National Owl Pellet SurveyBackground The Mammal Society organises a number of surveys to study various aspects of mammalian biology, one of which is the Mammal Society National Owl Pellet Survey. This survey - which started in January 1993 and is still continuing - analyses owl pellet batches from throughout the UK and records the results on a computer database. The data recorded are then used to support the objectives of the survey and other research. The vast majority of the pellets are from the barn owl (Tyto alba).
Owls are
highly efficient birds of prey, and small mammals provide a large part
of their diet, particularly for the Barn owl. When the owl ingests a prey
item such as a field vole, the soft tissues of the prey item are digested.
However, the digestive system of the owl has very little effect on the
bones, teeth and claws or on the fur. These indigestible remains cannot
pass beyond the stomach, and hence must be evacuated through the mouth.
Muscular action by the stomach kneads these remains into a pellet consisting
of a fur matrix containing the hard parts, and the pellet is then egested.
A barn owl will cast on average two pellets per day. These pellets can
then be collected and analysed to identify their content. The analysis
consists of extracting the skeletal remains from the matrix, and selecting
the skulls and dentaries (jawbones). These bones are then used to identify
the species of the prey items, and to produce a count of the number of
prey items per species extracted from the pellets. The survey
has a number of specific objectives, such as providing information on
the current distribution of small mammal species, and to identify seasonal
and annual variations in the "availability" of the prey species.
The availability is essentially the probability of capture of a given
prey species by a predator. It is a complex characteristic influenced
not only by the population density, habitat preferences and behaviour
of the species, but also by those aspects of other sympatric prey species. The results
of the survey also provide a baseline data set with which the results
of similar surveys carried out at other times can be compared. Such a
comparison was made between the results of this survey and those of a
survey carried out in the 1960/70's by David Glue of the BTO. The results
of this work were presented to the Mammal Society Easter Conference in
1999 by Charlotte Webbon, and published in the June 2000 issue of Mammal
Review. Comparisons were also made between different geographical locations
and between different habitat types. Results Pellet batches have so far been received from 168 locations, most providing more than one batch, and many providing monthly batches. The total number of pellets providing data to this survey as at October 2002 is 26,315, from which 93,649 prey items have been extracted. 18 small mammal prey species have been identified and are listed in Table 1. Bird, reptile and amphibian prey items have also been recovered from the pellet batches.
Table 1. Species recovered from the pellets The total results to October 2002 are shown in terms of percentage prey items in Figure 1. This chart shows that the three main prey species of the barn owl are field vole (Microtus agrestis), wood mouse (Apodemus sylvaticus) and common shrew (Sorex araneus). The field vole has provided the greatest contribution - approximately twice that of each the other two main prey species. These results are consistent with other similar surveys. The combined percentage contribution from these three species was 78.9% of the total number of prey items. The remaining species were all minor prey species. The most important of these by prey items were pygmy shrew (Sorex minutus), bank vole (Clethrionomys glareolus), harvest mouse (Micromys minutus) and house mouse (Mus domesticus), which between them provided a further 18.4%. Thus, 7 small mammal prey species provided 97.3% of the prey items. The pie chart shows the individual percentage contributions from these 7 small mammals and from the bird prey items. The remaining 1l species of small mammals, together with the reptile and amphibian prey items are combined to complete the picture. The small mammals provided 99.0% of all the prey items.
A number
of prey species unusual in owl pellets appear in the survey results. Three
species of bat have been recovered from pellet batches - pipistrelle bat
(Pipistrellus pipistrellus), brown long-eared bat (Plecotus
auritus) and Natterer's bat (Myotis nattereri). A number of
moles (Talpa europaea) have also been recorded. This species spends
most of the time in its underground burrow system, but does occasionally
appear on the surface. Juvenile moles also move across the surface when
seeking a new territory in which to start their burrow. A single weasel
(Mustela nivalis) was recovered from a location in Argyllshire,
and a small number of the elusive common dormice (Muscardinus avellanarius)
appeared in batches of pellets from locations on the Isle of Wight and
in Sussex. Annual
and Seasonal Variation in the Contributions from the Main Prey Species Given the
high reproductive and mortality rates of small mammals and the dependence
of these aspects on variable characteristics of their habitats, annual
and seasonal variations in their availability and hence their relative
representations in owl pellet batches are inevitable. The effects of habitat
variation also produce differences in the shape of the percentage contribution
histograms and curves from location to location. Figure 2
shows the annual variations in the three main prey species (field vole,
common shrew and wood mouse) for the total data. Although in each of the
years the field vole is the primary prey species, its percentage representation
varies, as does its margin over the other two main prey species. From
1995 to 1998 inclusive, The wood mouse is second in importance by prey
items, and is followed by the common shrew. During the other years to
date, the common shrew is the more important of these two species.
Figures 3 and 4 demonstrate regular seasonal variation for the field vole and common shrew respectively at three locations. Although there are differences in the scale of the proportions between the result sets at the locations, there is a considerable degree of synchrony in the seasonal variations. Figure 3 shows that at these locations, the field vole provides its main contributions during autumn and winter.
Figure 4 shows that the main contributions from the common shrew occur regularly during the spring and summer months. The average combined percentage contribution from these two species is 83.1%, 72.5% and 65.3% at the Argyllshire, Bedfordshire and Sussex locations respectively.
The above analyses are based on the proportions of the various prey species appearing in the owl pellets, and hence reflect the availability of these species to the owls. However, the importance a given prey species is to the owl diet depends not only on the relative numbers of individuals of the species captured, but also on the typical body weight of the species. Note that for small mammal species, body weight varies considerably during the year, and the average body weight for a species is also affected by recruitment of juveniles at the species breeding seasons. A factor proportional to the average body weight of each species is used to transform prey item counts for the species into a "food value" figure, allowing the relative importance of the species to the owl diet to be established. Figure 5 shows the results displayed in Figure 1 above transformed by these Prey Value factors.
Future
of the Survey The survey
is continuing, and will do so as long as contributors send in pellet batches
for analysis or analysis of their results. All the data is recorded on
a computer database developed for the survey. How
You Can Help Clearly the
success of this surveys depends on volunteers collecting and sending pellet
batches to the Project Co-ordinator for analysis. To support the investigations
of seasonal variations in availabilities, and the variations between locations
and habitat types, the pellets should be collected monthly. If this is
not possible, then other collection periods will be gratefully received.
Samples should be as large as possible. The contributor will receive a
report on each batch sent, giving details of the contents of the batch
and brief comments on the results. To date this has been a well supported
and hence successful survey. However, it does require pellet batches from
more locations particularly Scotland, Wales and South West and North England.
If you can provide pellet batches or analysis data, please contact me. R.Alasdair
Love. 4 Laurel Way, Totteridge, London N20 8HP.  Last
updated November 26 2002 |
