Mammal Records from Owl Pellets

Whilst live-trapping small mammals is useful because we can target specific sites, and exciting because it enables us to see and handle live creatures up close, there are significant downsides that we should not ignore: trapping is invasive and probably stressful for the mammals because it interferes with their movements while they are confined in traps: and it is quite labour-intensive for we humans.  These constraints should encourage us to look for alternative sources of small mammal records and, fortunately, there are some available in the form of the pellets regurgitated by avian predators such as owls.

Our two most widespread owls, the Tawny and Barn Owl, both forage for small mammals up to the size of a young Brown Rat.  Conveniently, Tawnies tend to hunt in tree-dominated habitats such as woodland, while Barn Owls forage mostly in open habitats such as rough grassland and wetlands such as fen and reedbeds; so, together they sample small mammals over a wide range of habitats. Adult owls swallow their small mammal prey whole without any chewing, so when the undigestible fur and bones are regurgitated as a pellet, this contains the undamaged skeletons of the prey neatly wrapped in a protective furry matrix.  This is what makes owl pellets so valuable to us mammal recorders, for it is not difficult to identify most small mammals from their skulls.

Each owl produces one or two pellets after a night’s foraging, and each pellet may contain the remains of two or more small mammals (my personal record is a huge Barn Owl pellet from a Scottish barn that contained ten small mammals!).  Typically, a large sample of owl pellets will contain abundant remains of Apodemus mice (Wood Mice or Yellow-necked Mice), Bank Voles, Field Voles and Common Shrews; depending upon the habitat, there may also be occasional occurrences of House Mice, Harvest Mice, Brown Rats, Water Voles, Pygmy Shrews and Water Shrews.   This means that, if we can find enough pellets from both species and identify their contents (by reference to published keys), we can gather valuable mammal records for the locality where the pellets were found.

So, how can we find good samples of owl pellets? Well, Barn Owls are relatively easy because they roost and breed in a variety of undisturbed farm buildings, owl boxes and in more natural situations such as hollow trees.  So, with landowner permission and a good torch, it is possible to find some of the chunky, shiny-black, fresh Barn Owl pellets beneath the birds’ roost sites (e.g. beneath a cross beam in a quiet, dark barn); I always look first for the sticky-white splash of their droppings as an indication of recent roosting activity. However, bear in mind that Barn Owls have declined in Britain to some 30% of their population 70 years ago, before the effects of agricultural intensification reduced the quality and extent of foraging habitat; consequently, many farms no longer support Barn Owls these days, so it is important to choose farms that support lots of rough grassland or wetland.  Also, beware the legal position through the spring and summer: it is an offence to disturb Barn Owls at their breeding sites without a licence; so, it is best to avoid searching buildings or hollow trees when the owls might be breeding.

Tawny Owl pellets are more challenging to find because the birds tend to roost by day secretively in woodland trees (often ivy-covered); again, I concentrate on searching for the sticky-white owl droppings on the woodland floor beneath a roost site (all bird of prey droppings have a viscous, sticky quality, usually without any black component, which helpfully sets owl droppings apart from other large tree-roosters like pheasants and woodpigeons), and sometimes these reveal a pellet or two (Tawny Owl pellets are slightly smaller and greyer than Barn Owl ones).  Bear in mind that, in addition to their day roosts, Tawny Owls occupy several nocturnal ‘listening’ perches, usually on a lateral tree branch with a clear acoustic ‘view’ of the woodland floor, which they use for detecting small mammals.  This static approach to foraging contrasts with that of the Barn Owl, which does most of its listening to detect prey while flying over suitable habitat.  These night-time Tawny perches are less likely to yield pellets than are the day roosts, but they sometimes do.

There are sometimes short-cuts that we can use to gather good samples of owl pellets: for example, some ornithologists are involved in monitoring nest box schemes for Barn Owls and/or Tawny Owls, so it is worth asking them for any spare pellets that they find at boxes during their visits.  However we acquire our pellets, it is crucial that we keep them in a sealable bag with a note of the date and grid reference; and the best way to make them last is to freeze them for a few days to kill off the clothes moth larvae that otherwise consume the fur that binds the pellets together.

We hope to organise a training session on owl pellet analysis one day, for which we will need lots of lovely owl pellets! So please keep in touch and let your committee know if you are able to collect any pellets.  In the meantime, there is good information on the Barn Owl Trust website:

https://www.barnowltrust.org.uk/barn-owl-facts/barn-owl-pellet-analysis/