CHAPTER 1
Introduction
1.1 Part One – The owls
1.1.1 Western Barn Owls Tyto alba alba
See Plate 1 Barn Owl portrait Plate 2 Barn Owl foraging in flight by day Plate 3 Barn Owl in flight at dusk silhouetted
With their characteristic heart-shaped face, Barn Owls Tyto alba are one of the most studied and best-known owl species in the world. Once considered the most common owls in Britain, they have suffered such a drastic decline in numbers that there were probably no more than 4,000 pairs left by the turn of the millennium (Toms, Crick & Shawyer 2000). Nevertheless, Barn Owls are still widely but thinly distributed across much of lowland Britain and, with the exception of urban areas and dense woodland, may forage over any habitat that supports small mammal populations.
Although not territorial, Barn Owls are highly faithful to their home range and to the roost and nest sites they use within it. This would typically be a vast area extending up to 5,000 hectares (ha), although during the nesting period only a small portion of this is used – perhaps about 350 ha. Within their range most Barn Owls use one nest site, several main roost sites and other occasional roost sites. Whilst some pairs roost together all year round, others seem to live quite separate lives and only come together to breed (see 2.4). They generally roost and breed in dry elevated spaces in buildings or tree cavities, but will make use of almost any structure or hole that meets their needs. Because of the loss of so many old farm buildings and hollow trees they are increasingly dependent on the availability of nestboxes (see Chapter 6).
Barn Owls usually breed once a year; most eggs are laid in April or May and the young fledge in July or August. However, there is a great deal of variation. In a good year, some pairs will breed twice or even three times and nesting has been recorded in every month of the year. The presence of a nest is often given away by the sound of the young calling for food; sometimes likened to 'snoring' it is a repeated 'shshsh shshsh shshsh', which may continue for many hours. The most noticeable call of the adult is a long drawn out shriek or screech that can sound either hoarse or tremulous. However, some individuals hardly ever screech, whilst others screech quite frequently, particularly in the spring. In spring or summer unpaired birds sometimes fly far and wide while screeching continuously (see 2.4).
Barn Owls have buff-coloured upper surfaces delicately marked with white and shades of grey. In most birds their distinctive heart-shaped face and under-parts are predominantly white. When seen in flight the overall impression is usually of a large pure-white bird. Foraging Barn Owls generally fly buoyantly back and forth, 'quartering' areas of rough grassland and other open prey-rich patches or strips. They may also hunt from fence posts or other fairly low perching places, usually less than 3 m above the ground. Although daylight hunting does occur, Barn Owls are mainly nocturnal and crepuscular, with peaks of activity around dusk and dawn. Their diet consists mainly of small mammals, particularly voles, mice and shrews, together with rats and the occasional bird or frog (Love 2009). Unlike Little Owls Athene noctua and Tawny Owls Strix aluco, Barn Owls do not eat worms and hardly ever eat insects. Their diet can vary considerably between sites and from year to year, depending on prey availability, but they are definitely specialist rather than generalist feeders and depend almost entirely on the availability of small mammals (see 2.1.4.2).
In a landscape with many isolated trees and copses, Barn Owl home ranges will contain numerous Tawny Owl territories; in mixed farming areas they are also likely to contain one or more pairs of Little Owls. In general, these species ignore each other. Direct competition for nest cavities is only likely to occur where there is a severe shortage of sites, as each species has a different cavity size preference. Although there is considerable overlap in their diet, direct competition over food resources is not a major problem, partly because Little Owls and Tawny Owls have a much more varied diet than Barn Owls, and partly because they are restricted to hunting from a perch. By hunting from the air Barn Owls are able to utilise parts of the landscape that perch-hunting owls cannot normally reach (see 2.1.3.1).
Most reports of Barn Owls received from the general public are of birds seen while driving at night. They are almost always described as 'white' because of the headlights and because the birds are usually in flight. Indeed, the front or underside of a Barn Owl sighted in any circumstances will almost always be described as 'white', though this is less likely to be the case if the bird is viewed from above, behind, or as a silhouette (see Plate 3).
1.1.2 Identifying owls and interpreting reports from the public
In order to identify Barn Owls correctly it is important to be able to recognise the other owls that may also be encountered. In addition, a good survey of any potential Barn Owl site or area will always include interviews with the people who live or work there in order to collect reports of owls they may have seen or heard (see 4.2.2). The ability to interpret the way people describe owl sounds and sightings is thus a very useful skill. Care will always be needed because words such as 'big' and 'screech' mean different things to different people and 'white owls' (Barn Owls) do not always look white. Furthermore, an owl sound reported as a 'screech' or 'shriek' may not always come from a Barn Owl (see 4.2.2).
The following species accounts provide an introduction to the four other owl species that are resident in Britain in order to:
• enable the reader to differentiate between Barn Owls and other owls
• summarise the extent to which other owl species may affect Barn Owls
• explain the relevance of each owl species to Barn Owl surveys and conservation work
1.1.2.1 Little Owls Athene noctua
See Plate 4 Little Owl portrait
Little Owls are the only other owl species in Britain commonly referred to as 'farmland birds' and, like Barn Owls, they mainly roost and nest in tree hollows and farm buildings. It is therefore highly likely that anyone surveying for Barn Owls will come across Little Owls at some point. The sound of a Little Owl calling may also be reported as a 'screech' and so there is considerable potential for confusion with Barn Owls.
With an estimated c. 5,800–12,000 pairs (Toms, Crick & Shawyer 2000), there are more Little Owls than Barn Owls in Britain. However, they are less widely distributed and occur mainly in southern lowland Britain, extending as far north as southern Scotland and west to the Welsh borders. Like Barn Owls, Little Owls are birds of open country and they are mainly found in mixed farming areas (partly cultivated and partly grassland) with hedgerows, orchards, copses and also in parkland. Little Owls are often referred to as 'locally common' and it is not unusual for one area to support several pairs breeding in close proximity whilst an adjacent area has none. Little Owls are largely sedentary with the birds generally faithful to their nest sites, and to each other, as long as both partners survive.
The type of roost and nest sites they use can vary considerably between individual pairs. Sites offering a small dark cavity are generally preferred, especially if the cavity has tunnel-like access. Agricultural and domestic buildings, trees, quarries and cliff faces have all been recorded as roost/nest sites. Little Owls have even been found making use of subterranean sites such as rabbit warrens and tunnels in sand and gravel pits. Nestboxes of a specific design, which replicate the species' nest cavity requirements, have been used successfully in many areas. Little Owls almost never nest on open ledges in buildings, in Barn Owl nestboxes, or in any other type of cavity with a large entrance hole and almost always choose a narrow entrance (about 70 mm across) leading to a small cavity.
The Little Owl nesting season is much more restricted than that of Barn Owls. Although both species lay their eggs in April or May, Little Owls rarely have a second brood and lost clutches are often not replaced. Even when occupied, Little Owl nests can be very difficult to pinpoint. The presence of a nest is often given away by the repeated calling of the nestlings but it can be very difficult to work out exactly where the sound is coming from. Whereas the call made by Barn Owl nestlings is a repeated and sometimes forceful 'shshsh shshsh shshsh', Little Owl nestlings generally utter a much less strident 'ssssss ssssss ssssss'. Mikkola (1983: 127) describes it as a 'continuous wheezing'.
Compared to Barn Owls or Tawny Owls, Little Owls are very small – roughly half the length, height and wingspan – and are very active during daylight. On an overhead wire or fence post a Little Owl appears as a dark grey/brown white-speckled 'blob' about the size of two small fists, roughly similar in size to a Song Thrush Turdus philomelos or a Common Blackbird Turdus merula. Their feathers are predominantly brown, grey and speckled with white, and lack the golden buff tones of a Barn Owl's upperparts (see Plate 1).
Unlike Barn Owls they always hunt from a perch, typically going directly from a fence post to pick up a food item and returning with their distinctive undulating flight, although at night their flight may be more direct (Mikkola 1983). Adult Little Owl calls may be heard at any time of day or night and include a plaintive 'kiew, kiew' and a 'wherrow, wherrow' reminiscent of a small dog barking. Male territorial vocalisations start during late winter, often from a favourite perch, and can occur throughout the nesting season (March to June).
Little Owl diet is far more varied than the predominantly small mammal-rich diet of Barn Owls. In addition to rodents and shrews, prey often includes beetles, worms and other invertebrates. Small birds are an important supplement in the breeding season, as are frogs. Material evidence in the form of pellets is therefore quite different from Barn Owls. Little Owl pellets are approximately half the size at 20–40 mm long and 10–20 mm wide and frequently contain the shiny remains of beetle carapaces. They can very easily be confused with Common Kestrel pellets (see Plate 24). Little Owls are the only owl species known to intentionally include plant matter in their diet and consequently pellets may also contain grass and seeds.
There appears to be little or no interaction between Little Owls and Barn Owls. Records do exist of Barn Owls eating Little Owls (Mikkola 1983), but this is rare and there are plenty of examples of Little Owls and Barn Owls nesting successfully in close proximity to each other, even in the same building at the same time. There is virtually no competition for nest cavities because their requirements are very different. Although there is some overlap, their diets are very different and there is little direct competition for food.
In spite of the fact that Little Owls are very active in daylight, they are unlikely to be reported unless by people who observe birds that are nesting very close to where they live or work. To birdwatchers their undulating flight is quite distinctive, but to less interested people they are just another flying bird. Although Little Owls spend a lot of time on fence posts and telegraph wires, the casual observer is much more likely to notice one perched on a roof or perhaps a tree close to their house. Amongst the general public the term 'little owl' is often taken to mean an owl that is not yet fully grown. As a result, this species may often remain nameless or may be known by another name, such as 'small owl', 'skritch owl', or simply 'the ones you see during the day'.
1.1.2.2 Tawny Owls Strix aluco
See Plate 5 Tawny Owl portrait and Plate 6 Tawny Owl in flight at night
Tawny Owls are by far the most common and most vocal of all British owl species and their calls 'kewick kewick' and 'hooo hu huhuhuhooo' are very familiar in Britain (twit twoo). Because they are relatively common and because they are the owls that most people hear, a very basic knowledge of Tawny Owls is absolutely essential for anyone carrying out a Barn Owl survey. Like Barn Owls, Tawny Owls are medium-sized birds but, unlike Barn Owls, they generally go unnoticed as they tend not to fly around in the open, are almost entirely nocturnal and are much darker in colour.
The UK population is estimated at approximately 19,000 breeding pairs (Burfield & Bommel 2004) and they are very widely distributed across England, Scotland and Wales but absent from all of Ireland. Tawny Owls are woodland birds that have adapted to live virtually anywhere there are trees: city parks, wooded gardens, farmland with only hedgerow trees or copses, parkland, and of course dense woodland and forestry. Whereas Barn Owls occupy a vast and undefended home range (up to 5,000 ha), by contrast Tawny Owls are highly territorial within their relatively small territories of 12–20 ha (Southern 1970). Simple arithmetic suggests that one Barn Owl home range (in an open landscape with lots of trees) could contain over 200 Tawny Owl territories!
The lives of Tawny Owls are very orientated towards trees. They generally hunt from tree branches, roost in trees and nest in trees, whereas Barn Owls are birds of open country and rarely venture into dense woodland. Tawny Owls are thought to roost regularly in a favoured tree, close to the trunk, in thick cover such as ivy, or in a similarly concealed position. Nests are often in tree hollows, but a deep, dry cavity is evidently not essential as they also make use of the old nests of other species such as Carrion Crows Corvus corone, Eurasian Magpies Pica pica, Eurasian Sparrowhawks Accipiter nisus or Common Buzzards Buteo buteo (Mikkola 1983) or nest in the open fork of a tree trunk or side branch (see Fig. 1.1). More rarely, nests on cliff ledges have been recorded, as have more terrestrial locations, including amongst tree roots and in rabbit holes. The floor area of a typical nest cavity is remarkably small when compared to a typical Barn Owl nest, sometimes with only just enough room for the female to sit on her eggs. As Tawny Owls tend to have only two or three young which leave the nest long before they are full-grown, large cavities are not needed. Smaller cavities may also be easier to defend. The floor size of a typical Tawny Owl nestbox is only 20 × 20 cm.
In the course of well over twenty years fieldwork, involving many thousands of visits to potential Barn Owl sites while searching for signs of occupation and checking boxes, Barn Owl Trust staff have come across only two cases of a Tawny Owl nesting in a Barn Owl nestbox. The species' clear preference for trees is also demonstrated by the fact that there is only one record of Tawny Owls nesting in a bale stack and only a few records of them roosting in farm buildings. Although reports of Tawny Owls trapped in chimneys are received several times a year, it is clear that the vast majority of Tawny Owls live in trees.
Breeding behaviour commences around mid-winter with territorial vocalisations by the male (often 'hooo hu huhuhuhooo'), followed by duetting with the female (usually 'kewick kewick'), and this becomes progressively more centred around the nest site as spring approaches. Tawny Owls are an early-nesting species, with eggs laid typically in late February to early April and hatching about a month later. Tawny Owls rarely lay more than three or four eggs (maximum = six, mean = three) and almost never breed twice in a year. Food begging calls are similar to those made by Barn Owl nestlings, but somewhat higher and thinner – more of a 'tssssp tssssp tssssp' than a 'shshsh shshsh shshsh'. Unlike Barn Owls, Tawny Owl young typically leave the nest several weeks before fledging and go through a phase known as branching (climbing/jumping amongst tree branches) during which they are often discovered on the ground (see Chapter 9). The young remain dependent on their parents for up to three months after fledging. A secondary peak in vocalisation occurs in early autumn as juveniles disperse and attempt to establish their own territories.
