Owls of Europe 9781472597175

Citation preview

Introduction

Owls are successful birds and they have dispersed to all continents except Antarctica and some remote ocean islands. The great majority of species occur in the tropics and subtropics. North America and the Palearctic zone of the Old World (Europe, Asia, except the southern-most tropical parts, and northern Africa) are inhabited by 33 species of owls, of which eight species occur both in North America and Eurasia. Thirteen regularly breeding species of owl are known from Europe, ten of which are on the British list. The basic aim of the book has been to review the available knowledge of the 13 owl species of Europe, and four additional species from countries adjoining the Mediterranean, some of which species may occasionally occur in Europe. This work, therefore, deals with 17 species out of the 134 world species at present known. The sequence and scientific nomenclature of Professor K. H. Voous in his List of Recent Holarctic Bird Species (1977) has been followed with some minor differences in English vernacular names. To avoid obvious confusion the older name of African Marsh Owl is preferred to Marsh Owl and Hume's Owl seems more suitable than Hume's Tawny Owl. In his article 'More about Desert Liliths: Hume's Tawny Owl' in the journal Israel Land and Nature, summer 1977, M. C. Jennings suggested that Hurne's Tawny Owl should be called Desert Tawny Owl, but this name, unfortunately, gives the impression that this very distinctive owl is no more than a desert form of the Tawny Owl, as I wrongly suggested myself in Owls of the World in 1973. The main body of the book consists of species descriptions, and preceding these are general summaries covering the most important features that distinguish owls from other groups of birds. The 17 species accounts each contain the following sections: Description; In the field; Voice; Behaviour; Food; Breeding biology; Distribution. The distribution of each species is shown by two maps, one for Europe, North Africa and the Middle East, the other for the world. Whilst I have tried to present as accurately as possible the current state of the existing knowledge of west Palearctic owls' ranges, it should be remembered that many species, such as Great Grey, Snowy, Hawk, Tengmalm's and Short-eared Owl, tend to be nomadic, nesting where food animals are most abundant. The maps for such species are, therefore, only a guide based on nests discovered in recent years, though they should give a good indication of the regions normally inhabited by each species. Part three of the book deals primarily with ecological relationships among European owls, and attempts to answer the question of how different owl species manage to live alongside each other. This question can be raised in a more general way: why are there only thirteen regularly breeding owl species in Europe? This has been and still remains a complex ecological problem in which many more questions may be posed than can be answered. In this part I have, however, tried to explain the complex interactions between predators and their prey. I have also included an analysis of sexual dimorphism, which is obviously a morphological isolation mechanism to reduce possible intraspecific competition. Part three also deals with the interspecific relationships among European owls and other birds of prey, including all

PART I

SPECIAL CHARACTERISTICS O F OWLS

I:

The origin of owls

Birds are poorly represented as fossils, compared with fish, reptiles and mammals, partly because bird bones are thin-walled, hollow, fragile and easily destroyed by predators or scavengers. Often they are so badly damaged during the process of fossilisation that they cannot be identified. That is why palaeornithology remains one of the most neglected of natural history subjects. However, we know that in geological terms, birds are relative newcomers to the earth's fauna. Archaeopteryx lithographica, the first known birds, appeared in the Upper Jurassic about 170 to 160 million years ago, but it is not possible to say how soon after this some sort of owl evolved. Rich and Bohaska (1976) recently published their discovery of what they believed to be the oldest known owl, Ogygoptynx wetmorei, which lived in North America during the mid-Paleocene (about 60 million years ago). This owl clearly does not belong to any of the known families of Srrig$onnes and may represent a new higher category of owls that provides a link between the Strigidae and the Tytonidae. However, Walker (1973) has suggested that, in Europe, the modern families of owls or owl-like birds had already evolved during the latter half of the Mesozoic era, between 135 and 70 million$ years ago. At least two tarsometatarsi found in the Upper Cretaceous deposits of Rumania have Strigifm characters. If this identification is correct, it will prove to be the earliest known owl in the world. There is good evidence that between 70 and 50 million years ago, when southern England and parts of France were covered with sub-tropical or tropical forest, and temperate plants grew in what is now the Arctic, a larger number of owl species lived

2:

Taxonomy

Although the members of the Sm'gifonnes may be easily distinguished from those of other orders, the familial, subfamilial and generic relationships within the order are much less certain. Owl taxonomy is currently in a state of flux. For instance, Norberg's (1977, 1978) studies on the occurrence and evolution of bilateral ear asymmetry in owls will almost certainly bring about some revision. Peters' (1940) owl classification is the most widely used. He separated modern owls into two families: the typical owls, the Strigidae, and the barn owls, the Tytonidae. Tytonidae The skull is long and narrow, the orbits small and the furcula unpneumatic (Fig. 2). The rear edge of the sternum has one pair of notches. The bill is elongated and relatively slender. The coccygeal gland is plumose. The ears are symmetrical in shape and size, but the left one, together with the skin flap in front of it, is located higher than the right ear-hole and skin flap. There does not seem to be any individual difference in this asymmetry (no left- or right-eared owls) and it is restricted to the ear-opening without affecting the middle and inner ears. The second and third toes are of equal size. The claw of the middle toe is serrated along the inner edge and the tarsometatarsus is elongated. Postembryonic development consists of two downy plumages, as in diurnal raptors, but a mesoptile stage is lacking. The eggs are elongated. The Tytonidae is subdivided into the Tytoninae and the Phodilinae. In the Tytoninae there is one genus Tyto consisting of eight species, of which only one, Tyto alba, occurs in Europe. The Phodilinae has one genus Phodilus which contains two species, one occurring in south-east Asia and the other in Africa. The last taxon has been the source of debate for many years, with some authors choosing to ally Phodilus with the strigids rather than the tytonids. Recently Feduccia and Ferree (1978) have shown similar derived bony stapes (Columella) occurring in Tyto and Phodilus supporting the concept of their close affinity. Strigidae The skull is round, the orbits large, the furcula is pneumatic and the rear edge of the sternum has two pairs of notches (Fig. 2). The bill is strong, short and sharply decuwed. The coccygeal gland is naked. The ears are frequently asymmetrical in shape and size, the right ear being larger than the left (see Fig. l), with the asymmetry also extending to the bones of the skull. However, Norberg (1978) has stressed that the outer ears are perfectly symmetrical in the majority of Strigidae genera, and that an asymmetrical arrangement is known to involve parts of the skull in four species only: Ural Owl, Great Grey Owl, Tengmalm's Owl and Saw-whet Owl Aegolius acadicus. Moreover, both the Tyto and Pholidus genera of the Tytonidae show a clear bilateral asymmetry of the external ears. It must be concluded that these parts of the owl's skull are unreliable factors in separating the Strigidae and the Tytonidae. The third toe is longer than the second, with smooth cutting edges to the claw of.the