THE USE OF SOUNDS IN BIRD SYSTEMATICS
PER ALSTRÖM
Introductory Research Essay No. 2.
Uppsala 2001
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The use of sounds in bird systematics
Per Alström
Department of Systematic Zoology, Evolutionary Biology Centre, Uppsala
University, Norbyvägen 18 D, 752 36 Uppsala, Sweden
E-mail: per.alstrom@ebc.uu.se
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The number of bird species in the world was estimated to be c. 8600 by Mayr (1946),
c. 9000 by Bock & Farrand (1980), and c. 9700 by Sibley & Monroe (1990). This
increase widely exceeds the number of newly described species in that time period.
Part of this increase can be attributed to the growing knowledge of bird vocalizations
in combination with the current trend to recognize allopatric taxa with distinctive
songs as species rather than as subspecies (Sibley & Monroe 1990, Parker 1991, Price
1996, Peterson 1998).
Although sounds have mainly been of use in the ranking of closely related
allopatric taxa, vocalizations have also been used to infer relationships, both within
and between genera, and have been crucial in the discovery of several new species.
I here review the use of vocalizations (and other acoustic signals) in systematics.
See also Payne (1986) for a thorough review and Morel & Chappuis (1992) for a
review of West African taxa.
The importance of vocalizations in the discovery of new species
Several new species have been discovered because of their distinct vocalizations.
Some of these escaped attention because they are highly secretive and difficult to see,
and others because they are sibling species which are morphologically similar with
other species. Several examples are given here.
A rallid heard in September 1997 on a steep mountain slope in primary forest in the
Talaud archipelago, Indonesia later proved to be an undescribed species, the Talaud
Bush-Hen Amaurornis magnirostris (Lambert 1998).
The existence of two new species of Andean pygmy-owls, Subtropical Pygmy-Owl
Glaucidium parkeri (Robbins & Howell 1995) and Cloud Forest Pygmy-Owl G.
nubicola (Robbins & Stiles 1999), was first indicated from tape recordings of their
voices.
The Cryptic Warbler Cryptosylvicola randrianosoloi was first detected in 1992 in
eastern Madagascar by its voice (Goodman et al. 1996) and was subsequently found
to be common (Morris & Hawkins 1998). The same applies to Jocotoco Antpitta
Grallaria ridgelyi from the Andes in southern Equador (Krabbe et al. 1999).
The Ancient Antwren Herpsilochmus gentryi was discovered by José Alvarez
Alonso and Bret Whitney when Whitney identified it as a new species from among
Alvarez’s unclassified tape recordings of unseen canopy birds from Peru (Whitney &
Alvarez Alonso 1998). They later managed to find it in the field (and to collect two
birds), confirming its distinctness.
Three new species of Scytalopus tapaculos from South America, Chocó Tapaculo S.
chocoensis, Chusquea Tapaculo S. parkeri (Krabbe & Schulenberg 1997) and
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Diademed Tapaculo S. schulenbergi (Whitney 1994), were discovered because their
songs differed from other known species.
The song of the Nepal Wren-Babbler Pnoepyga immaculata (Martens & Eck 1991)
was long thought to be a variant of the song of Scaly-breasted Wren-Babbler P.
albiventer. It was later realised that the individuals with this kind of song also differed
morphologically from Scaly-breasted Wren-Babbler, and the existence of a previously
unknown species was thus revealed.
Three previously unknown species of warblers were discovered in China during the
1990s as a result of their vocalizations. Emei Leaf Warbler P. emeiensis, restricted to
a small area in central China, was first noted because its song and call differed
markedly from two other sympatric, similar-looking species, Blyth’s Leaf-Warbler P.
reguloides and White-tailed Leaf-Warbler P. davison (Alström & Olsson 1995).
Vocalizations were also of paramount importance in the discovery of two new species
of Seicercus warblers, S. soror and S. omeiensis, and in the elucidation of a group of
sibling species (Alström & Olsson 1999, 2000, Martens et al. 1999; see below). Other
new warblers have been found because of their distinct vocalizations, e.g. Dorst’s
Cisticola Cisticola dorsti (Chappuis & Erard 1991) and River Prinia Prinia fluviatilis
(Chappuis 1974), both from West to Central Africa.
The Taiwan Bush-Warbler Bradypterus alishanensis was previously treated as the
subspecies melanorhynchus of Russet Bush-Warbler B. mandelli (=seebohmi), but
was described as a new species (Rasmussen et al. 2000a) when it was realised that its
song differed markedly from other populations of Russet Bush-Warbler (and that no
name was available for this population).
The discovery of the Rock Firefinch Lagonosticta sanguinodorsalis in Nigeria is a
remarkable story (Payne 1998). It was not discovered because of its own song, but
because of the song of another species. It was realised that the song of the Jos Plateau
Indigobird Vidua maryae differed from the songs of all other indigobirds, and it was
predicted that there must be an unknown firefinch song model in the area (indigobirds
are species-specific brood parasites that mimic the songs of their host species, mainly
firefinches Lagonosticta spp; e.g. Payne 1968, 1973a, 1973b, 1982, Payne & Payne
1994).
No less than three indigobirds, Jos Plateau Indigobird Vidua maryae, Goldbreast
Indigobird V. raricola and Barka Indigobird V. larvaticola, were discovered mainly
because of their songs (Payne 1982, 1998, Payne & Payne (1994).
At least two new species of corvids, Sinaloa Crow Corvus sinaloae from western
Mexico (Davis 1958) and Little Raven C. mellori from southern Australia (Rowley
1967a, b) were discovered because of differences in vocalizations from other species,
in the second case a sympatric species (Australian Raven C. coronoides).
The importance of vocalizations in the assessment of taxonomic rank
There are many cases, in a wide range of genera, where allopatric taxa have been
”upgraded” from the rank of subspecies to species, or even from colour morph to
species, because of differences in their acoustic signals. The converse is also rather
common. Some examples are presented here.
The North American Western Grebe Aechmophorus occidentalis and Clarks’ Grebe
A. clarkii were previously considered to be colour morphs of the same species
(Western Grebe A. occidentalis). However, Nuechterlein (1981) showed that the
”advertising call”, that is used in mate attraction, differs significantly in the two
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”colour morphs”, and that in an area of sympatry, both ”morphs” responded almost
exclusively to their own ”morph’s” call. This confirmed the studies of Storer (1965)
and Ratti (1979), which had shown strong assortative mating of the ”morphs”.
In many seabirds, especially those that breed in burrows and only visit their
breeding islands at night, females probably identify conspecific males by their
vocalizations, at least at long range (James & Robertson 1986, Bretagnolle 1990,
Bretagnolle et al. 1990, Bretagnolle & Robinson 1991, Bretagnolle1995). Studies of
sounds have sometimes revealed differences between populations that were
considered conspecific, and have been used in conjunction with other evidence to
show that these taxa are better considered to be separate species. The Antarctic Prion
Pachyptila desolata, Salvin’s Prion P. salvini, Broad-billed Prion P. vittata, Slenderbilled
Prion P. belcheri and Fairy Prion P. turtur have been treated differently by
different authors on the basis of morphological data. Bretagnolle et al. (1990) studied
these taxa on their breeding islands, where two to four occurred in sympatry. They
showed that their voices (as well as morphology, phenology of breeding and diet)
differed consistently, especially in sympatric taxa, and they therefore considered all to
be separate species.
Bretagnolle (1995) analyzed the vocalizations of Soft-plumaged Petrel Pterodroma
mollis from several different localities. Based on this (in conjunction with
morphological characters), he concluded that the Soft-plumaged Petrel should be split
into two species, one in the northern Hemisphere, P. feae, and one in the southern
hemisphere, P. mollis. (It should be mentioned that others, e.g. Bourne 1983, believe
that the North Atlantic taxa feae and madeira should be treated as separate species
based on morphological differences, although Bretagnolle 1995 remarked that their
calls were similar and overlapped.)
The Herald Petrel Pterodroma heraldica was formerly believed to have a darkbellied
and a pale-bellied colour morph. However, Brooke & Rowe (1996) noted
consistent differences in the vocalizations between pale and dark birds (especially in
the rate of delivery of the calls in a series). These differences, in combination with
evidence of assortative mating and lack of gene flow, led them to propose that the two
morphs are in fact separate species, Herald Petrel P. heraldica and Henderson Petrel
P. atrata.
The Least Tern Sterna antillarum was split from Little Tern S. albifrons primarily
based on differences in calls (Massey 1976).
The 22 species of megapodes Megapodidae currently recognized are little
differentiated in plumage, but markedly in bare part colours and proportions (Roselaar
1994, Jones et al. 1995). Although their vocalizations are imperfectly known, there
are ”minor differences between the races and sometimes marked ones between
species”, supporting the proposed classification (R.W.R.J. Dekker pers. comm. in
Roselaar 1994).
Even in groups such as bustards Otididae, that are not very vocal, voice has proved
useful in taxonomic assessments. It has been suggested that the Crested Bustard
Eupodotis ruficrista involves three allopatric species due to differences in
vocalizations and morphology (Chappuis et al. 1979, Morel & Chappuis 1992, Payne
et al. 1997). Gaucher et al. (1996) proposed that the Houbara Bustard Chlamydotis
undulata is better treated as two allospecies, C. undulata and C. macqueenii, due to
differences in ”display call” and courtship display (supported by morphological and
genetic differences).
Acoustic signals have comparatively rarely been used in wader taxonomy, despite
that most waders have distinct sound displays. The Amami Woodcock Scolopax mira
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is an exception. It was once treated as a subspecies of the Eurasian Woodcock
Scolopax rusticola, but Brazil & Ikenaga (1987) pointed out differences in (among
other things) vocalizations and the apparent lack of a display flight.
Thönen (1969), Olsson (1987) and Miller (1996) remarked that the ”drumming”
made by the tail-feathers during flight display differed between North American and
European populations of Common Snipe Gallinago gallinago. Based on this (in
combination with morphological differences and disjunct distributions), they
suggested that these should be considered separate species.
American Golden Plover Pluvialis dominica and Pacific Golden Plover P. fulva
were formerly treated as conspecific (under the name American Golden Plover P.
dominica). Connors et al. (1993) studied these taxa in an area of sympatric breeding
in western Alaska and showed that there were consistent differences in vocalizations
(both ”song”, alarm calls and other calls) and in habitat choice, and that mating was
assortative. Byrkjedal & Thompson (1998) came to the same conclusions. These
results supported the proposition by Connors (1983), based on a study of specimens,
that these taxa are separate species.
Miller (1996) noted differences in five parameters in the display vocalizations of
Common Ringed Charadrius hiaticula and Semipalmated C. semipalmatus Plovers,
while only very slight intra-taxon differences over large areas. These findings lent
support to the widely accepted notion that these taxa are best treated as separate
species.
Song characteristics have been used to re-estimate species limits in some cuckoos:
Asian Drongo-Cuckoo Surniculus lugubris and Philippine Drongo-Cuckoo S.
velutinus (Payne 1997); Horsfield’s Cuckoo Cuculus optatus (=horsfieldi) and
Oriental Cuckoo C. saturatus (Payne 1997); Common Cuckoo C. canorus and
African Cuckoo C. gularis (Payne 1986, Morel & Chappuis1992); and Hodgson’s
Hawk-Cuckoo Hierococcyx fugax and Philippine Hawk-Cuckoo H. pectoralis (Payne
1997).
In owls, voice has often been of major importance in the assessment of taxonomic
rank. The classic study by Marshall (1978) on small Asian owls, in which he
classified taxa with dissimilar vocalizations as separate species and, conversely, taxa
with similar vocalizations as conspecific, led to a multitude of taxonomic
rearrangements. For example, he suggested species status for no less than seven scops
owls (genus Otus) that were previously treated as subspecies. Other studies on the
voices of Asian scops owls have been important in resolving taxonomic matters, and
have further increased the number of recognized species (Roberts & King 1986,
Marshall & King 1988, Becking 1994, Lambert & Rasmussen 1998).
Another example is that all the taxa previously associated with Otus rutilus of
Madagascar have been shown to differ in voice (and morphology), and have been
suggested to be treated as separate species: O. moheliensis (Lafontaine & Moulaert
1998), O. capnodes (Safford 1993), O. pauliani (Herremans et al. 1991), O. [r.]
mayottensis (Lewis 1998), O. madagascariensis and O. rutilus (Rasmussen et al
2000b). Chappuis (1974–1985) and Morel & Chappuis (1992) suggested that
European Scops-Owl O. scops and African Scops-Owl O. senegalensis sould be
considered to be separate species based on differences in voice.
The Least Pygmy-Owl Glaucidium minutissimum, that is widely distributed in
South America, was formerly treated as a polytypic species. However, Howell &
Robbins (1995) analysed vocalizations and (in conjunction with morphology and
other evidence) suggested that it ought to be treated as four separate species.
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Vocalizations have been used extensively in taxonomic revisions of nightjars. For
example, Jerdon’s Nightjar Caprimulgus atripennis, Sulawei Nightjar C. celebensis
and Philippine Nightjar C. manillensis were all split from Large-tailed Nightjar C.
macrurus because of their distinctive vocalizations (Mees 1985, Ripley & Beehler
1987, Rozendaal 1990). Likewise, Tawny-collared Nightjar C. salwini, Yukatan
Nightjar C. badius and Silky-tailed Nightjar C. sericocaudatus were considered to be
specifically different due to differences in voice (Hardy & Straneck 1989).
Conversely, Dowsett & Dowsett-Lemaire (1993) pointed out that songs of Fierynecked
Nightjar C. pectoralis and Black-shouldered Nightjar C. nigriscapularis were
similar, and suggested that these should be treated as conspecific. They also showed
that the taxa ruwenzorii (Ruwenzori Nightjar), guttifer (Usambara Nightjar) and
poliocephalus (Abyssenian Nightjar), which have at one time or another been
considered separate species, are best treated as conspecific, Montane Nightjar C.
poliocephalus, due to basically similar vocalizations. However, Cleere (1995), also
using vocal characters, disagreed with this assessment.
Voice has been important in the assessment of species status of a taxon that is
believed to be extinct in the wild: Grayson’s Dove Zenaida graysoni from Socorro
Island south-west of Baja California (Baptista et al. 1983). It was established that its
voice (and visual display) differ significantly from the Mourning Dove Zenaida
macroura, with which it has often been considered to be conspecific. It was also noted
that it only rarely interbreeds with Mourning Dove in captivity.
Vocalizations have been used to assess the taxonomic rank in other doves. The
Grenada Dove Leptotila wellsi was shown to differ vocally (as well as
morphologically) from Grey-fronted Dove L. rufaxilla (Blockstein & Hardy 1989).
Playback tests were also of importance in this re-evaluation. Chappuis (1974–1985)
and Morel & Chappuis (1992) showed that the vocalizations of the morphologically
closely similar Eurasian Collared-Dove Streptopelia decaocto and African Collared-
Dove S. roseogrisea differ markedly, and proposed species status for those.
Pittas (Pittidae) have loud, relatively simple ”songs” that have been used (in
conjunction with especially morphology) to show that the mainly allopatric Fairy Pitta
Pitta nympha, Blue-winged Pitta P. moluccensis, Indian Pitta P. brachyura and
Mangrove Pitta P. megarhynchus are best treated as separate species (Lambert 1996,
Lambert & Woodcock 1996).
The Neotropical Tyrannidae include many species that are poorly differentiated
morphologically, and vocalizations have often been of great importance in the
recognition of species. Lanyon (1978) extensively used vocalizations and playback
tests in his monumental revision of the genus Myiarchus, because he was ”convinced
that the use of vocal characters, in conjunction with more conventional morphological
characters, would be the key to any successful attempt to determine specific limits and
relationships within the genus”. He proposed several taxonomic rearrangements based
on this research.
Willow Flycatcher Empidonax traillii and Alder Flycatcher E. alnorum were
formerly considered to be conspecific, but Stein (1958, 1963) showed that they
differed in vocalizations (and other aspects), that they did not respond to playback of
each other’s songs and were partly sympatric. Similarly, studies of vocalizations
(together with morphology and allozymes) showed that the Western Flycatcher of
western North America was in fact two separate, partly sympatric, species: Pacificslope
Flycatcher E. difficilis and Cordilleran Flycatcher E. occidentalis (Johnson
1980, Johnson 1994).
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The species in the South American suboscine genus Scytalopus (tapaculos) are
extremely similar in plumage and structure and are very secretive and difficult to
observe (e.g. Fjeldså & Krabbe 1990, Ridgely & Tudor 1994). The former
classification was based on comparative studies of museum specimens (Zimmer 1939,
Peters 1951). A review by Krabbe & Schulenberg (1997) using vocalizations (in
combination with morphology and distribution) led to a literal ”explosion” of species:
from 11 recognized by Zimmer and Peters to no less than 37. Three of these, Chocó
Tapaculo S. chocoensis, Ecuadorian Tapaculo S. robbinsi and Chusquea Tapaculo S.
parkeri, were new to science, and an additional two or three were considered to be
undescribed. Several of the species were shown to be sympatric. Krabbe &
Schulenberg classified allopatric taxa with unique songs as species, an approach that
was supported by DNA data presented by Arctander & Fjeldså (1994).
The use of vocalizations has led to the recognition of many South American
Thamnophilidae and Formicariidae as species. For example, Isler et al. (1997)
suggested that the widely distributed polytypic Slaty Antshrike Thamnophilus
punctatus is better treated as at least six separate allospecies; Isler et al. (1999) argued
that Myrmotherula surinamensis should be treated as four species; and Whitney et al.
(2000) concluded that the Herpsilochmus pileatus complex consists of three species,
of which one, H. Sellowi, was previoulsy undescribed because it had been confused
with pileatus for almost a century.
Isler et al. (1998) analyzed vocalizations of eight syntopic, similar-looking and
similar-sounding antbirds (Thamnophilidae). Based on this, they proposed that when
deciding the rank of allopatric antbird taxa, three diagnosable vocal characters (the
minimum number that distinguished the syntopic pairs in the study) should be used as
a point of reference. They recommended that for taxa that are very poorly
differentiated in other respects, more than three vocal characters are required to allow
classification as species, whereas for taxa that differ strongly in other ways, fewer
vocal characters may suffice.
The Bengal Bushlark Mirafra assamica was previously treated as a polytypic
species ranging from India to Sri Lanka and Vietnam. However, Alström (1998)
showed that there are pronounced differences in songs, calls and song-flights (as well
as morphological differences, and in one case in habitat) between four allopatric taxa.
Based on this, he proposed that they be treated as four separate species.
Bicknell’s Thrush Catharus bicknelli has received much interest lately, because it
has been shown to differ from Grey-cheeked Thrush C. minimus (with which it was
formerly considered conspecific) in a number of aspects, including song (and lack of
response to playback to two other subspecies of Grey-cheeked Thrush – minimus and
aliciae) (Ouellet 1993).
Old World warblers are renowned for being morphologically poorly differentiated,
though the different species usually differ more clearly by their songs. This was noted
more than 200 years ago, when White (1789) remarked that ”I have now, past dispute,
made out three distinct species of the willow-wrens (motacillæ trochili) which
constantly and invariably use distinct notes.” He was referring to Willow Warbler
Phylloscopus trochilus, Common Chiffchaff P. collybita and Wood Warbler P.
sibilatrix, of which only the first one had been named. In fact, in the genus
Phylloscopus (leaf warblers) in Eurasia excluding the Philippines, Greater Sundas
and Wallacea, the number of recognized species has gone up by 31% in the last
decade, and in all except two species, songs were important in the assessment of their
taxonomic rank (Irwin et al. 2001). Two examples are given below.
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Pallas’s Warbler Phylloscopus proregulus used to be considered a wide-ranging
polytypic species, breeding in Siberia, northern Mongolia and north-east China
(proregulus); central China and the Himalayas west to central Nepal (chloronotus);
and western Himalayas (simlaensis). A fourth taxon, kansuensis, from northern
central China, was treated as a synonym of either proregulus or chloronotus. Alström
& Olsson (1990) proposed that proregulus and chloronotus/simlaensis should be
treated as two separate species based on pronounced differences in vocalizations and
lack of response to playback of each other’s songs. Alström & Olsson (1997) pointed
out that also kansuensis differed much in vocalizations from the others, and did not
respond to playback of song of these, and concluded that it ought to be treated as a
separate species. In addition to these splits, Alström et al. (1992) found a species with
unique vocalizations that was sympatric with chloronotus in central China (differing
also in morphology, habitat choice and nest site). It was described as a new species,
Chinese Leaf-Warbler P. sichuanensis, though it was later realised that this name was
predated by yunnanensis (Martens & Eck 1995, Alström & Olsson submitted).
The taxonomy of the Common Chiffchaff Phylloscopus collybita complex has
received much attention in recent years. It was formerly considered to be a single
polytypic species, though extensive studies of its vocalizations and other data (e.g.
Martens & Hänel 1981, Martens 1982, Salomon 1987, 1989, 1990, Martens &
Meincke 1989, Helbig et al. 1996) have led to the suggestion that at least four species
ought to be recognized: Common Chiffchaff P. collybita, Iberian Chiffchaff P.
brehmii, Canary Islands Chiffchaff P. canariensis, and Mountain Chiffchaff P.
sindianus. The relationships of the Siberian taxon tristis were unresolved (Helbig et
al. 1996). These species are allopatric, except for Mountain Chiffchaff and Common
Chiffchaff which occur together in western Asia, and the latter meets Iberian
Chiffchaff in a narrow zone in the Pyrenees. Hansson et al. (2000) showed that
Swedish populations of Common Chiffchaff of the subspecies collybita and abietinus
responded more strongly to song of their own than to the other subspecies, and in
conjunction with e.g. differences in habitat choice, they predicted that there would be
only limited hybridization if these taxa met in the future.
The Golden-spectacled Warbler was until recently treated as a single polytypic
species, Seicercus burkii, with a wide distribution in mountains of southern Asia
(mainly the Himalayas and China). Alström & Olsson (1999, 2000) and Martens et al.
(1999) unravelled that this is actually a complex of no less than six sibling species,
with up to four species occurring at different altitudes on the same mountain (two of
these, Seicercus soror and S. omeiensis, were previously undescribed; see above).
Differences in vocalizations and playback tests were of major importance in the
elucidation of this situation.
Song has been used extensively in the classification of African cisticolas Cisticola
spp., both to split and to lump taxa (e.g. North 1964, Chappuis 1974, 1980, Erard
1974, Morel & Chappuis 1992).
Keith & Gunn (1971), Chappuis (1974–1985), Chappuis (1980) and Morel &
Chappuis (1992) used vocalizations to review the taxonomy of some Apalis Apalis
spp. species.
One remarkable case where cryptic species were revealed by their vocalizations is
the study of North American Common (Red) Crossbill Loxia curvirostra sensu lato
by Groth (1988, 1993a, 1993b). He (1988, 1993a) studied a large number of
individuals from across the continent, and correlated sonagrams of calls with
measurements of the same individuals. Based on these variables, the birds clustered in
eight different groups. Several of these are sympatric, e.g. six in the Pacific
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Northwest. Strong assortative mating was shown to occur in two different populations
in the Appalachians (1993b). He concluded (1993a) that ”L. curvirostra is a group of
sibling species” but, due to the morphological similarity and overlap in
measurements, he was unable to assign names to all of these species. The American
Ornithologists’ Union (1998) interpreted his results as the existence of at least nine
different species in North America. Studies of vocalizations by Robb (2000) have
suggested that there may be cryptic species of crossbills also in Europe.
One of the most amazing discoveries involving vocalizations concerns the African
indigobirds Vidua (e.g. Nicolai 1964, Payne 1968, 1973, 1976, 1982, 1990, 1998,
Payne & Payne 1994, 1995, Payne et al. 1992, 1993). The males of these speciesspecific
brood-parasites mimic the songs of their host species (mainly firefinches
Lagonosticta spp.), and the females mate with indigobirds that sing the songs of their
hosts (and lay their eggs in the nests of their hosts). Although most indigobird species
are morphologically poorly differentiated, their songs are often markedly different.
This insight has led to the recognition of several ”forms” as distinct species (all 10
indigobird species now recognized were at one time or another considered to be either
subspecies or colour morphs of Village Indigobird V. chalybeata, or overlooked).
Another fascinating result of this research is that there is strong evidence that new
species of indigobirds may evolve sympatrically as a result of accidental hostswitching
(Payne & Payne 1994, 1995, Payne et al. 1993, 1998, 2000, Klein & Payne
1998).
Crows Corvus are morphologically relatively poorly differentiated, although their
voices are often clearly different. Vocalizations have been of major importance in the
classification of the North American species American Crow C. brachyrhynchos,
Northwestern Crown C. caurinus, Tamaulipas Crow C. imparatus, Sinaloa Crow C.
sinaloae and Fish Crow C. ossifragus (Brooks 1942, Davis 1958, Hardy 1990).
Use of vocalizations in inferring relationships
Although vocalizations have mainly been used to answer questions of species status,
some authors have used voice to judge relationships among species. In a few cases,
features of songs and calls have been used as characters to infer phylogenetic
relationships.
Bretagnolle (1995) compared the vocalizations of several different Pterodroma
species and drew conclusions about their relationships based on
similarities/dissimilarities between them.
The calls of Anatidae ducklings were analysed by Kear (1968), who concluded that
they had phylogenetic information. The shape and frequency range of the distress call
tended to be similar in closely related species and to be more divergent in more
distantly related ones. For example, she remarked that the call of White-backed Duck
Thalassornis leuconotus, whose taxonomic position had been in doubt, was ”very like
those of Dendrocygna and quite unlike the distress call of Oxyura.”
Andersson (1973, 1999) studied calls and displays of skuas (Stercorariini) and
concluded that some of these are synapomorphies for Great Skua Stercorarius skua
and Pomarine Skua S. pomarinus – supporting the controversial, though now wellsupported
(see Andersson 1999), view that the latter is more closely related to the
large skuas (which are often placed in the genus Catharacta) rather than to the smaller
species Arctic S. parasiticus and Long-tailed Skua S. longicaudus.
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Miller (1996) used characteristics of nuptial vocalizations to infer relationships
among Pluvialis plovers and some Calidris sandpipers. He also concluded that
”acoustic characters seem to have great potential for resolving species relationships at
various levels” in Gallinago snipes and Charadrius plovers.
Acoustic data was used by Winkler & Short (1978) to infer relationships among
pied woodpeckers (Picoides/Dendrocopos). In many cases, their analysis corroborated
previous studies (e.g. the probable monophyly of the New World group). In other
cases the vocal data was in conflict with other evidence (e.g., Middle Spotted
Woodpecker D. medius was considered to be more distantly related to White-backed
Woodpecker D. leucotos than previously thought, and the same applied to Blackbacked
Picoides arcticus and Three-toed P. tridactylus Woodpeckers).
Vocalizations were used to resolve relationships of some antwrens in the genus
Myrmotherula (Whitney & Pacheco 1997).
Songs, calls and display flight were used in addition to other data to show the close
relationship between Berthelot’s Pipit Anthus berthelotii, endemic to the Canary
Islands and Madeira, and Tawny Pipit A. campestris (Alström & Mild 1993; later
corroborated by molecular data by Arctander et al. 1996 and Voelker 1999).
Based partly on vocalizations, Whitney (1992) suggested that Thamnomanes
occidentalis be placed in the genus Dysithamnus instead. Whitney & Pacheco (1994)
also used vocalizations in discussing the affinities of the little-known monotypic
genera Gyalophylax and Megaxenops.
King (1989) showed that the different species in the genera Tesia and Urosphena
clustered in two groups according to characteristics of their songs. Based on this (in
conjunction with other behavioural and morphological differences) he proposed a new
classification of these genera.
Helbig et al. (1996) used characters of songs and calls to study the relationships
within the Common Chiffchaff complex. There was some conflict between this tree
and the tree derived from their analysis of molecular data, especially with respect to
the Siberian taxon tristis.
The relationships of various treecreepers Certhia spp. have been discussed based on
their vocalizations (Martens 1981, Martens & Geduldig 1988), and the affinities of
Brown Creeper Certhia americana to Short-toed Treecreeper C. brachydactyla and
Eurasian Treecreeper C. familiaris have recently been studied using sounds (Baptista
& Krebs 2000).
The Black-collared Bulbul Neolestes torquatus from the Afrotropics has variously
been treated as a bulbul (Pycnonotidae) or a ”shrike” (Malaconotidae, Laniidae or
Prionopidae). A recent investigation (Dowsett et al. 1999) used vocalizations
(together with morphology and DNA) to show that it is not a shrike, but most closely
related to bulbuls.
Güttinger (1978) used characteristics of song to show that three species of
greenfinches (European Carduelis chloris, Yellow-breasted C. spinoides and Greycapped
C. sinica) are more closely related to each other than to two (sub)species of
goldfinch (European C. c. carduelis and Grey-capped C. [c.] canis), and that the
morphological similarity between the Grey-capped Greenfinch and the Grey-headed
Goldfinch are due to parallelism rather than to common ancestry.
Payne (1986) stated that ”similarities in song quality may express genetic
similarities” even in species in which song is learned, and accordingly may be of use
in phylogenetic analyses. He used vocal characters to reconstruct the phylogeny of the
Black-throated Green Warbler Dendroica virens complex. Since his tree was largely
congruent with a previous hypothesis of relationships (Mengel 1964), he concluded
10
that ”the distribution of song traits among species indicates that cultural changes may
have followed the same branching events as in the genetic differentiation of the
species”.
Characteristics of song were also used by Stein (1968) to analyze relationships
among North American Vermivora warblers.
Discussion and conclusions
As has been shown above, acoustic signals have been of great use in a wide range of
birds in (1) the discovery of new species, (2) assessment of taxonomic rank of
allopatric taxa under the biological species concept (Mayr 1942) and (3) in
phylogenetic analyses.
The importance of vocalizations in the discovery of new species is widely
acknowledged, and it seems likely that more sibling species will be discovered in the
future as a result of thorough analyses of vocalizations, especially in geographical
areas that have been poorly surveyed. However, it seems unlikely that the discovery
of new species will substantially increase the total number of bird species.
In contrast, the growing knowledge of the vocalizations of different taxa (which is
largely due to the increased use of tape recorders and sound analysis software and the
more easy travelling in recent years), in combination with the current trend to afford
species status to allopatric taxa with distinctive vocalizations, will probably lead to a
continued steady increase in the number of recognized species. A large proportion of
all bird taxa (estimated at 27,000–28,000 by Mayr & Gerloff 1994) is poorly known
with respect to their vocalizations, and the taxonomic status of many of these species
will undoubtedly be re-evaluated when their voices become better known. It is thus
vital that more taxa are tape recorded, especially those that are currently treated as
subspecies, and that recordings are properly documented, curated and made
accessible.
Acknowledgements
I am indebted to Claude Chappuis, Robert B. Payne, Fredrik Ronquist and Bret
Whitney for their valuable comments on the manuscript, to Vincent Bretagnolle,
Robert J. Dowsett, Alain Guimond, Alan Knox, Lars Larsson, Pamela C. Rasmussen
and Kees Roselaar for suggesting some useful references, or for checking certain parts
of the text, and to Effie Warr for much help with references throughout the
preparation of this paper.
-
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