J. Zool., Lond. (2001) 254, 207±218 # 2001 The Zoological Society of London Printed in the United Kingdom
A comparative analysis of the avifaunas of different
zoogeographical regions
I. Newton and L. Dale
Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE 17 2 LS, U.K.
(Accepted 12 July 2000)
Abstract
This paper provides a comparison of the landbirds of all main zoogeographical regions, based on the most
recent (Sibley±Monroe) listing and classi®cation of the world's birds. This classi®cation arranges 9416
landbird species (i.e. excluding seabirds) into 2002 genera, 140 families and 23 orders. On this basis, the
Neotropical region holds 36% of all known landbird species and 45% of genera, the Afrotropical region
holds 21% of species and 24% of genera, the Indomalayan region 18% of species and 22% of genera, the
Australasian region 17% of species and 23% of genera, the Palaearctic region 10% of species and 14% of
genera and the Nearctic region 8% of species and 15% of genera. These major continental regions thus
show 4.6-fold variation in species numbers or 9.1-fold variation in species numbers per unit area. The
region of Oceania, comprising many Paci®c Islands, holds only 2% of the world's bird species and 4% of
genera. About 92% of all bird species on the continental parts of the Neotropical, Afrotropical and
Australasian regions are endemic to those regions, compared to 64% of the Indomalayan, 54% of the
Nearctic and 46% of the Palaearctic species. The Oceania region has the smallest number of endemics, but
these form 87% of all species occurring naturally in this region. About 91% of all landbird species breed in
only one zoogeographic region, another 8% in two regions, with the remaining 1% in three to seven
regions. Only four species breed in all seven regions. Similarities in the species composition of different
regions were compared using Jaccard and Simpson indices. As expected, each region shares the greatest
number of species with the closest other region and the fewest species with the most remote region. As in
previous analyses, the Neotropical and Australasian regions emerged as having the most distinctive
avifaunas. Regions that hold large numbers of landbird species also hold large numbers of genera and
families, as well as high species-per-genus and species-per-family ratios. Comparable levels of diversity thus
extend through all these taxonomic categories. This implies that, whatever factors have promoted
particular levels of avian diversity in the different regions, they are of long standing. As found in previous
studies, species-per-genus and species-per-family ratios are lower in island than in continental avifaunas.
No relationship is apparent between the size of each zoogeographical region and the numbers of species,
genera and families found there; rather those regions with tropical forest have many more bird taxa overall
than those without.
Key words: avifaunas, biogeographic regions, biodiversity, island birds, species numbers, zoogeography
INTRODUCTION
The species numbers, distributions and phylogenetic
relationships of birds are probably better known on a
world scale than those of any other group of organisms.
This permits widescale biogeographical analyses to be
conducted for birds with greater precision and reliability
than is possible for most other types of organisms.
Beginning with Sclater (1858) and Wallace (1876),
attempts have been made to compare the avifaunas of
the different zoogeographic regions, both in terms of
species numbers and of species similarities and differences. Some of the most recent comparative accounts
include those of Mayr (1994) for the Neotropical and
Nearctic regions, Keast (1972) for the Australasian,
Afrotropical and Neotropical regions, and Udvardy
(1958) for the Nearctic and Palaearctic regions. Several
other authors have provided assessments of the bird
faunas of individual regions (e.g. Vaurie (1959, 1965)
for Palaearctic, Mayr & Short (1970) for Nearctic,
Inskipp, Lindsey & Duckworth (1996) for Indomalayan,
Moreau (1966) and Dowsett & Forbes-Watson (1993)
208
I. Newton and L. Dale
Palaearctic
Palaeartic
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O
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Af
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In
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ay
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Antarctic
Antartic
Fig. 1. Main zoogeographical regions used in this avifaunal analysis. From various sources.
for Afrotropical, Ridgely & Tudor (1989, 1994) for
Neotropical), or for parts of regions, as in many arearestricted checklists and handbooks.
In this paper, we provide a comparative analysis of
the landbirds of all the different zoogeographic regions,
using the most recent listing and classi®cation of the
world's bird species (Sibley & Monroe, 1990, updated
version by Sibley 1996 on CD-ROM), and also calculate
statistical measures of regional similarities. For the
breeding birds of each region, comparisons are made at
four different taxonomic levels, of orders, families,
genera and species.
The species list of Sibley & Monroe (1990) was
arranged under the classi®cation of orders and families
produced by Sibley & Alquist (1990), based mainly on
the method of DNA±DNA hybridization. For the lower
taxonomic levels of genera and species, Sibley &
Monroe (1990) used as a basis the most recent available
revision of each group. However, no analysis of this
type can ever be de®nitive, because of the continual
(slow) discovery of new species, and more particularly
because of the continual taxonomic revision and regrouping of already known forms. Aspects of the
Sibley±Alquist classi®cation have been challenged, and
some groups have been subject to more recent revision.
Nevertheless, we have opted to stick with the Sibley±
Monroe list because the CD-ROM version is still the
most recent and complete, because it applied similar
taxonomic criteria throughout, and because some of the
initially controversial ®ndings in the Sibley±Alquist
classi®cation have already been supported by subsequent work. Moreover, any modi®cations to bird
classi®cation that might result from future work, while
they might require some changes to the ®gures in our
tables, are unlikely to greatly alter the main conclusions.
Our analysis is concerned only with landbirds (including both terrestrial and freshwater forms, but
excluding marine forms), a total of 9416 species arranged in 2002 genera, 140 families and 23 orders. For
these birds, we calculated the number of species, genera,
families and orders that are known to breed in each
zoogeographic region and the numbers of species,
genera, families and orders that each region shares with
each other region. Then, at each of these four taxonomic
levels, we calculated indices of similarity between each
pair of zoogeographic regions. This provided, at each
taxonomic level, a measure of the degree of resemblance
between the avifaunas of different regions. We also
calculated the number of endemic taxa in each zoogeographic region. The ®ndings are discussed brie¯y in
relation to the known zoogeographic histories of each
region.
Following the usual zoogeographical convention, six
main regions were recognized, namely Palaearctic,
Afrotropical (formerly Ethiopian, Benson et al., 1979),
Indomalayan (formerly Oriental, Clark et al., 1988),
Australasian, Nearctic and Neotropical, with approximate boundaries as shown in Fig. 1. In each region we
followed what seemed to be the most usual convention for birds, with the border between the Nearctic±
Zoogeography of birds
Neotropical regions corresponding to the northern limit
of the tropical forest in Mexico, that between the
Indomalayan and Palaearctic regions corresponding to
the Himalayan mountain chain and Yangtze River, and
that between the Indomalayan and Australasian regions
corresponding to Wallace's Line running north-east
between the islands of Bali and Lombok and between
Borneo and Celebes (Sulawesi). The eastward limit of
Australasia was drawn so as to include the islands of
New Ireland, Solomons, New Hebrides, New Zealand,
Chathams and Campbell, while other Paci®c Islands
eastward almost to Galapagos were grouped as a
seventh region, Oceania. The Galapagos Islands
themselves were classed as Neotropical.
Between each of these regions, the landbird fauna
intergrades, and drawing the lines between regions in
somewhat different positions (as some authors have
done) would make small differences to the species totals
in the tables, but would not affect the main conclusions.
For the purposes of this paper, we have considered each
of the seven main regions as separate units, and have
made no attempt to divide them into subregions (e.g.
Malagasy is included in the Afrotropical region and
New Zealand in Australasia).
METHODS
Our totals of 9416 landbird species, 2002 genera, 140
families and 23 orders were obtained by summing all the
species, genera, families and orders listed by Sibley
(1996), but for species and genera the totals are slightly
different from those given by the author himself (his
®gures for landbird genera 2003 and species 9413, but
note the inconsistencies in his contents list). Apparent
errors were present in his ®gures for genera in Phasianidae (his 47, ours 49), Odontophoridae (10 vs 9),
Psittacidae (80 vs 81), Tyrannidae (148 vs 146), Meliphagidae (42 vs 41), Muscicapidae (69 vs 70), Sturnidae (38
vs 37), Zosteropidae (13 vs 14), Silviidae (97 vs 95),
Fringillidae (239 vs 240), and for species in Caprimulgidae (77 vs 78), Meliphagidae (182 vs 181), Vireonidae
(53 vs 54), Cisticolidae (120 vs 122), Zosteropidae (95 vs
96), Silviidae (562 vs 560) and Passeridae (388 vs 389).
One species, Apolopteron familiare, was listed twice by
Sibley (1996), in the Meliphagidae as well as (more
appropriately) in the Zosteropidae. The 320 species of
seabirds that we excluded from analysis were taken as
all species of Laridae (skuas, skimmers, gulls, terns, and
auks), Phaethontidae (tropicbirds), Sulidae (gannets),
Phalacrocoracidae (cormorants), Fregatidae (frigatebirds), Spheniscidae (penguins), Gaviidae (loons) and
Procellariidae (petrels and other tubenoses).
With the regions de®ned as in Fig. 1, we compiled a
spreadsheet, listing each species and marking each zoogeographic region in which it naturally breeds (i.e.
excluding regions in which it occurs only as a nonbreeding migrant or has been introduced by human
agency). Species which only occur on islands in a given
region were marked separately from those that occur on
209
the main continental landmass (some of the latter also
occur on islands). From these lists we could calculate
the numbers of species occurring in each region, and the
number shared between each region and each of the
other regions. We could also calculate the numbers of
species occurring in different numbers of regions from
1 to 7 (including Oceania). The same procedure was
repeated for each genus, family and order.
One way to compare the avifaunas of 2 regions is to
calculate the number of taxa they have in common. But
this procedure takes no account of the total number of
taxa in each region, and the proportion of the total that
shared taxa form. To take account of these aspects,
various indices of similarity have been devised (Cheatham & Hazel, 1969; Pielou, 1979; Brown & Gibson,
1983). We chose to use 2 different indices, namely those
of Jaccard (1901) and Simpson (1940), because they are
the most different of all indices from one another. All
other indices known to us are functionally related to
one or other of these 2 indices, so would be expected to
give broadly similar results. Moreover, the Jaccard and
Simpson indices have been widely used in other zoogeographic comparisons, so our ®ndings should be more
directly comparable with some previous ones for other
taxa (e.g. Holloway & Jardine, 1968; Connor & Simberloff, 1978; Flessa & Miyazaki, 1978). By calculating
2 different indices of zoogeographic similarity (as well
as the numbers of shared taxa), we could examine how
much our conclusions were in¯uenced by the analytical
method used.
For each method, N1 = total number of species in
region 1, N2 = total number of species in region 2, and
Nc = number of species common to both regions. The 2
indices of similarity are given by:
Jaccard: J = Nc /(N1 + N2 - Nc)
Simpson: S = Nc /N1, where N1 N2
The relationship between J and S depends on the ratio,
R = N1/N2 (N1 N2) and is given by J = RS/[1 + R(1 - S)].
A consequence of this relationship is that the rankings
of similarities between pairs of regions can be reversed
depending on the relative numbers of species present.
For example, consider the 3 sites x (Nx = 10), y
(Ny = 10), z (Nz = 20), Ncxy = 8, Ncxz = 9.
Using Jaccard: Jxy = 0.66 and Jxz = 0.42
Using Simpson: Sxy = 0.80 and Sxz = 0.90
On these ®gures, then, sites x and y seem most similar to
one another on the Jaccard index, and sites x and z on
the Simpson index. Calculation of both indices in our
analysis enabled us to compare the ®ndings from both.
Another difference between the 2 indices is that S is
standardized to take the value 1 when all the species in 1
region occur in the other (i.e. Nc = N1). By contrast, the
maximum value of J depends on the ratio R = N1 /N2,
such that Jmax = R.
Because the biggest problem of intergrading faunas
occurs on islands, as between Asia and Australasia
(Wallacea) or between Australasia and Oceania, 3 comparative analyses were conducted: (1) for regional
210
I. Newton and L. Dale
Table 1. Numbers of landbird taxa that breed in different zoogeographical regions, with the numbers listed separately for: (a)
each region as a whole; (b) the main continental part of each region together with its land bridge islands; (c) the oceanic islands
associated with each region. Species that occur on both continent and oceanic islands of the same region are listed under
continentala (b). Percentages of the total known numbers of extant landbird taxa in the world (taken as 9416 for species, 2002
for genera, 140 for families and 23 for orders, see text) in parentheses. +, < 1%
(a) All birds
Species
Genera
Families
Orders
(b) Continental birds
Species
Genera
Families
Orders
(c) Island birds
Species
Genera
Families
Orders
Palaearctic Indomalayan Afrotropical Australasian
Nearctic
Neotropical Oceania
Overall
937 (10)
288 (14)
58 (41)
14 (61)
1697 (18)
431 (22)
73 (52)
17 (74)
1950 (21)
473 (24)
75 (54)
19 (83)
1592 (17)
457 (23)
73 (52)
16 (70)
732 (8)
302 (15)
52 (37)
15 (65)
3370 (36)
893 (45)
71 (51)
18 (78)
187 (2)
82 (4)
23 (16)
10 (43)
9416
2002
140
23
903 (10)
286 (14)
58 (41)
14 (61)
1259 (13)
398 (20)
70 (50)
16 (70)
1714 (18)
412 (21)
70 (51)
19 (83)
926 (10)
340 (17)
65 (46)
15 (65)
723 (8)
301 (15)
52 (37)
15 (65)
3170 (34)
847 (42)
70 (50)
18 (78)
0 (0)
0 (0)
0 (0)
0 (0)
7789
1784
130
23
34 (+)
29 (1)
17 (12)
8 (35)
438 (5)
168 (8)
47 (34)
16 (70)
236 (3)
121 (6)
42 (30)
13 (57)
666 (7)
224 (11)
49 (35)
16 (70)
9 (+)
9 (+)
8 (6)
6 (26)
200 (2)
104 (5)
29 (21)
14 (61)
187 (2)
82 (4)
23 (16)
10 (43)
1627
532
81
19
a
Areas of different continental regions are taken as (6100 000 km2): Palaearctic 46, Indomalayan 9.6, Afrotropical 21,
Australasian 8.9, Nearctic 21 and Neotropical 18.2.
avifaunas as a whole; (2) for taxa found only on the
main continental land masses (some of which also occur
on associated islands); (3) for taxa found only on islands.
RESULTS
Species numbers
By far the richest region ornithologically is the Neotropical, which holds at least 36% of all known landbird
species and 45% of genera (Table 1). This is followed by
the Afrotropical region (21% of species and 24% of
genera), the Indomalayan region (18% of species and
22% of genera) and the Australasian region (17% of
species and 23% of genera), and then by the Palaearctic
region (10% of species and 14% of genera) and the
Nearctic region (8% of species and 15% of genera).
These major continental regions thus show 4.6-fold
variation in the number of known bird species they
hold, or 9.1-fold variation in the numbers of species per
unit area (based on land areas given in the footnote to
Table 1). The region of Oceania, comprising a large
number of Paci®c Islands, holds only 2% of the world's
landbird species and 4% of genera.
At higher taxonomic levels, the ranking of regions is
slightly different, with the Afrotropical region emerging
as the richest, with 54% of families and 83% of orders
represented, although the Indomalayan, Australasian
and Neotropical regions are not far behind (Table 1).
Oceania again falls well below the others, with only 16%
of families and 43% of orders represented. Also, the
differences between regions are less marked, with
1.4-fold variation in numbers of families and of orders
between the main continental regions, excluding
Oceania.
The exclusion of species that occur only on islands
does not substantially alter the picture, although the
number of species in this category varies greatly
between regions (Table 1b). Australasian islands hold
7% of the world's total known landbird species, Indomalayan islands hold 5%, Afrotropical islands 3% and
the Neotropical islands 2%. These ®gures compare with
the 2% on Oceanian islands, and < 1% on Palaearctic
and Nearctic islands (Table 1c). Throughout, they refer
to species which occur only on islands, and exclude
those which also occur on continents.
Endemic species numbers
The numbers of endemic species, and the proportions
they form of the total regional avifauna, varies greatly
between regions (Table 2a). Overall, the Neotropical
region has the greatest number of endemics at all
taxonomic levels, followed after a large gap by the
Afrotropical, Australasian and Indomalayan regions,
and then after another large gap by the Palaearctic and
Nearctic regions. The Oceanian region has fewest
endemics (163 species in 31 genera, with no endemic
families or orders); although these species form only 2%
of the world's birds, they form 87% of all species
occurring naturally in Oceania. In other words, Oceania
has relatively few species, but those that occur show
very high levels of endemism, as expected of remote
oceanic islands.
If we exclude islands from the picture, and look only
at the species that occur on continents (Table 2b), about
92% of all the continental species that occur in the
Neotropical, Afrotropical and Australasian regions are
endemic to those regions, compared with 64% of Indomalayan species, 54% of Nearctic species and 46% of
Zoogeography of birds
211
Table 2. Numbers of landbird taxa that are endemic as breeders in different zoogeographical regions, with the numbers listed
separately for: (a) each region as a whole; (b) the main continental part of each region together with its land-bridge islands; (c)
the oceanic islands associated with each region. Species that occur on both continent and oceanic islands are listed under
continental (b). Figures in brackets are percentages of the total known taxa in that region or subregion (from Table 1)
(a) All birds
Species
Genera
Families
Orders
(b) Continental birds
Species
Genera
Families
Orders
(c) Island birds
Species
Genera
Families
Orders
Palaearctic
Indomalayan Afrotropical
Australasian
Nearctic
Neotropical
Oceania
442 (47)
26 (9)
0 (0)
0 (0)
1184 (70)
126 (29)
3 (4)
0 (0)
1807 (93)
293 (62)
16 (21)
2 (11)
1415 (89)
280 (61)
18 (25)
0 (0)
395 (54)
58 (19)
0 (0)
0 (0)
3121 (93)
686 (77)
20 (28)
2 (11)
163 (87)
31 (38)
0 (0)
0 (0)
411 (46)
25 (9)
0 (0)
0 (0)
801 (64)
108 (27)
3 (4)
0 (0)
1572 (92)
240 (58)
11 (15)
2 (11)
848 (92)
221 (65)
14 (22)
0 (0)
387 (54)
58 (19)
0 (0)
0 (0)
2925 (92)
642 (76)
19 (27)
2 (11)
0 (0)
0 (0)
0 (0)
0 (0)
31 (91)
1 (3)
0 (0)
0 (0)
383 (88)
18 (11)
0 (0)
0 (0)
567 (85)
59 (26)
4 (8)
0 (0)
8 (89)
0 (0)
0 (0)
0 (0)
196 (98)
44 (42)
1 (3)
0 (0)
163 (87)
31 (38)
0 (0)
0 (0)
235 (100)
53 (43)
5 (12)
0 (0)
Table 3.Number of bird species occurring in different numbers of zoogeographic regions (total 7, including Oceania). Numbers
listed separately for: (a) all birds; (b) continental birds; (c) oceanic island birds. Species that breed on both continents and
oceanic islands are listed under continental (b). +, < 0.05%. Rounded percentages of total known numbers of landbird taxa in
the world, or on continents or islands (as appropriate) in parentheses (from Table 1)
(a) All birds
Species
Genera
Families
Orders
(b) Continental birds
Species
Genera
Families
Orders
(c) Island birds
Species
Genera
Families
Orders
1
8527 (91)
1500 (75)
57 (41)
4 (17)
2
784 (8)
304 (15)
19 (14)
1 (4)
3
76 (+)
86 (4)
9 (6)
2 (9)
4
19 (+)
49 (2)
16 (11)
3 (13)
5
1 (+)
28 (1)
11 (8)
1 (4)
6
5 (+)
20 (1)
11 (8)
4 (17)
7
4 (+)
15 (+)
17 (12)
8 (35)
6944 (89)
1294 (72)
47 (36)
4 (17)
740 (9)
292 (16)
19 (15)
1 (4)
76 (1)
86 (5)
9 (7)
2 (9)
19 (+)
49 (3)
16 (12)
3 (13)
1 (+)
28 (2)
11 (8)
1 (4)
5 (+)
20 (1)
11 (8)
4 (17)
4 (+)
15 (1)
17 (13)
8 (35)
1583 (97)
206 (39)
10 (12)
0 (0)
44 (3)
12 (2)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
Palaearctic ones. Bigger differences between regions
emerge at the genus and family levels, and the ranking
of regions changes. Only the Neotropical and
Afrotropical regions have endemic orders. These include
the Tinamiformes and Galbuliformes in the Neotropics,
and the Coliiformes and Musophagiformes in the
Afrotropics.
Turning to island endemics, the greatest number
occur on Australasian islands (with 567), followed by
Indomalayan islands (383), Afrotropical islands (235),
Neotropical islands (196) and Oceanian islands (163).
Far fewer occur on Palaearctic islands (31) and Nearctic
islands (8). These ®gures depend not only on the
numbers and isolation of the islands associated with
each major zoogeographic region, but also as stressed
above, on where the boundaries between regions are
drawn. By de®nition, all the species that occur only on
oceanic islands are endemic to islands, but some occur
on the islands of neighbouring regions. This explains
why not all island species are marked as endemic to
particular regions in Table 2c.
Distribution patterns of species and higher taxa
Some 91% of all landbird species breed in only one
zoogeographic region, and another 8% in two regions,
with the remaining 1% in three to seven regions
(Table 3). The four most widespread species, which
breed in all seven regions, include the barn owl Tyto
alba, black-crowned night heron Nycticorax nycticorax,
common moorhen Gallinula chloropus and peregrine
falcon Falco peregrinus. Each of these species belongs to
a different order.
Similar highly-skewed distributions are found among
both continental and island species. In fact, no island
212
I. Newton and L. Dale
Table 4. Numbers of species shared between different pairs of regions, and indices of similarity in avifaunal composition (see text)
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
±
361
±
116
83
±
33
167
24
±
107
29
15
8
±
15
16
17
9
239
±
Palaearctic
±
Indomalayan
0.039
±
Afrotropical
0.123
0.050
±
Australasian
0.035
0.104
0.014
±
Nearctic
0.146
0.040
0.022
0.011
±
Neotropical
0.016
0.009
0.009
0.006
0.333
±
Indomalayan
0.159
±
Afrotropical
0.042
0.023
±
Australasian
0.013
0.053
0.007
±
Nearctic
0.069
0.012
0.006
0.003
±
Neotropical
0.003
0.003
0.003
0.002
0.058
±
Simpson index = Nc /N1
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
Jaccard index = Nc /N1 + N2 - Nc
Palaearctic
Palaearctic
±
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
Nc ± number of taxa shared by areas
N1 ± number of taxa in ®rst area (with the smallest number)
N2 = number of taxa in second area (with the largest number)
species occurs naturally in more than two regions.
Progressing through genera, families and orders, distributions become progressively more widespread, and
eight (35%) different orders are represented in all seven
regions, more than are found in 1, 2, 3, 4, 5 and 6
regions (Table 3).
Similarities between regions
This analysis, excluding the Oceanian region, was
conducted at only two taxonomic levels, and involved
calculating the numbers of species and families shared
by each pair of zoogeographic regions. The general
pattern was as expected, in that each region shares the
greatest number of landbird species with the closest
other region, and the fewest species with the most
remote region (Table 4). For example, the Palaearctic
region shares 361 species with the adjoining Indomalayan region and only 15 species with the remote
Neotropical region, while the Nearctic region shares 239
species with the Neotropical region and only nine with
the remote Australasian region. The Nearctic and
Palaearctic regions share 107 species.
A much greater proportion of families than of species
extend to more than one region, but the distributions of
families follow the same general trends as shown by
species (Table 5). Thus, the Palaearctic still shares most
landbird families (56) with the adjoining Indomalayan
region and 34 families with the remote Neotropical
region, while the Nearctic shares 46 families with the
Neotropical region and 33 with the remote Australasian
region. The Nearctic and Palaearctic regions share 37
families. The correlation coef®cient (r) between the
degree of sharing at species and family levels in the six
main regions is 0.63, P = 0.012. However, whereas at the
species level the Afrotropical and Indomalayan regions
each share most species with the Palaearctic (Table 4),
at the family level they share more families with one
another than either do with the Palaearctic (Table 5).
At both species and family levels, the Jaccard
similarity indices of species composition in different
pairs of regions are highly correlated with the Simpson
indices (despite their propensity to give different results
in some types of data, see Methods). At the species level,
the correlation coef®cient (r) between the two indices
was 0.915 (0.030 ‹ P < 0.0001) and at the family level
the coef®cient was 0.936 ‹ 0.023 (P < 0.0001). None the
less, at the family level the relative ranking of the
Indomalayan region with respect to most other regions
varied slightly between the two indices (Table 5).
Unlike the raw ®gures on numbers of shared species
quoted above, the indices take account of the total
numbers of species in the regions concerned (the
Simpson index incorporates the total from whichever of
the two regions compared has the smallest total, while
the Jaccard index incorporates the totals from both of
the regions compared). None the less, the indices
present a broadly similar picture of faunal resemblances
to the shared species total, but the relative ranking of
some of the regions with respect to others is changed
slightly. At the species level, most such differences relate
Zoogeography of birds
213
Table 5. Numbers of families shared between different pairs of regions, and indices of similarity in avifaunal composition
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
±
56
±
51
59
±
46
55
50
±
37
40
36
33
±
34
39
36
31
46
±
Palaearctic
±
Indomalayan
0.966
±
Afrotropical
0.879
0.808
±
Australasian
0.793
0.753
0.685
±
Nearctic
0.712
0.769
0.692
0.635
±
Neotropical
0.586
0.534
0.480
0.425
0.885
±
Indomalayan
0.747
±
Afrotropical
0.622
0.663
±
Australasian
0.541
0.604
0.510
±
Nearctic
0.507
0.471
0.396
0.359
±
Neotropical
0.358
0.371
0.327
0.274
0.597
±
Simpson index = Nc /N1a
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
Jaccard index = Nc /N1 + N2 - Nc
Palaearctic
Palaearctic
±
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
a
Nc ± number of taxa shared by areas; N1 ± number of taxa in ®rst area (with the smallest number); N2 = number of taxa in
second area (with the largest number).
Table 6. Number of species per genus and species per family in different zoogeographical regions
All species
Palaearctic
Oriental
Afrotropical
Australasian
Nearctic
Neotropical
Oceania
Island species
Mean no. of
species per genus
Mean no. of
species per family
Mean no. of
species per genus
Mean no. of
species per family
3.25
3.93
4.12
3.48
2.43
3.77
±
16.12
23.26
26.00
21.80
14.06
47.52
±
1.17
2.60
1.95
2.97
1.00
1.92
2.28
1.71
3.60
2.88
4.57
1.13
3.59
3.57
to the Afrotropical and Australasian regions, and at the
family level to the Palaearctic region (Tables 4 & 5).
None of these minor differences alter the fundamental
point that the greatest levels of species sharing and
family sharing occur between regions that are geographically close together, and the smallest levels
between regions that are far apart. In other words,
whether one uses the numbers of shared species, or the
Simpson or Jaccard indices, the conclusions on landbird
faunal similarities and differences are broadly similar.
The same holds if the analysis is restricted to continental
avifaunas, omitting species which are found only on
islands (data not given).
Numbers of species per genus and per family
Regions that hold large numbers of landbird species
also tend to hold large numbers of genera and families,
although only the former relationship is signi®cant
statistically (Table 6). They also tend to hold more
species per family than other regions. In this respect the
Neotropical region is exceptional, with an average of
47.5 species per family, and four families each
containing more than 200 species.
Regional variations in species-per-genus and speciesper-family ratios are re¯ected in the signi®cance levels
of the Kruskal±Wallis tests used to compare these
ratios between regions (Table 7). On this basis, the
Oceania region shows fewer species per genus than all
other regions, the difference emerging as statistically
highly signi®cant in the comparisons with the Afrotropical and Neotropical regions. Oceania also has
fewer species per family than other regions, with the
differences highly signi®cant in the comparisons with
the Indomalayan, Afrotropical, Australasian and Neotropical regions. The Neotropical region shows more
species per genus and more species per family than the
214
I. Newton and L. Dale
Table 7. Differences between regions in the numbers of species per genus and per family, as examined by analyses of variance,
using Kruskal±Wallis tests. P values show the individual signi®cance levels of each pairwise test. For a total of 21 independent
tests and an overall experimentwise signi®cance level of 5%, the signi®cance levels for individual comparisons should be 0.0024
or less (Sokal & Rohlf, 1981: 241±242). Values that achieve signi®cance on this basis are bold
Species per genus
Afrotropical
Indomalayan
Australasian
Nearctic
Neotropical
Oceanian
Species per family
Afrotropical
Indomalayan
Australasian
Nearctic
Neotropical
Oceanian
Palaearctic
Afrotropical
Indomalayan
Australasian
Nearctic
Neotropical
H = 1.86
P = 0.17
H = 0.64
P = 0.42
H = 0.91
P = 0.34
H = 1.09
P = 0.30
H = 4.24
P = 0.04
H = 3.50
P = 0.061
H = 0.29
P = 0.59
H = 0.17
P = 0.68
H = 4.41
P = 0.04
H = 0.53
P = 0.47
H = 10.13
P = 0.001
H = 0.02
P = 0.89
H = 2.31
P = 0.13
H = 1.53
P = 0.22
H = 6.76
P = 0.09
H = 2.96
P = 0.09
H = 1.32
P = 0.25
H = 8.17
P = 0.004
H = 7.70
P = 0.006
H = 1.24
P = 0.23
H = 14.92
P < 0.001
H = 1.42
P = 0.23
H = 1.70
P = 0.19
H = 1.65
P = 0.20
H = 0.35
P = 0.55
H = 3.50
P = 0.06
H = 4.68
P = 0.03
H = 0.01
P = 0.91
H = 0.01
P = 0.93
H = 3.16
P = 0.08
H = 0.47
P = 0.50
H = 10.84
P = 0.001
H = 0.00
P = 0.98
H = 4,55
P = 0.03
H = 0.33
P = 0.57
H = 11.55
P = 0.001
H = 3.51
P = 0.061
H = 0.36
P = 0.55
H = 11.48
P = 0.001
H = 5.99
P = 0.014
H = 2.52
P = 0.11
H = 15.46
P < 0.001
Table 8. Differences between the continental and island avifaunas of each region in the numbers of species per genus and per
family, as examined by analyses of variance, using Kruskal±Wallis tests. P values show the individual signi®cance levels for each
pairwise test. For a total of six independent tests and an overall experimentwise signi®cance level of 5%, the P values for individual
comparisons should be 0.0085 or less (Sohal & Rohlf, 1981: 241±242). Values that achieve signi®cance on this basis are bold
Species per genus
Palaearctic
Indomalayan
Afrotropical
Australasian
Nearctic
Neotropical
H = 16.43
H = 2.79
H = 13.35
H = 0.00
H = 14.91
H = 21.16
Species per family
P < 0.001
P = 0.095
P < 0.001
P = 0.996
P < 0.001
P < 0.001
Nearctic and the Palaearctic, but, allowing for the
number of tests involved, these differences could be
regarded only as tending towards statistical signi®cance. None of the other regional differences in species
numbers per genus or per family emerged as statistically signi®cant, allowing for the numbers of tests
involved.
Treating islands separately, the numbers of species
per genus and per family are signi®cantly lower than on
the continental parts of each region in four of the
regions, but not in the Indomalayan and Australasian
regions (Table 8). Lower species per family ratios extend
even to sizeable islands, such as Madagascar and New
Zealand when compared with their nearest continents
(Fig. 2).
H = 12.71
H = 1.08
H = 7.52
H = 0.19
H = 14.66
H = 10.72
P < 0.001
P = 0.300
P = 0.006
P = 0.663
P < 0.001
P = 0.001
Number of taxa and land area
The numbers of landbird taxa found in each of the six
continental regions are not correlated with the land
areas of those regions (Table 9). The slight (but nonsigni®cant) negative relationships, found at the levels of
species, genera and families, are greatly in¯uenced by
the relatively small size of the Oriental and Australasian
regions (which are rich in species) and the large size of
the Palaearctic region (which is poor in species).
DISCUSSION
Broadly speaking, this analysis upholds the general
conclusions of previous studies. At the same time, it
Zoogeography of birds
215
(a)
400
300
400
Africa
Median = 2
Mean = 4.1
200
200
100
100
Number
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 >15
100
75
Australia
Median = 2
Mean = 3.5
300
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 >15
100
Madagascar
Median = 1
Mean = 1.4
New Zealand
Median = 1
Mean = 1.4
75
50
50
25
25
0
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 >15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 >15
Species per genus
(b)
40
40
0
0
40
Madagascar
Median = 2
Mean = 3.7
New Zealand
Median = 2.5
Mean = 3.3
20
10
10
0
0
86– 90
20
1– 5
6–10
11–15
16– 20
21–25
26– 30
31– 35
36– 40
41– 45
46–50
51– 55
56– 60
61– 65
66– 70
71– 75
76– 80
81– 85
30
86– 90
40
86– 90
10
1– 5
6– 10
11– 15
16– 20
21– 25
26– 30
31– 35
36– 40
41– 45
46– 50
51– 55
10
86– 90
20
1– 5
6– 10
11– 15
16– 20
21– 25
26– 30
31– 35
36– 40
41– 45
46– 50
51– 55
56– 60
61– 65
66– 70
71– 75
76– 80
81– 85
20
30
Australia
Median = 4
Mean = 21.8
30
56– 60
61– 65
66– 70
71– 75
76– 80
81– 85
Africa
Median = 5.5
6
Mean = 26.0
1– 5
6–10
11–15
16– 20
21–25
26– 30
31– 35
36– 40
41– 45
46–50
51– 55
56– 60
61– 65
66– 70
71– 75
76– 80
81– 85
Number
30
Species per family
Fig. 2. (a) Species-per-genus and (b) species-per-family ratios in: (left) Madagascar compared to Africa; (right) New Zealand
compared to Australia. Comparisons using Kruskal±Wallis tests, adjusted for ties, species-per-genus: Madagascar vs Africa,
H = 0.01, P = 0.91; New Zealand vs Australia, H = 0.44, P = 0.51; species-per-family: Madagascar vs Africa, H = 108.21,
P < 0.001; New Zealand vs Australia, H = 59.57, P < 0.001.
quanti®es the similarities and differences between all
zoogeographic regions together, in a way that previous
analyses have not, and it does so on a recent classi®cation and species list. In other words, the basic regional
division of the world's avifauna, originally proposed by
Sclater (1858), has stood the test of time, despite the
many additional species discovered since then, the
extensive revisions of avian taxonomy, and the greatly
improved knowledge of bird distribution patterns. As
stressed at the outset, future changes to the classi®cation
and species list for world birds are likely to alter the
®gures in the tables, as are changes to the chosen
boundaries of the zoogeographical regions, but such
changes are unlikely to affect the main conclusions on
216
I. Newton and L. Dale
Table 9. Results of regression analyses (r values) of number of taxa in relation to: (a) area; (b) number of genera; (c) number of
families; (d) number of species per genus, (e) number of species per family in different zoogeographical regions (including
Oceania). Figures show Pearson's coef®cients. *P < 0.05, **P < 0.01, ***P < 0.001. Asterisks, individual signi®cance levels for
each pairwise test. For a total of 14 independent tests (as in species comparisons), and an overall experimentwise signi®cance
level of 5%, P values for individual comparisons should be 0.0036 or less. For a total of 11 independent tests (as in genera
comparisons) the equivalent ®gure should be 0.0046 or less, and for eight independent tests (as in families comparisons) the
equivalent ®gure should be 0.0064 (Sohal & Rohlf, 1981: 241±242). Correlations that are signi®cant on this basis are bold
Overall no. of species
Continental species
Island species
Overall no. of genera
Continental generab
Island generac
Overall no. of families
Continental families
Island families
Area (km2)a
(a)
No. of genera
(b)
No. of families
(c)
No. of species
per genus
(d)
No. of species
per family
(e)
70.355
70.162
±
70.346
70.234
±
70.574
70.472
±
0.981***
0.979***
0.981***
±
±
±
±
±
±
0.650
0.635
0.892**
0.520
0.553
70.953***
±
±
±
0.645
0.698
0.933**
0.487
0.538
0.946**
0.926**
0.761
0.871*
0.989***
0.995***
0.947**
0.993***
0.983***
0.943**
0.532
0.552
0.826*
a
Analysis excludes Oceania.
Genera containing island species.
c
Families containing island species.
b
the diversity ranking of the regions or on the general
levels of resemblance between their avifaunas.
The various regions differ greatly in the numbers of
bird families, genera and species they support as breeders, both overall and per unit area. Of the three
southern regions, the Neotropical is by far the richest,
with about 3370 species (or 185/1 000 000 km2), while
Afrotropical has 1950 species (95/1 000 000 km2) and
the Australasian has 1592 (179/1 000 000 km2). Such
great differences between the southern realms have been
attributed to: (1) the different past opportunities for
avifaunal build-up, by a combination of autochthony,
colonization and speciation (relative to extinction)
within the regions; (2) differences in the past and
present capacities of the regions to support large
numbers of different taxa. The latter depends in turn
partly on topographic and climatic variation, which
in¯uences the vegetation, and hence the range of habitats available. Of the three northern realms, the mainly
tropical Indomalayan region is by far the richest, with
1697 species (or 177/1 000 000 km2), while the
Palaearctic (937 species or 20/1 000 000 km2) and
Nearctic (732 species or 35/1 000 000 km2) realms are
relatively poor, a fact attributed largely to latitude and
to recent glaciations which may well have eliminated
many species, along with their habitats
The main conclusion on avifaunal distinctiveness is
already well-established, that neighbouring zoogeographic regions share more species, genera and families
than more distant ones. This trend is barely apparent
for orders, most of which have representatives in more
than one region. Only four orders are endemic to
particular regions, with two in the Neotropics and two
in the Afrotropics. The most distinctive landbird faunas,
in terms of endemism, are the three southern ones: the
Neotropical, Afrotropical and Australasian. This re¯ects the long period of isolation of these regions, all
derived from the ancient southern landmass of Gondwanaland, and the large sea distances that have long
separated them from one another.
Since the zoogeographical regions were de®ned by
Sclater (1858) on the basis of the distinctiveness of their
avifaunas, it is not surprising that each region shares
only small proportions of its species with other regions.
Moving through the taxonomic hierarchy, from orders,
through families to genera and species, distributions
change progressively as expected, from widespread to
restricted. Eight (35%) out of the 23 landbird orders are
represented in all seven zoogeographic regions, and
another four (17%) are represented in six regions; while
only four (17%) are represented in only a single region.
In contrast, only 29 (1%) species are represented in four
to seven regions, while 8527 (91%) species are found in
only one region. This difference in distribution between
taxonomic levels can be attributed to the greater age of
the higher taxa, and the greater opportunities for wider
dispersal that have occurred during their evolutionary
history. As revealed by both fossil and molecular
evidence, some extant orders of birds have been represented on Earth since the Cretaceous, > 65 million years
ago (Cooper & Penny, 1997), while many passerine
species date back < 2±3 million years (Klicka & Zink,
1999). When most of the bird orders and many of the
families evolved, the continental land masses were in
very different relative positions than they are today.
This is held to account for the presence of some orders,
notably the ¯ightless Struthioniformes, on all three
southern continents, despite the current wide separation
of these continents (Cracraft, 1973).
Because birds as a group are among the most mobile
of terrestrial animals, they are likely to show wider
distribution patterns and more overlap between regions
than are many other groups. But only when similar
analyses, using up-to-date taxonomic and distributional
Zoogeography of birds
data, have been conducted on other types of animals,
will it be possible to check this view.
Scoring species merely by presence or absence in
particular zoogeographic regions gives a generous impression of the distributions of all species, most of
which occur in only small parts of only one region, but
especially for those that occur primarily in one or two
continents, and occupy only a small adjacent part of
another. Examples include the several species that
extend from Eurasia across the Bering Strait into
Alaska (e.g. yellow wagtail Motacilla ¯ava, willow
warbler Phylloscopus trochilus), and the several that
extend in the opposite direction from North America
into north-east Siberia (e.g. grey-cheeked thrush
Catharus minimus, sandhill crane Grus canadensis). In
fact, every region has some species that occupy a small
area adjacent to another continent from which they are
presumed to have spread. If such (presumably recent)
range extensions were excluded, species distributions
would seem even more zoogeographically restricted
than indicated in the tables. Moreover, even widespread
species vary greatly in the proportion of each region
they occupy, depending largely on the distribution of
their habitats.
Similar points could be made concerning the distribution of higher taxa. For example, several families are
endemic to particular single regions apart from one or a
few species that extend to a neighbouring region. Of the
231 species of Furnariidae, all are restricted to the
Neotropics as here de®ned, apart from two which
extend north into the southern Nearctic region. Of the
60 species of Formicariidae, only two species extend
into the Nearctic and of the 50 species of Cracidae only
two extend into the Nearctic. Similarly, of the 68 species
of Pardalotidae, only one species extends from
Australasia into the Indomalayan region. Other
examples occur among families centred on other
regions, all of which tend to give a generous impression
of familial distributions, and a minimal measure of
familial endemism.
Any comparison of species-per-genus and speciesper-family ratios carries the implicit assumption that
fairly uniform taxonomic criteria have been applied
across the world's avifaunas. With this caveat, regions
that hold the largest numbers of families also hold the
largest numbers of genera and species, as well as the
largest species-per-genus and species-per-family ratios.
Evidently, levels of diversity extend through all these
taxonomic ranks, which in turn implies that whatever
factors promote this diversity, they are of long standing.
The fact that species-per-genus and species-per-family
ratios are lower on islands than on continents is already
well known, and would be expected on a hypothesis of
random colonization, i.e. random draws from a source
species pool would be expected to have fewer species per
genus than the pool itself (Simberloff, 1970; JaÈrvinen,
1982). Taking account of number of species on a large
sample of islands, Simberloff (1970) concluded that
species-per-genus ratios were actually higher than
expected on random colonization. He attributed this to
217
genera differences in ecology and dispersal ability,
which made it probable that species in certain genera
were more likely than species in other genera to reach
islands and survive on them. Most of the islands in his
sample were close to mainland and had no endemic
species. The islands of Madagascar and New Zealand
have many endemic genera and families, some of which
may have evolved in situ. However, taking account of
the number of taxa involved, species-per-genus ratios on
Madagascar did not differ signi®cantly from those in
Africa, but species-per-family ratios were signi®cantly
lower. The same was true for New Zealand compared
with Australia.
In contrast to the usual species±area relationship, bird
species numbers do not increase with increasing size of
zoogeographic region (instead showing a non-signi®cant
decline). This ®nding presumably arises because the
regions occupy different latitudinal spans, have somewhat different vegetation formations, and, in particular,
differ in the extent of tropical forest and other speciesrich habitats that they hold. Two of the smallest regions
(Indomalaya and Australasia) not only contain large
areas of tropical forest, but are also fragmented, with
many separate islands holding endemic species. Both
these regions are extremely rich in species. In contrast,
the largest region (the Palaearctic) has no tropical
forest, and very few islands holding endemic species.
This region is poor in species. With such differences in
habitats and land fragmentation, it is perhaps not
surprising that the species±area relationship does not
hold at the level of whole zoogeographic regions.
Hence, further work in explaining differences in avifaunal richness between regions might pro®tably
concentrate on exploring the role of past and present
climatic±vegetation factors in in¯uencing current diversity patterns.
Acknowledgements
Our indebtedness to the late Charles Sibley and the late
Burt Monroe for compiling a recent list of the World's
birds is obvious. We also thank Peter Rothery for
statistical help.
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