Owl knocking about the upper Finniss River back in 2012. Let's hope there are still a few about in 2016.

So there you go, next time you’re at an owl roost and you see some suspiciously non-bird-poo-looking lumps on the ground beneath it, you can have a fair idea of the sorts of things those pellets might contain.

CBW

 

*This was collected under permit in the course of a commercial fauna survey. It is illegal to collect pellets without the proper wildlife research permits. In fact, it is technically illegal to collect any animal remains, including bones, feathers and road-killed or beach-washed carcasses. This is an area of wildlife legislation that I don’t entirely agree with; feathers, bones and other animal remains can be great teaching aids and kids get a real kick out of collecting them. But it’s not that simple either. Wildlife permits and regulations are probably a topic for another blog post on another day.

 

Buy Tracks, scats and other traces: a field guide to Australian mammals by Barbara Triggs from Andrew Isles.

Buy Field companion to the Mammals of Australia by Van Dyck, Gynther and Baker from Andrew Isles.

Support The Grip