Bijdragen tot de Dierkunde, 60 (3/4) 163-170 (1990)

SPB Academie Publishing bv, The Hague

Species boundaries in non-tropical Northern Hemisphere Owls

K.H. Voous

Institute of Taxonomie Zoology, University of Amsterdam, P.O. Box 4766, 1009 AT Amsterdam,

The Netherlands

Keywords: Northern Hemisphere Owls, species boundary, species concept

will be made based North- Abstract attempt on non-tropical

ern Hemisphere owls on which a comprehensive

in non- book has been 1988). A survey is presented of the status of species boundaries published recently (Voous,

Northern Hemisphere owls in order to investigate the restricted to discussion of tropical The survey will be a

reality of the biological and geographicalspecies concept applied stages in the evolution of owl-species without mak- to these owls in current handbooks. At the same time the prac- ing special reference to expressions of doubt which ticability ofevolutionary systematics as opposed to phylogenetic in recent years have been raised against the objec- synthesis is elaborated on. of the tivity of the concept geographically polytypic

species. Résumé

2. On passe en revue la situation des limites entre espèces chez les Monotypic genera

Hiboux non-tropicaux du hémisphère Nord, afin d’examiner la réalité du et de concept biologique géographique l’espèce ap- Nyctea scandiaca - Snowy Owl - circumpolar arctic

circulation pliqué à ces oiseaux dans les traités en courante. tundra

D’autre on insiste sur la de la part, praticabilité systématique Surnia ulula - Northern Hawk Owl - circum-

à la évolutive, en tant qu’approcheopposée synthèse phylogéné- global boreal forest tique. Micrathene whitneyi - Elf Owl - Sonoran Saguaro desert

“Speotyto” cunicularia — Burrowing Owl - American grass-

lands and semi-deserts

1. Introduction

The taxonomie limits of species allocated to mono-

the and Accepting that according to biological ge- typic genera offer no problems. In addition to the ographical species concept reproductive behaviour differential nature of their morphological charac- and geographical variationare the keystones of spe- ters, theirspecificity is supported by theirlife styles cies formation processes in terrestrial animals which are directly related to their relatively uncom-

worthwhile (Mayr, 1953: 19), it seems to investigate plicated geographical and ecological distribution.

of boundaries in selected the status species taxo- The Snowy Owl is an arctic descendant of the Eagle nomie groups on which enough knowledge on be- Owls Bubo; the NorthernHawk Owl is a somewhat haviour and distribution is available. In it aberrant of either Glaucidium Ninox fact, ecotype or or

the of the in of means survey applicability nature a of both, and the Elf Owl is a minor deviation of still Years I have tried do important theory. ago to Pygmy Owl Glaucidium. this in relation to Northern Hemisphere ducks, The Burrowing Owl is a case by itself. Tradition- geese and swans (Voous, 1964); at present another ally placed in the monotypic genusSpeotyto with an 164 K.H. Voous - Species boundaries in non-tropical Northern Hemisphere Owls

exclusively American distribution, there has been a body length 46 cm) of more than 1 Vi recent tendency of incorporating this species in the times its size. This situation is realized in the island

World Athene 1988: of where small form Old genus (see Voous, 199). Hispaniola, a indigenous (T.

DNA-analysis seems to indicate, however, that a. glaucops ) seems to be forced to pair up with, or

relatives Burrowing Owls have no known close succomb under an alien giant recently arrived on its

bio- USA (C.G. Sibley, pers. comm.). Thus, providing own from the Bahama Islands or the (T. a. chemical similarity is the ultimateindicator of rela- pratíncola) with a wing length almost 1 Vi times that tionship, the Burrowing Owl should be considered of the indigenous Barn Owl. In this case one could to represent a monotypic genus. If placed in the ge- wonder indeed whether these Barn Owls will

ultimate terrestrial nus Athene, in which it is the manage to act as independently reproducing popu-

of the and that This view representative, the application biological lations, is, as two separate species. geographical species concept remains equally clear has been advocated by Olson (1978). It is more like- and unchallenged. ly, however, that in the long run one of the Barn

Owl types will oust the other (not necessarily the

smallerone) and that only one survives, best adapt- 3. Monotypic species ed the local conditions of habitat and food to sup-

ply, eliminating the local species problem in the Ketupa flavipes - Tawny Fish Owl - Sino-Himalayan sub-

Another similar situation occurs on Lord montane mountain streams process.

- Strix butleri - Hume's Owl Wadis in Middle East stone- Howe Island, halfway between Australia and New

deserts Zealand, where large Barn Owls from California

and smaller from Australia (T. a. pratincola) ones As in the monotypic genera the eco-geographical of (T. a. delicatula, body length about 72% the distributionsof these morphologically well-defined North American owls) have been introduced for ro- species are of a restricted nature and the species dent control, allegedly without interbreeding (see limits are unchallenged. Digging deeper into the Voous, 1988: 12). This, then, would be a case of a history of these species, the Tawny Fish Owl could first attempt to species duplication in an insular oc- be an ecoform of the early Asian fish owls Ketupa,

currence of the Barn Owl. and more specifically of the Malay Fish OwlKetupa Indeed, it is rumbling within the Barn Owl com- is ketupu, whereas Hume's Owl most probably a plex of forms. The North American Barn Owl is at desert derivative of early palaearctic Tawny Owls least 20% larger in size than the corresponding Eu- Strix aluco.

ropean form (T. a. alba). A reason for it has been

recently explained. Mammal prey supply in North

4. Holarctic distributions America includes larger species than in Europe. In

Europe no resident owl species in between the size (a) Holarctic species with wide global distributions of the Barn Owl and the Long-eared Owl Asio otus

Tyto alba - Barn Owl - near-cosmopolitan exists, whereas in North America the parapatric Asio flammeus - Short-eared Owl - semi-cosmopolitan Western and Eastern Screech Owls Otus kennicottii

The Barn Owl is the bird with the vir- among species and O. asio of intermediatesize are preying on widest Some 35-40 the same of the Barn global ranges. geographical tually types prey as Owl, press-

from the Barn Owl take the races are currently recognized. Apart a gliding ing to on average larger prey variation in darkness of the plumage and spotting and growing to larger size in the process. Thus, the of the underparts, body size is the most obvious ge- body weights, and therefore the prédation forces, ographically varying character. One may wonder of the Barn Owl and the Long-eared Owl in Europe whether bird of the each other than a tiny race inhabiting Curaçao relate to as slightly more 1:1, as in the South Caribbean (T. a. bargei, body length against almost 2:1 in North America. Would this

29 would cm) have a chance to survive when con- situation permit or obstruct a mutual reproductive

with Owls fronted a Barn Owl from the USA (T. a. behaviourof Old World and New WorldBarn

pratincola, - 1990 165 Bijdragen tot de Dierkunde, 60 (3/4)

underneath rather than in case these populations would ever meet? As it is barred longitudinally

is As there is biogeographical evi- unlikely that this will happen, the problem more striped (Eck, 1968). academic than real. dencethat boreal forests have been continuous over

of Another interesting point is the fact that differ- the Bering Sea area during one or more periods

of feather lice extension in the Pleistocene, connecting the ent groups parasitic Strigiphilus glacial

and northern of Asia and the (Mallophaga) occur on American, Australian parts America, conspeci-

the hand and of boreal forest owls is understanda- south Asian Barn Owls on one on ficity present

differences African, Middle East and European Barn Owls on ble (Voous, 1958). No behavioural are the other hand (Clay, 1966). This would indicate known to doubt this conclusion (see also Voous, that the geographical separation of these popula- 1988a on Asio otus).

could tions is of old date. However, one hardly

Holarctic of doubtful visualize where and when Old and New WorldBarn (c) species or species groups

taxonomic status Owls have been in contact with each other. Is this,

- - Owls and then, finally, a reason to split the Barn Owl into Otus scops/flammeolus Scops warm-temperate

mountain forests the more than one species? I think not. But con-

Glaucidium passerinum/ - Pygmy Owls - boreal and moun- cannot be either. specificity of the groups proved gnoma tain forests should realize that in than Instead, one more one Strix uralensis/ varia - Ural and Barred Owls - boreal

place the Barn Owl seems to be on the verge of and temperate forests bursting out of its species limits.

Old World In the Short-eared Owl no geographical differ- Though being a member of the mainly

have been between of owl" rather than of the New World ences apparent population type "scops

owls" der 1975; groups inhabiting the wide expanse of cold and "screech (Van Weijden, Hekstra,

the the FlammulatedOwl has temperate Eurasia and North America. Even 1982), so many morpho-

Short-eared Owls nesting in similar climates in logical, structural, behavioural and vocal charac-

North Ameri- South American mountains and plains, as well as teristics of its own that its status as a

endemic those in the tropical Galapagos islands, differ only can species is at present widely accepted

from those 1966: 240; Voous, 1988: very slightly in their plumage characters (Marshall, 53-58).

and World Northern inhabiting the Northern Hemisphere. As in addi- Though Old World New tion zoogeographical evidence exists in favour of Pygmy Owls differ in but few structural feather

extension of the their is different. the theory of a post-Tertiary range characteristics, feeding ecology

Short-eared Owl from North America into the The North American Pygmy Owl is semi-in-

the Old World Owl South American continent, there is no reason to sectivorous, whereas Pygmy

challenge the generally accepted species limits in probably hardly if ever catches insects or other ar-

this owl. thropods. This adds a distinct ecological asset to the

taxonomy of these owls which was lacking former-

Holarctic treated (b) species traditionally as one ly. In contrast to earlier views according to which species Old and New World Pygmy Owls were conspecific

Surnia ulula - Northern Hawk Owl - boreal forests (Voous, 1960), the New World Northern Pygmy

Strix nebulosa - Great Grey Owl - boreal forests Owl is now treated as a separate species G. gnoma,

- Asio otus - Long-eared Owl south boreal and tem- whereas the Old World Pygmy Owl G. passerinum perate forests is considered the most advanced northern member

Aegolius funereus - Tengmalm's Owl - boreal forests

of this otherwise mainly tropical group and the

World carnivorous of the In these species Old World and New races most exclusively species genus at have been traditionally recognized. In Surnia ulula that.

resemblance of Ural and Aegolius funereus the differencesare trivial. In On account of a superficial

Owls from Asia in and Barred Owls Strix nebulosa and Asio otus there is a parallel ten- east particular

World from North Otto Kleinschmidt dency of the New owls to be more clearly eastern America, 166 K.H. Voous - Species boundaries in non-tropical Northern Hemisphere Owls

(1934), originator of the concept of Artenkreise or evolutionary systematics.

superspecies, united these owls into one of his illus- To summarize this paragraph: species limits in

trated "geograms", whereas his admirer Siegfried northern owls with holarctic distributions have

Eck (1968) followed track by recognizing them as been traced easily on the basis of morphology,

This the geographical members of one species. vocalizations, habitat choice, food and geographi-

of former cal At the time intermediate in would indicate the theory a geographical history. same stages

the of formation have been connection or even an eco-geographical continuity. process species strongly

otherwise? Though a connection may have existed at some time in evidence. Could it have been expected

of in the early Pleistocene, the presence a third wood owl species, the Spotted Owl Strix occidenta- lis in mixed conifer forests in the Pacific Northwest 5. Complicated taxonomic situations in Old World of North America, disturbs the simplicity of Owls

Kleinschmidt's and Eck's views. The vocal qualities

*' of the Spotted Owl seem to resemble those of the Tyto capensis/longimembris

- Grass Owl - one or two Ural Owl more than do these of the Barred Owl. species?

*»»iOtus sunia/brucei/scops/senegalensis it is if unrealistic Evidently premature not to con-

- Owls - how Scops many species? sider one or the other of the North American wood **,Bubo bubo/bengalensis/ascalaphus owls as the American representative ofthe Eurasian - Eagle Owls - one or more species?

Ural Owl. **Athene noctua/brama

- Little Owl and Spotted Owlet - conspecifics?

* < Strix aluco/nivicola Holarctic treated (d) species group traditionally as — Tawny Owl - one or two species? two * < species Strix uralensis /davidi

Bubo bubo/B. virginianus - Eurasian Eagle Owl / Great - Ural Owl - one or two species?

* Horned Owl - all types of dense- Asio otus/abyssinicus

ly and sparsely forested regions - Long-earedOwl - one or two species?

In the cases indicated by (*), strict geographical The Eurasian Eagle Owl and the Great Horned Owl vicariance in basically similar habitats has been are each others taxonomie and ecological counter-

considered by some authors as convincing proof of parts. Body size and more strongly barred versus conspecificity. Others have used plumage differ- longitudinally streaked or marmorated underparts

ences and geographic separation, supposedly of are considered sufficient evidence for the recogni-

long duration, as a basis for the recognition of two tionof distinct New Worldand Old World species.

species. In the species marked (**), shared geo- The vocal performances of these owls seem to be the hand and insufficient the graphical history on one same. Geographical variation in the virtually have continuous of the Horned knowledge on marginal geographical overlap range Great Owl from led these species to be treated usually as one (Eagle Canadain the north to Tierra delFuego in the south

Owls Bubo) or as two species (Owlets Athene), but is considerable in (differences body size of up to opinions among authors differ. Future field work 20-30%), but does not provide a basis for specific

more than anything else will have to decide in these separations. Apparently tradition, rather than any- border-line the main cases. thing else, is reason for treating the Great The Owls them- Horned Owl and the Eurasian Scops (***) are a problem by Eagle Owl as distinct

selves. The ranges of the Oriental Scops Owl Otus species, unlike the situation in the Great Grey and suniaand theEuropean Scops Owl Otusscops seem Long-eared Owls. There is nothing to say against

to in and at these overlap Afghanistan western Mongolia different taxonomie decisions, as long as the

the birds occur in different habitats. and least, though reasons are recognized considered practicable.

Their songs are slightly differentin rhythm, but not Evidently the Great Horned and Eurasian Eagle

in quality. have been treated as but are Owls border-line in of They one, represent a case the sense - 167 Bijdragen tot de Dierkunde, 60 (3/4) 1990

The Striated of Western and Eastern now mostly consideredas two species. The recognition a species

brucei of Screech Owl of differentvocali- or Pallid Scops Owl Otus seems to differ (***)on account

and in from the sympatric European Scops Owl Otus zations an only marginal overlap Colorado

in both habitat arid forested and the Rio Grande on the Texas-Mexican scops (open versus along regions) and vocalization and there is at present border(Marshall, 1967) may have been accepted re-

is weak. sufficient positive evidence to treat these owls as cently, but the strength of the arguments

and African In the taxonomie and behavioural nature separate species. The European (O. addition,

South American senegalensis) Scops Owls differhardly in characters of the relation with Central and of plumage and structure, but decidedly in voice congenerics, including the Vermiculated Screech

Screech (Van der Weijden, 1973). The geographically inter- Owl (**) is uncertain. All over the world the mediate Arabian Scops Owl Otus (scops) pamelae Owls seem to be in a stage of hyperactive geograph-

as in the American seems to resemble the Eurasian and the African ic speciation. Similar questions

Owls be asked with reference the Scops Owls to an equal degree, but its vocalizations Screech can to

be- which of the and are a performance of its own, being halfway Pygmy Owls (**): subtropical

of African Owls Owls is the Northern Owl's tween the stuttering song Scops tropical Pygmy Pygmy

root descendant and thepurrof Oriental Scops Owls from Thailand nearest relative, either as a or a

(Marshall, 1978: 8; Voous, 1988: 38). Evidently the (Voous, 1988: 157-160). The answer depends on

studies of these owls and two options, either one Afro-European species or comparative life-history two allopatric species, have equal rights. The whole the reconstruction of their distributional history. species complex is in a state of decomposition rather than of geographic radiation. 7. Island distributions is In summary: the geographical species concept

of island practicable, but should be treated in close combina- Problemson the taxonomiestatus popula-

in Northern owls. tion with ecological and behavioural data, hence tions are scarce Hemisphere

Those involved the and extra-limital with caution, care and reason. are marginal

ranges of Scops Owls Otus in the Philippines and

Indonesiaand elsewhere in adjacent parts of the In- 6. Complicated taxonomic situations in New dian and Pacific Oceans. More particularly, they World Owls relate to the Mountain Scops Owl Otus spilocepha-

Oriental Owl Otus Collared ** lus, Scops sunia, Scops Tyto alba/glaucops/punctatissima Owl Otus bakkamoena and the south Asian Cuck- - Barn Owl - oneor more species?

* * * Otus£ kennicottii/asio oo Owlet Glaucidium cuculoides. There is atenden-

- Screech Owls - conspecifics? in literature to treat well-marked cy ornithological **(Otus guatemalae insular races as separate species, but in most cases -Vermiculated Screech Owl - conspecific relations? neither proof nor doubt can be raised as to the pos- ￿*

* < Strix varia/fulvescens lar or continental populations as required by the

- Barred and Fulvous Owls - conspecifics? biological and geographical species concept. Most

* Asio otus/stygius decisions therefore remain subjective, but in the

- Long-eared and Stygian Owls - conspecifics? case of the Scops Owls some have been based on

Different opinions on the significance of strict al- similarity of vocalizations (Marshall, 1978). lopatry have led to the recognition of either one or two species in the species pairs indicated with (*). 8. Vocalizations

None of these decisions seems to be more reason- able than the other. It has becomealmost a fashion to use vocalizations

The New World Barn Owl (**) has been dis- for setting taxonomie problems. More and more

voice is much cussed earlier; see section 4 (a). one seems to forget that as subjected 168 K.H. Voous - Species boundaries in non-tropical Northern Hemisphere Owls

to geographic variation as is any other variable West Indies, Brazil, New Zealand, etc.

(see Voous, 1988: 12) character in organic beings and occurring not un-

Bubo bubo - Eagle Owl, S. France (Mourer- frequently in birds at that. Thus, only those differ- Chauviré, 1975), Azerbaijan(Caucasus) ences that have been proved to represent an unsur- Bubo virginianus - Great Horned Owl, North America barrier in behaviour mountable reproduction can (Brodkorb & Mourer-Chauviré, 1984)

Differences - be considered as species-specific. that Nyctea scandiaca Snowy Owl, S. France (Mourer- can stand this test are fewer than often supposed. Chauviré, 1975)

Athene noctua - Little Owl, S. France (Mourer- the Collared There is therefore no need to split up Chauviré, 1975) Scops Owl into an Indian Scops Owl Otus bak- Strix uralensis/aluco - Ural and Tawny Owls, Strix intermedia: and Collared Owl Otus kamoena a Scops lempiji on central and S. Europe (Janossy, 1972;

voice differences advocat- reasons of geographic as Mourer-Chauviré, 1975)

otus - Long-eared Owl, America (see ed by Roberts & King (1986). Similar geographic Asio North Voous 1988: 261) variations have been noticed in the Oriental Scops

Owl Otus sunia in eastern Asia (see Voous, 1988:

35) and in the Western North American Screech

Owl Otus kennicottii (Marshall, 1967). Mixed pairs Virtually all of these owls are larger in size than the of Western and Eastern Screech Owls have been respective species from to-day and most of them described from the area of geographic contact of can be considered as chronological- or palaeo- these owls on the Rio Grande border between Mexi- subspecies or variations in time. Most probably the co and the USA (Marshall, 1967), suggesting that impoverished mammalian prey fauna and the song differences are not unsurmountable in general diminution of the size of mammals are reproductive behaviour. On the other hand, voice responsible for the decrease in body size in Recent rather than outward appearance is the communica- owl species (e.g. Bubo; see Voous, 1988: 86). This tion method most frequently employed by noctur- situation is also indicative of the fact that size alone nal animals, including owls. Therefore a compari- need not be a species-specific character, though it son of territorial songs in insular populations of might have been in cases of sympatry such as in the

Scops Owls in the Philippines and Indonesia has probably synchronic Tyto robusta and T. gigantea,

the latter than the been used with apparent success to unravel larger even present-day Eagle problems of taxonomy and relationship (Marshall, Owl Bubo bubo, from Upper Miocene deposits in

1978). The remarkably deep song of the Flammu- Italy (Ballmann, 1973, 1976) and in the wood owls lated Owl Otus flammeolus from western North during and after the process of species differentia-

America is so strongly differentfrom the song types tion of the Ural and Tawny Owls in Europe during of Old World Scops Owls that the once proposed the Middle Pleistocene (see Voous, 1988: 238).

interest conspecificity between these owls is now almost Of special is the presence of an extremely forgotten. long-legged and probably flightless Little Owl

Athenecretensis, not unlike an American Burrow-

ing Owl, from Pleistocene cave deposits on Crete

which may have lived alongside a Little Owl of re- 9. Species variation in time: owl fossils cent proportions (Weesie, 1982, 1987). This de-

velopment seems to have started at a time that the With thealmost explosive increase of discoveries of mammalianfaunawas more diverse and richer than avian fossils, theknowledge on extinct owls has im- today. The situation ended when the ecological in proved a surprising way. In the present context condition on the island of Crete grew less varied the selection of sub-Recent and Pleisto- following island. along with a gradual decrease in size of the cene owls be made. can On the data fossil whole, present on owls are not

in conflict with a - the of geographical Tyto alba Barn Owl, S. Europe, Israel, Mediterra- concept spe-

nean islands, North America, Mexico, cies changing in time and place. 169

- 1990 Bijdragen tot de Dierkunde, 60 (3/4)

10. Hybrid owls References

in between Vögel aus dem Neogen der Halbin- Hybrids have been raised captivity Ballmann, P., 1973. Fossile

sel (Italien). Scripta Geol., 17: 1-75. other Gargano among species: 1-59. Ballmann, P., 1976. Idem. 2. Scripta Geol., 38: Bubo bubo male Great (a) a female Eagle Owl X Brodkorb, P. & C. Mourer-Chauviré, 1984. Fossil owls from Zoo, England, HornedOwl B. virginianus (Dudley 55: Early Man sites of Olduvai Gorge, Tanzania. Ostrich,

Risdon, 1951) 17-27.

female Owl Strix aluco X male Ural Clay, T., 1966. The species of Strigiphilusparasitic on the Barn (b) a Tawny Owls J. Entom. Soc. Queensland, 5: 10-17. Owl Strix uralensis (Scherzinger, 1983). Tyto. Eck, S., 1968. Der Zeichnungsparallelismus der Strix varia. has fertile with The hybrid progeny proved par- Zool. Abhandl. Staatl. Mus. Tierk. Dresden, 29: 283-288. the close be- ent species, confirming relationship fertile Flieg, G.M., 1971. Tytonidae x Strigidaecross produces

these as in the previous 178. tween species suggested eggs. Auk, 88:

paragraphs. Hekstra, G.P., 1982. A revision of the American Screech Owls

(Otus). Doct. Thesis, Free Univ. Amsterdam. Most remarkable is a case of hybridization be- Janossy, D., 1972. Die mittelpleistozäneVogelfauna der Stráns- x tween a femaleStriped Owl Asio clamator male ká Skála. Anthropos, 20: 35-64. Barn Owl Tyto albain the St. Louis Zoo, Missouri, Kleinschmidt, O., 1934. Die Raubvögel der Heimat. Leipzig.

if not USA (Flieg, 1971). This suprageneric Marshall, J.T., 1966. Relationships of certain owls around the

suprafamilial mesalliance ended with the death of Pacific. Nat. Hist. Bull. Siam. Soc., 21: 235-242. Marshall, J.T., 1967. Parallel variation in North and Middle two embryos at the 15th day of incubation. The ex- American screech owls. Monogr. West. Found. Vertebr. traordinary event illustrates that genetical struc- Zool., 1: 1-72. theeffects of tures are overruled in nature by acting Marshall, J.T., 1978. Systematics of smaller Asian night birds boundaries. species based on voice. Orn. Monogr., 25: 1-58.

Mayr, E., 1953. Animal species and evolution. Cambridge,

Mass.

11. Conclusion Mourer-Chauviré, C., 1975. Les oiseaux du Pleistocène moyen

et supérieur de France. Doet. Thesis, Univ. Claude Bernard, like owls of in Even in a group which, comparison Lyon. details with other birds, relatively incomplete on Olson, S.L., 1978. A paleontologicalperspective of West Indian

Publ. Acad. Sei. Philadel- structure, distribution, habitat, and life style are birds and mammals. Spec. nat.

phia, 13: 99-117. available, the biological and geographical species

Risdon, D.H.S., 1951. The rearing of a hybrid Virginian x Eu- retained its value and concept has fully applicabili- Avicult. 57: ropean Eagle-Owl at Dudley Zoo. Mag., almost ty. As is to be expected in variable species, 199-201. it any border-line case is represented: is, by analyz- Roberts, T.J. & B. King, 1986. Vocalizations of the owls of the

if could observe the in Pakistan. Ornis Scand., 17: 299-305. ing these species, as one species genus Otus

Scherzinger, W., 1983. Beobachtungen an Waldkauz- in the process of changing in place and time. As a Habichtskauz-Hybriden (Strix aluco x Strix uralensis). Zool. matter of fact this does not include an insight in the Garten, (NF) 53: 133-148. in the direction of forma- mechanism, nor species Voous, K.H., 1958. European breeding birds of holarctic distri- and needed tion processes specializations. Having bution. Arch, néerl. zool., 13 (suppl. 1): 270-276.

and included more than solely quantifiable mor- Voous, K.H., 1960. Atlas of European breeding birds.

Edinburgh-London. phological data, the biological and geographical Voous, K.H., 1964. Stages of geographic species formation in contains and in species concept more assets, a more Univ. northern hemisphere ducks, geese, and swans. Free realistic way at that, than can be provided by any Quarterly, 9: 133-147. based cladistic theories. There is species concept on Voous, K.H., 1988. Owls of the northern hemisphere. London.

New Old no apparent need to elaborate on sister-groups, nor Voous, K.H., 1989. Vocal apparatus in World and

Studies in honour of Dr. Pieter and World Long-eared Owls. on such subjective concepts as apomorph Wagenaar Hummelinck. Pubis. Found, scient. Res. Surinam plesiomorph characters for arriving at a synthetic Neth. Antilles, 123: 29-31. dis- view on the dynamic nature of the owl-species Weesie, P.D.M., 1982. A Pleistocene endemic island form

cussed. within the Athene: Athene cretensis from genus n. sp. 170 K.H. Voous - Species boundaries in non-tropical Northern Hemisphere Owls

Crete. Proc. kon. ned. Akad. Wet., Ser. B) 85: 323-336. Weijden, W.J. van der, 1975. Scops and Screech Owls: vocal

Weesie, P.D.M., 1987. The quaternary avifauna of Crete, evidence for a basic subdivision in the genus Otus. Ardea, 63:

Greece. Doct. Thesis Univ. Utrecht. 65-77.

Weijden, W.J. van der, 1973. Vocal affinities ofthe African and

European Scops Owls. Bull. IFAN, (Ser. A) 35: 716-722. Received: 20 November 1989