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IBIS
128:
266-277
Sensory capacities and the nocturnal habit
of
owls
(Strigiformes)
GRAHAM R. MARTIN
Departments
of
Zoology and Comparative Physiology, and
of
Extramural Studies,
University
of
Birmingham, P.O.
Box
363,
Birmingham
B15
2TT
Accepted
5
July
1985
Behavioural studies show that in the eye
of
the Tawny Owl
Strzx
alum
both absolute visual sensitivity and
maximum spatial resolution at low light levels are close to the theoretical limit dictated principally by the
quanta1 nature of light and the physiological limitations on the structure
of
vertebrate eyes. However,
when the owl’s visual sensitivity in relation to naturally occurring ligh levels is analysed, it is concluded
that at night there will often be occasions when vision can only be used to control the owl’s behaviour with
respect to large objects.
Owls are capable of detecting and catching prey by hearing alone. However, absolute auditory
sensitivity is not superior to that of mammals (including Man), but does appear to have reached the
absolute limit on sensitivity in the aerial environment, which is dictated by the minimum ambient sound
level.
An explanation
of
the owl’s ability to be active at night based only upon high sensory sensitivity
is
thus untenable. Many features of the natural behaviour
of
the Tawny Owl (e.g., the high degree
of
territoriality, prey catching technique, dietary spectrum) may be interpreted as reflections
of
an additional
requirement
for
the nocturnal habit beyond high sensory sensitivity: detailed knowledge of local
topography.
The nocturnal habit amongst birds is rare. Probably less than 250 species
(c.
30/) are
regularly active outside daylight hours and the majority of these species, such as
certain nightjars (Caprimulgidae), are best regarded as crepuscular rather than
strictly nocturnal in their activity. Some bird species (swiftlets
Collocalia,
and the
Oil Bird
Steatornis caripensis)
are able to guide themselves in the totally dark interior
of caves, using echolocation (Griffin
&
Suthers 1970, Konishi
&
Knudsen 1979);
however, such environments are structurally simple.
It is only amongst the Strigiformes (owls and barn owls) and the Caprimulgi-
formes (frogmouths and their allies) that strictly nocturnal, non-echolocatory birds
which fly in structurally complex environments, such as forests, are found. Not all
species in these orders are nocturnal; indeed, probably less than
40
of the 135 species
of owl may be regarded as strictly nocturnal, in that they are capable of completing all
aspects of their life cycle between sunset and sunrise (Burton 1985).
The nocturnal environment would therefore seem to pose particular limiting
problems for flying birds, and it is typically supposed that these problems are sensory
in nature (Walls 1942, Tansley 1965). This paper presents available data on the
visual and auditory capacities of owls and views these in the context of the ultimate
limits on sensitivity and the critical features of the nocturnal environment. It is
concluded that while the principal problems posed for the nocturnal behaviour of
owls are indeed sensory, the solution
to
those problems involves more than sensory
adaptations. Habitual nocturnal behaviour amongst birds would seem to be
dependent upon behavioural, as well as sensory, adaptations.
I
986
SENSORY
CAPACITIES
OF
OWLS
267
The principal species discussed here
is
the Tawny Owl
Strix
aluco.
The visual
and auditory capacities of this species have been more comprehensively studied than
those of any other bird, and many aspects
of
its natural history have been studied in
detail. The Tawny Owl has a wide but discontinuous distribution in the palearctic
and oriental zoogeographical regions. Its preferred habitat
is
closed canopy broad-
leaved deciduous woodland, but it will inhabit coniferous and more open woodland
types, parkland and the centres
of
large cities as long as mature trees are present
(Beven 1956, Mikkola 1983). It is argued that the sensory adaptations of this and
other nocturnal species are very similar and most of the problems discussed here
concerning the Tawny Owl apply generally to nocturnal owl species.
Vision
at
low light levels
Absolute sensitivity
Field observations have long been interpreted as evidence that the visual sensitivity
in owls
is
higher than in Man. Indeed the assumption
of
high sensitivity, coupled
with evidence that owl retinae contain many rod photoreceptors was part
of
the
original evidence used
to
support the duplicity theory of
vision
(Schultze 1867). The
assumed high visual sensitivity of owls was correlated with the large size, tubular
shape and frontal placement of owl eyes compared with those typically found in
diurnal birds (Fig.
l),
and these anatomical features became regarded as adaptations
concerned with the nocturnal habit. However, how these particular features were
supposed to facilitate high sensitivity was not discussed in detail (e.g., Walls
1942,
Tansley 1965).
The assumption of high visual sensitivity in owls was questioned by evidence
which suggested that the visual detection
of
infra-red radiation could mediate prey
capture in the Tawny Owl (Vlanderpianck 1934). However, the ocular media
of
this
Parus
Figure
1.
Drawings
of
horizontal sections through the head of the Black-capped Chickadee
Parus
atricapillus
and the Great Horned
Owl
Bubo
viriginianus.
Redrawn to scale from Wood
(1917).
In
Parus
eye shape, size and position are typical of diurnal Passeriforme species, in
Bubo
eye shape and position are
typical
of
other
owl
species, and eye size is similar to that
of
the Tawny
Owl.
268
G.
R.
MARTIN
IBIS
128
bird were subsequently shown to be virtually opaque to infra-red radiation
(Matthews
&
Matthews 1939).
The first experimental studies of visual sensitivity suggested that owl eyes were
between
10
and
100
times more sensitive than that of Man (Hecht
&
Pirenne
1940,
Dice 1945). However, neither of these studies measured absolute sensitivity directly,
nor was it claimed that stimuli were calibrated with a high degree of accuracy. More
recent analysis of visual sensitivity in Tawny Owls has led to a more modest
conclusion in that absolute sensitivity in these birds was found to be higher than that
of Man by an average of 2.5-fold (Martin 1977). This difference is within the normal,
five-fold, range of absolute visual sensitivity found in the healthy human population
and also within the range by which individual human thresholds may differ on a daily
basis (Pirenne
et
al.
1957). Thus we might expect to find individual human subjects
with visual sensitivity greater than that of individual Tawny Owls.
The 2.5-fold average difference in sensitivity between Owl and Man can be
accounted for by a difference in the maximum brightness
of
the retinal image
produced in the owl and human eye (Martin 1982). This suggests that the retinae of
these two eyes are of similar sensitivity; there are good theoretical reasons for
believing that this should be the case. For example, it has been argued that the
human retina has attained the absolute limit of visual sensitivity, which is dictated by
the quanta1 nature of light and the signal-to-noise limitations of extracting
information from an array of photoreceptors which are responding at the limits of
their sensitivity (Pirenne 1962, Barlow 1981). Since the rod photoreceptors of Man
and Owl are very similar in respect of three parameters-dimensions, spectral
sensitivity and specific absorbance of their visual pigments (Bowmaker
&
Martin
1978, Bowmaker
&
Dartnall 1980)-they would absorb a similar proportion of the
light quanta that reaches them (Martin 1982). Therefore unless maximum retinal
image brightness can be greatly increased over that of the human eye, visual
sensitivity much in excess of that in humans is unlikely to be found in any owl
species.
An hypothetical vertebrate eye with
a
focal length similar to that of the Tawny
Owl
[
E
17
mm; (Martin 1982)] but whose visual sensitivity was
10
or
100
times that
of Man, would require entrance pupil diameters of
25
mm and 81 mm respectively.
Clearly such eyes would have to be both absolutely large as well as peculiar in their
general design-quite unlike the typical tubular shape of the owl eye (Fig.
l),
and
indeed quite unlike any vertebrate eye
so
far described (Martin 1983). Furthermore,
it has been argued that a relatively long focal length, such as that found in the Tawny
Owl’s eye, is an essential feature
of
an eye designed to function throughout the
naturally-occurring range of night time luminance levels (Martin 1982). This is
because a long focal length will produce a large retinal image and thus permit
flexibility in the way the retinal image is sampled by the photoreceptor array. Such
flexibility is necessary to maximise the amount of information which can be extracted
from the retinal image over a wide range
of
luminances (Snyder
et
al.
1977). [The
naturally occurring nocturnal luminance range is discussed below]. Thus, given this
constraint of a long focal length, it would seem that an owl eye with an image
brightness even
10,
let alone
100
times higher than that of Man, is unlikely to have
evolved.
Absolute visual sensitivity in the Tawny Owl is approximately
100
times higher
than that
of
the Pigeon
Calumba
livia
(Blough 1955) (Fig.
2),
a strictly diurnal bird
species. This difference can be attributed principally to differences in retinal neural
mechanisms, since pigeon and owl eyes differ little in both the maximum brightness
of
their retinal image and in the parameters of individual rod photoreceptors (Martin
1982).
1986
I
0
0
-I
-
-
h
+
3
.-
4"
-2
-3
I
c
1
I
I
I
I
I
I
I
I
I
I
I
SENSORY
CAPACITIES OF
OWLS
I
269
mst
mxs
mm
sr
mrn
-
hobrtats
ODed-;
'
Cloud
-
mst
mxs
-
mm
+
sr
Brood leoved
woodlond conopy
-
Cloud
-
Figure
2.
Top. Minimum separable visual acuity (expressed as loglo of the reciprocal
of
the minimum
separable angle) as
a
function of luminance in Man, Pigeon and two owl species (cdm-*, candela per
square metre).
Bottom. Naturally occurring luminance levels of a grass
or
leaf litter substrate during day-time and night-
time. Luminance ranges within open habitats and under
a
broad leaved woodland canopy are shown. Each
horizontal bar indicates the maximum likely range
of
luminance which can be experienced in the habitat
under the conditions indicated. Day-time luminance levels (the four horizontal bars to the right) are for
latitude
50"
at the time of the summer solstice, and show the luminance range
from
sunrise
(sr)
to
maximum midday sun (ms). Night-time luminance levels (the four horizontal bars to the left) show the
range from maximum moonlight
(mm)
to minimum starlight
(mst);
maximum starlight without moon
(mxs)
is also indicated. Luminance levels experienced under maximum cloud cover are shown for both
day- and night-time in the two habitat types. The total
diurnal luminance range
of
5.6 log,, units
is
the
range which may be experienced between maximum sunlight without cloud cover in an open habitat, and
at sunrise with cloud cover beneath woodland canopy. The total
nocturnal luminance range
of 6.31
loglo
units is the range from maximum moonlight without cloud in an open habitat, to minimum starlight with
cloud beneath the woodland canopy. Points
A,
B and
C
indicate the mean
behaviourally-determined
absolute oisual threshold
of Pigeon, Man and Owl respectively (modified from Martin, 1982).
(0-0)
Human; Pirenne
et
al.
(1957);
(0-0)
Human; Shlaer (1937);
(U----U)
Owl
Bubo;
Fite (1973);
(V----V)
Owl
Strix;
Martin and Gordon (1974);
(Op)
Pigeon;
Columba,
Blough (1971);
(0..
.
,
.O)
Pigeon; Hodos and Leibowitz (1977). Hodos
et
al.
(1976).
G.
R.
MARTIN
IBIS
128
2
70
Spatial resolution
Theoretical considerations of retinal image analysis (Snyder
et
al.
1977) suggest that
spatial resolution at low light levels in Man and Owls should be similar, and there is
experimental evidence that this is
so
(Fig. 2). Spatial resolution of any image must
decrease with luminance level regardless of the device (e.g., photoreceptors,
photographic emulsion, photodiodes, etc) used to analyse the image. High spatial
resolution of an image of low luminance is not possible theoretically, regardless
of
the
degree of contrast in the image (Snyder
et
al.
1977).
There is commercially available equipment which serves
to
enhance human
visual resolution at night; it does
so
either by increasing the size and brightness of the
image, and/or by unemploying a non-visible part of the electromagnetic spectrum
which is at a higher intensity than the visible wavelengths
(e.g.,
infra-red). Because
of the similarities in rod photoreceptor parameters in human and owl eyes, it may be
seen that an owl could achieve spatial resolution superior to that of Man by
employing only one element of the strategy used by the commercial devices, i.e.,
increased image-size and brightness. However, as argued above it would seem that
there is little possibility that this can occur.
Vision at high light levels
Behavioural (Martin
&
Gordon 1974a,b), electrophysiological (Martin
et
al.
1975)
and anatomical (Bowmaker
&
Martin 1978) studies all furnish evidence that the
Tawny Owl visual system functions adequately at high, day-time, light levels.
Evidence from analysis
of
the visual pigments of cone photoreceptors and their
associated oil droplets (Bowmaker
&
Martin 1978) corroborates behavioural
investigations of wavelength discrimination (Martin 1974) which suggest that the
owl is not capable of such subtle wavelength discriminations as diurnal bird species
such as the Pigeon
Colurnba
livia (Wright 1979). However, Figure
2
shows that
maximum spatial resolution in two owl species-(the Tawny Owl and the Great
Horned Owl
Bubo
virginianus)-and in the pigeon is very similar and is attained in all
three species at a similar luminance level. This implies that, during the day-time,
owls are not at a visual disadvantage compared with at least some obligate diurnal
bird species, such as the Pigeon. All of these species are, however, inferior in their
maximum spatial resolution compared with either man or with a diurnal bird of prey,
the American Kestrel
Falco
sparvevius
(Hirsch 1982). Maximum resolution in these
latter species is approximately five times higher than in the tawny owl at a similar
high luminance level.
Binocular vision
The owls in general are often regarded as having the most frontally placed eyes
amongst the birds (Fig. 1) and that this has resulted in them possessing some of the
largest binocular fields to be found in this vertebrate class. However, in the Tawny
Owl the optic axes of the eyes are not parallel, but diverge by
55"
and maximum
binocular field width equals only 48" (Martin 1984a). By comparison, the optic axes
in Man and Pigeon diverge by 10" and 132" respectively and the maximum widths of
their binocular fields are approximately 140" and 22". The function of binocularity
and the factors which may account for the known interspecific differences in
binocular field width amongst vertebrates has received much discussion, but there is
little concensus on either of these topics (Martin 1984a). Thus it is not clear whether
the relatively large binocular field
of
the owl compared with that
of
the Pigeon can be
271
correlated directly with the nocturnal habit, as has often been assumed (Walls 1942,
Tansley
1965,
Mikkola 1983, Burton
1985).
I
986
SENSORY
CAPACITIES
OF
OWLS
Vision in relation to naturally occurring light levels
Figure
2
shows that the light levels which may be experienced within the nocturnal
environment are highly variable. Depending upon such factors as the degree of
vegetation cover, the altitude and phase of the moon and the presence of cloud,
incident light levels at night may differ by over one million-fold. In addition, the
reflectance of natural substrates can differ by a factor of up to 100-fold [e.g., fresh
snow compared with black soil (Krinov
1947)]
so
the range of luminance levels an eye
might be exposed
to
after sunset may cover eight orders of magnitude. Thus, for the
purposes of understanding the sensory problems of the nocturnal habit the simple
‘nocturnal’ label can be misleading. For example, nocturnal bird species which live
in predominantly open habitats such as the majority of the Caprimulgiformes
(nightjars, etc.), and some owls, may never experience the lowest naturally occurring
luminance levels to which the nocturnal woodland owls are frequently exposed.
Figure
2
indicates that, even under cloud cover, the sky itself should always be
visible to both Man and Tawny Owl (though not to the Pigeon) and thus some kind
of pattern vision should be possible by viewing objects in silhouette against the sky.
However, under a woodland canopy the minimum natural luminance even of grass
or leaf litter, let alone black earth or similar low reflectance substrates, is low enough
to render these substrates invisible both to Man and to Tawny Owl.
Also,
because
the reflectances of many natural substrates and objects are very similar (Krinov
1947), the visual scene will be of low contrast.
So
there will be many occasions,
especially under a vegetation canopy, when even large objects will be invisible
despite being viewed at average luminance levels which are above absolute visual
threshold. While this invisibility of natural objects can be readily verified by personal
‘observation’, predicting the nature, size and distance at which various objects might
just be detected at a given luminance is problematic, principally because there is little
information on spatial resolution beyond measures of minimum separable acuity
(involving high contrast stimuli) at low luminance levels. However, something
of
the
problem can be appreciated by the following prediction which can be made from the
data of Figure 2: at a luminance level approximately
10
times higher than absolute
visual threshold (i.e., in the middle of the luminance range which commonly occurs
under a broad-leaved woodland canopy at night) even a dark-coloured rodent
60
mm
in length walking across snow will be invisible to both Man and
owl
until within
a
viewing distance of
3
m.
It would seem, therefore, that at the lower naturally occurring light levels the
visual system can only be used both by Man and Owls
to
control behaviour with
respect to large objects, and cannot detect such objects as small branches or prey
items.
Audition
Auditory
localization
It has been clearly demonstrated that audition can play an important part in prey
capture by owls. Barn Owls
Tyto
alba
can detect and capture live rodent prey by
pouncing from a perch in total darkness, guided only by the sound that the animal
G.
R.
MARTIN
IBIS
128
272
produces as
it
moves through leaf litter (Payne 1971). The outer ears of many owl
species are large, and exhibit complex bilateral asymmetry in size, shape and position
(Norberg 1977). It has been proposed that these anatomical features are an essential
part of the auditory localization system, functioning principally in the vertical
location of sounds in the frontal plane (Norberg 1978, Knudsen
&
Konishi 1979).
While studies of auditory localization in the Barn Owl (Knudsen
et
al.
1979), make it
clear that audition itself is sufficient to mediate prey capture, it is also clear that the
Barn Owl’s ability to localize white noise sound sources (which approximate leaf
litter rustle) is not superior to that of Man (Mills 1958, Roffler
&
Butler 1968).
Auditory sensitivity
As
is
the case with visual sensitivity, it was proposed that owls were markedly
superior to man in their absolute auditory sensitivity (Konishi 1973). However, this
conclusion was based upon the result obtained from a single subject, and it is known
(Sivian
&
White 1933, Masterton
et
al.
1969) that measures of minimum auditory
thresholds exhibit large variability due to both intraspecific and procedural
differences.
More detailed interspecific comparisons of auditory thresholds in birds and
mammals have led to the conclusion that both owls and mammals, including Man,
have similar absolute auditory sensitivity and that this sensitivity coincides with the
auditory masking level produced by minimum naturally occurring ambient sound
(Martin 1984b). This suggests that the ultimate limit on auditory sensitivity in
vertebrates
is
determined not by physiological factors, such as thermal agitation
of
fluids within the cochlea (Harris 1967)
,
or ‘self-noise’ (Diercks
&
Jeffress 1962), but
by a source common to all non-aquatic vertebrates: the minimum ambient sound
level. Auditory sensitivity greater than that dictated by the minimum ambient sound
is unlikely to evolve, since sounds below this minimum will always be inaudible due
to the auditory masking produced by the ambient noise.
It may be concluded, therefore, that under any given environmental conditions
the ability
of
both owls and man to detect and locate a particular sound is likely to be
very similar.
The sensory problem
of
nocturnality
The conclusion to be drawn from the above discussion of both the visual and
auditory capacities of owls is that an explanation of the ability of these birds to be
active at night, based only upon high sensory sensitivity, is untenable. Owls possess a
visual system whose absolute sensitivity and resolution approach the theoretical
maxima (dictated principally by the quanta1 nature of light). Even
so,
vision cannot
function adequately to mediate prey detection
or
flight in the spatially complex
environment
of
a woodland habitat under the full range of nocturnal light
conditions. Also, although auditory sensitivity in
OW~S
appears
to
have reached the
ultimate limit possible in the aerial environment, audition cannot of itself permit the
detection of obstacles, although
it
can clearly serve in the detection
of
prey items
which either emit sounds
or
make sounds as they move.
It has been hypothesized that much,
if
not all, human perception is dependent
upon cognition as well as immediate sensory input (see, for example, Gregory
(1974), Frisby (1979)). This is exemplified by studies of car-driving at night-time. In
completing this task people frequently, sometimes habitually, drive in a manner
which is beyond the control of information immediately available via their visual
system (Hills 1980). Accidents are usually avoided, however, since the visual
I
986
SENSORY
CAPACITIES
OF
OWLS
273
information that is available (e.g., detection of road markings and lights on other
vehicles) is supplemented by general knowledge of the nature of roads and traffic and
specific knowledge of local topography.
Since both Man and Tawny Owl are equally restricted in their main tele-
receptive sensory capacities, a similar analysis would seem applicable to the
nocturnal behaviour of the Tawny Owl. Thus, it may be proposed that knowlege of
both the general characteristics of the environment and specific details of the local
topography are important for the mediation of the bird’s behaviour under nocturnal
conditions. The nocturnal habit
of
owls must be dependent upon behavioural and
cognitive adaptations, as well as sensory adaptations.
Natural history
of
the tawny owl
Long-term studies of the natural history of Tawny Owls (Southern 1970, Hirons
1976, 1985, Hardy 1977) have shown that this species exhibits a number of unusual
behavioural traits. Perhaps the most important single feature of the Tawny Owl’s
natural history is its sedentary habit which is manifested
by
its high degree of
territoriality.
The birds are territorial throughout the year and territorial boundaries alter
little, if at all, during a bird’s life time. The defended area supplies all of the food
requirements
of
the individual bird. Also, both male and female share the same
territory throughout the year, although there is some evidence that females
occasionally move territories. In these features the Tawny Owl differs not only from
the majority of bird species, including sympatric diurnal raptors of similar habitat
preferences (e.g., the Sparrowhawk
Accipiter nisus
and Goshawk
A. gentilis
(Newton
1979, Marquiss
&
Newton 1981, Kenward 1982)), but also appears to differ from the
majority
of
other owl species.
No other owl species has received such detailed study as the Tawny Owl, but
summarised data for Western Palearctic species of owl (Cramp 1985) indicates that
only in the Ural Owl
Strix uralensis
do individuals behave like the Tawny Owl and
typically defend an exclusive hunting and breeding territory throughout the year.
Both the Tawny and Ural Owls are nocturnal, with extensive woodland the preferred
hunting and breeding habitat.
There are, however, species in which individuals
or
pairs have, on occasion, been
reported as maintaining a territory throughout the year (Barn Owl, Eagle Owl
Bubo
bubo,
Little Owl
Athene noctua,
Long-Eared Owl
Asio
otus,
Tengmalm’s Owl
Aegolius funereus).
These species are also regarded as nocturnal but they prefer
habitats which are essentially more open than those frequented by the Tawny and
Ural Owls, with scattered trees or woodland glades and margins an essential
component. It would be of interest to know more detail of the particular
circumstances (especially habitat characteristics) which are correlated with the
occurrence
of
a sedentary habit in certain individuals
of
these species.
No
diurnal and/or crepuscular species of Western Palearctic owl (Snowy Owl
Nyctea scandiaca,
Pygymy Owl
Glaucidium
passerinurn,
Hawk Owl
Surnia
ulula,
Great Grey Owl
Strix
nebulosa,
Short-Eared Owl
Asio flarnrneus)
appears to be as
territorial as the nocturnal owls. In these species the majority
of
pairs maintain a
territory (which need not coincide with the hunting range) during the breeding
season only.
Of the Western Palearctic species only the Scops Owl
Otus
scops
presents an
exception to these general conclusions in that the species is regarded as essentially
nocturnal, but there are no records of sedentary individuals
or
pairs. Scops Owl
requires the cover of trees for roosting and nesting but hunt large insect prey over
274
G.
R.
MARTIN
IBIS
128
open ground. Northern populations of the species are migratory, while southern
ones are partially migratory or resident. However, it is not known whether any of the
resident individuals maintain a territory outside the breeding season although local
populations are known
to
be gregarious at this time.
In the case of the Tawny Owl, possession of a territory seems essential for the
survival of the bird during the annual cycle.
A
high proportion
(?600/)
of
young
birds die each year, many
of
starvation, and it is believed that the majority of these
birds do not hold territories and that their starvation is directly attributable to this
factor (Hirons
et
al.
1979,
Hirons
1985).
Mortality amongst territory holders is less
than
15%
per annum. Once established in its territory (in Southern England, in the
preferred habitat type, approximatrely
12
ha) an individual owl (both male and
female alike) is likely to remain in that area all of its life (average life expectancy,
5
+
years: oldest ringed bird,
18
years
7
months). Territorial boundaries are defended
vigorously against neighbours and non-territorial intruders are excluded from
territories. However, if a territory becomes vacant it is normally re-occupied before
the adjacent resident birds expand into it.
The dietary spectrum of the Tawny Owl is broad. It seems that since Tawny
Owls forage exclusively within the boundaries
of
their territory, they must turn to
alternative prey items during the annual cycle. Thus, unlike the owls of similar body
weight, but which live in open habitats and tend to be specialist feeders (e.g., the
Short-Eared Owl), the Tawny Owls’ response to a shortage of prey of optimum size
(small mammals and birds) is not to forage further afield, but to forage within its
territory for smaller items, such as earthworms, which at certain times of the year can
form a substantial proportion of the diet (Southern
1970).
Not only do the birds remain in their territory throughout their life, but they are
also consistent in the way that the territory is used (Hardy
1977).
Thus, hunting,
roosting and feeding tend to take place at regularly-used perches. The hunting
technique is predominantly ‘perch-and-pounce’, where the owl waits for prey to
come into the vicinity of the chosen perch. It does not quarter for prey on the wing as
do owls
of
open habitats (e.g., the Short-Eared Owl) although hunting Tawny Owls
have been recorded as flying into a roost of small passerines (Cramp
1985).
Hunting
on the wing has been recorded in the Tawny Owl but this was apparently anomalous
behaviour (Nilson
1978).
It was recorded in summer at a high latitude when light
levels were higher than the normal nocturnal range.
It is also noteworthy that Southern
et
al.
(1954)
concluded that young Tawny
Owls, during the protracted period of dependence on their parents (approximately
2i-3
months post fledging), make little or no effort to feed themselves, but
concentrate on learning their way about the parental territory. Also, there is evidence
that even adult Tawny Owls may not always be able to negotiate obstacles within
their territory. Thus, when disturbed or surprised at night, Tawny Owls have been
known to fly into branches and even tree trunks (Hirons, pers. com.) thus suggesting
that vision was not sufficient to guide these birds about obstacles.
Conclusion
Territoriality and the nocturnal
habit
Many features
of
the natural history of the Tawny Owl becomes explicable in the
light of the sensory limitations and the behavioural and cognitive aspects
of
nocturnality discussed above. Thus, the high degree of territoriality may be
interpreted as essential to permit prey capture and general mobility when light levels
become limiting for the immediate visual guidance of flight and other behaviour.
To
275
stray out of the territory (for example in response to a shortage
of
optimal sized prey)
is of no advantage since
it
is specific knowledge of landmarks and the regularly used
perches that is essential for prey capture and movement under restricted sensory
input. For similar reasons invasion of an adjacent territory is
of
little value. The use
of the ‘perch-and-pounce’ hunting technique, employing a limited number of
perches, is also comprehensible, since the use of these regular perches will facilitate
the accumulation of the topographical knowledge required to mediate prey capture
using audition alone.
I
986
SENSORY CAPACITIES
OF
OWLS
The value
of
topographical knowledge
The adaptive function of avian territoriality is not generally understood, This is
because the proximate and ultimate factors underlying territoriality are not the same
in all species and indeed more than one factor may be operating simultaneously
(Wilson 1975, Davies 1978). Hinde (1956: 349) proposed that ‘familiarity with food
sources and refuges from predators’, could be regarded
as
one of the functions
of
territorial behaviour. However, most recent work on territoriality has tended to
favour analyses using metaphores drawn from economics in which emphasis has
been upon the costs and benefits involved in the defence of resources within the
territory.
Familiarity with food sources and refuges from predators are likely to be benefits
which accrue as the result of protracted residence in any one site. However, in the
case of the Tawny Owl it would seem that to be an habitual nocturnal species,
detailed topographical knowlege, gained through protracted residence in a restricted
area, is essential for survival. If an economical metaphor is to be employed, then
rather than regard the high degree of territorality in the Tawny Owl as involving only
the defence of a resource (average territory size in Tawny Owls is related primarily to
average abundance of prey within an area (Southern 1970, Hirons 1976, Hardy 1977,
Hirons 1985), and nest sites may sometimes be
a
limited resource (Lundberg 1979,
Wardhough 1984)), it might be convenient to view the situation as also involving the
‘protection of an investment’.
Defence of a resource implies that some commodity within the territory has a
value which can be readily appropriated by other individuals. However, topographi-
cal knowledge of a territory has value to the resident individual only. It cannot be
transferred to another individual nor can it be transferred by the resident to another
site. However, topographical knowledge, and hence the value of holding a particular
territory
is
likely to increase with length of residence in the same way that an
investment increases its value over time. (In the Tawny Owl Southern
&
Lowe
(1
986) have argued that continued occupancy of a territory results in greater hunting
skill. This
is
reflected in the data of Southern (1970) which suggest that the
probability of successful breeding increases with the length
of
time that the territory
has been occupied.) At some point the value of this investment may come to
outweigh that of the defended resources. Whether it does
so
for a particular species
or individual will depend upon many factors concerning the quality of the defended
resources and the value of topographical knowledge in the natural behaviour of the
species.
In the Tawny Owl, because of the necessity
of
detailed topographical knowledge
for nocturnal mobility and prey capture thoughout the year, the point at which the
value of this knowledge exceeds the value of the defended resources may be reached
frequently. The observations that resident Tawny Owls remain on territory and
accept a diversity of prey items, many of which are below optimal size, e.g.,
earthworms and beetles (Southern 1970) and/or use less favoured nest sites (e.g., a
276
G.
R.
MARTIN
IBIS
128
scrape on the ground rather than
a
hollow
tree
(Mikkola
1983)
may both be explained
in
these
terms.
I thank the following for their most helpful discussions and comments: J. Cohen, H. Kruuk, H. N.
Southern, K. Voous and
S.
R. Young.
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