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Title: How many species of birds have existed? (FSM Bulletin)
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BULLET


IN


OF THE


FLORIDA STATE MUSEUM


BIOLOGICAL


SCIENCES


Volume 5


Number 3


HOW MANY SPECIES OF BIRDS HAVE EXISTED?

Pierce Brodkorb


UNIVERSITY


OF FLORIDA


Gainesville







The numbers of THE BULLETIN OF THE FLORIDA STATE MUSEUM,
BIOLOGICAL SCIENCES, are published at irregular intervals. Volumes contain
about 300 pages and are not necessarily completed in any one calendar year.

UNIVERSITY OF FLORIDA

I1IIhlIIl II1 11111111111 III
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1





SCIENCE
LIBRARY
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be addressed to the Curator of Biological Sciences, Florida State Museum, Seagle
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BULLETIN, Flint Hall, University of Florida, Gainesville, Florida.


S-Published 27 January 1960


Price for this issue $.25










HOW MANY SPECIES OF BIRDS HAVE EXISTED?

PIERCE BRODKORB 1

SvNoPsls: The present world avifauna is composed of about 8650 species.
During the Pleistocene and late Tertiary the avifauna was larger, with about
11,600 species living at any given time. Before the adoption of the granivorous
habit in the mid-Tertiary, the avifauna was smaller, with an estimated 10,200
contemporaneous species. The Crataccous avifauna consisted mainly of aquatic
birds and contained about 1000 contemporaneous species. In the late Jurassic
the avifauna was negligible, with probably not more than 100 species existing
at any one time.
The average longevity of avian species is considered to be the equivalent
of the time needed to replace half the fauna. Based on the rate of extinction
in Pleistocene faunas, the average longevity is estimated at about 500,000 years.
Multiplying the number of contemporaneous species by the duration of a
given epoch and dividing by the average species longevity gives the number of
species evolved during that epoch. Addition of the epochal totals gives the num-
ber of species that have existed since the origin of class Aves. This is estimated
to be about 1,634,000 species. The described living and fossil birds total about
one-half of one percent of those potentially knowable.

The living birds are a numerous and well-known group, but knowl-
edge of their fossil history is still fragmentary. It is nevertheless of
interest to speculate on the number of species of birds that have
existed throughout time, and to estimate the ratio of presently known
to potentially knowable species of birds.
The most recent counts or estimates of the number of known living
species of birds total 8590 (Mayr and Amadon, 1951), 8809 (Brodkorb,
1957b), and 8548 (Van Tyne and Berger, 1959). The mean of these
three estimates is 8649 species, and they differ by less than 3 per-
cent. As the number of living species still unknown is negligible and
probably amounts to less than 100 undescribed species (Mayr, 1946),
we may use the figure of 8650 as the approximate number of living
species of birds, described and undescribed. It is doubtful that this
figure will change radically.
The quarter-century since the publication of Lambrecht's "Hand-
buch der Palacornithologie" (1933) has been a prolific one in the de-
scription of new species, but a recently completed manuscript catalog
of the known fossil birds of the world includes only 834 species ex-

1 The author. Professor of Biological Sciences at the University of Florida,
first presented this paper at the Seventy-seventh Stated Meeting of the American
Ornithologists' Union at Regina, Saskatchewan, 26 August 1959. Manuscript
received 1 Nomember 1959.








BULLETIN FLORIDA STATE MUSEUM


tinct prior to the technical description of the living fauna. Although
in other classes of tetrapods the number of known extinct species ex-
ceeds the number of living ones, in birds the known extinct species
total less than one-tenth those living today.

METHODS
Three types of information are needed to form an estimate of
how many species of birds have existed. First, we need an estimate
of the number of environmental niches available to and used by birds
during the different epochs. This gives the approximate number of
species living simultaneously in any given epoch. Second, we need
an estimate of the longevity of avian species, the time needed for one
species to be replaced by another. The average longevity of species
gives a figure on the rate of replacement or turnover of species. Third,
we need an estimate of the duration of the various epochs in years.
For a given epoch, the number of contemporaneous species multi-
plied by the duration of the epoch in years, divided by the average
species longevity, gives the total number of avian species that orig-
inated during that epoch. This may be expressed by the following
formula:
SD
-=N
L
where
S = number of contemporaneous species per epoch;
D = duration of epoch in years;
L = longevity of species in years;
N = number of species evolved during the given epoch.

The totals for the various epochs may then be added to obtain
the approximate number of species evolved since the class Aves ap-
peared.
PLEISTOCENE AVIFAUNAS
All living species of birds are thought to have been in existence
during the Pleistocene (Howard, 1950; Wetmore, 1959), and about
732 living species are already recorded from that epoch. As 272
extinct Pleistocene species are also recognized, the avifauna during
the Pleistocene was larger than now, and the birds as a group have
passed their climax (Wetmore, 1951).


Vol. 5







BRODKORB: HOW MANY SPECIES OF BIRDS


An estimate of the size of past avifaunas, to be meaningful, must
include an analysis of past environmental conditions. Those that
contributed to extinction during the Pleistocene include changes in
temperature, rainfall, sea level, and vegetational types.
Lowered temperatures during the glacial ages of the Pleistocene
and elevated temperatures during the interglacial ages undoubtedly
caused some shifting of the ranges of bird species, either directly or,
more probably, indirectly through changes in vegetational zones.
Yet effectually lowered temperatures apparently did not extend far
beyond the southern limits of glaciation. In North America, the
most heavily glaciated area in the world, effectual cooling during
times of glaciation reached only about 700 miles south of the southern
limits of the ice sheets, for many warm-adapted species remained that
close to the ice front (Brodkorb, 1957a, 1959a). Furthermore, in the
tropics temperature changes must have been negligible, for while a
northern crane is known to have crossed the equator to Java (Wet-
more, 1940), most species recorded from Pleistocene sites in the Great-
er Antilles (Wetmore, 1922, 1937), Brazil (Winge, 1887), and India
(Lydekker, 1886) are of species living in exactly the same places today,
with little or no indication of an influx of temperate zone types. There-
fore it is concluded that refrigerative effects in the Pleistocene were
confined to relatively narrow bands in the temperate zones, and the
tropics were of minor importance as refugia for species fleeing low-
ered temperatures during times of glaciation.
Other environmental factors may have been of more importance
and seem to have operated on a world-wide basis. The repeated
pluvial and interpluvial stages must have had profound effects on the
vegetation, both in the tropics and in the temperate zones. Low-lying
lands experienced repeated submergence and emergence with fluctu-
ations in ocean levels, which retarded or altered the seral succession
of the vegetation (Brodkorb, 1959a). The exposure of the continental
shelves during the glacial stages afforded broader connections be-
tween formerly isolated areas and allowed entrance of competitors
and predators at the expense of less aggressive types. The effects of
fluctuating ocean levels must have been greatest on islands such as
the Bahamas, where the extinction of the Pleistocene fauna is thought
to have resulted from a rising sea which so fragmented and inundated
the former large land mass that insufficient area remained to support
the fauna (Brodkorb, 1959b). All these factors must have contributed
to the extinction of forms in the tropics as well as elsewhere during
the Pleistocene.







44 BULLETIN FLORIDA STATE MUSEUM Vol. 5

In calculating the size of the avifauna living contemporaneously
during the Pleistocene, the probability must be taken into account that
certain species were restricted to different parts of the epoch. Thus
data for the late Pleistocene alone give a more reliable estimate of
the size of the contemporaneous avifauna than data for the entire
epoch.
From the late Pleistocene of North America and the West Indies
Wetmore (1956) lists 248 species, of which 185 are living forms and
63 extinct. Most Pleistocene avifaunas from other parts of the world
are small, inadequately dated, or insular. Nevertheless, the numer-
ous extinct species recorded from New Zealand, Australia, and Mad-
agascar suggest that the late Pleistocene avifaunas of other parts of
the world were comparably large. In the absence of adequate in-
formation from other continents, the North American ratio of living
to extinct species is assumed to be typical of the world situation.
By projecting to a world-wide basis the ratio of living to extinct
species in the late Pleistocene avifauna of a part of the world, such as
North America, a proportion may be formed which allows approxi-
mation of the size of the late Pleistocene avifauna of the whole world.
The ratio of living to total species recorded from the late Pleistocene
of North America should be roughly proportional to the ratio between
the number of living species in the world and the total late Pleisto-
cene world avifauna. This relationship may be expressed by the
formula:
NL : NT :: WL : WT
where
NL = North American living species recorded from late
Pleistocene;
NT = North American total species recorded from late
Pleistocene;
WL = world living species today (and hence in the late -'
Pleistocene);
WT = world total species in late Pleistocene.

Substituting,
185 : 248 :: 8650 : WT
WT = 11,596

Thus approximately 11,600 species of birds are thought to have
lived during the late Pleistocene.






BRODKORB: HOW MANY SPECIES OF BIRDS


LATE TERTIARY AVIFAUNAS
In the history of plant evolution the major event of the Cenozoic
era was the expansion of alpine and xeric habitats (Stebbins, 1947).
As these habitats became established during mid-Tertiary times, there
is little reason to believe that the number of species of birds existing
simultaneously during any given portion of the Pliocene or Miocene
was markedly less than in the Pleistocene. It may in fact have been
considerably greater than the number during the Pleistocene, when
the environmental factors already discussed became operative. There-
fore the Pleistocene figure of 11,600 may be used as a conservative
estimate of the number of contemporaneous species of birds during
the late Tertiary epochs.

EARLY TERTIARY AVIFAUNAS
As pointed out above, the mid-Tertiary was characterized by the
expansion of alpine and xeric habitats. At this time a great increase
and radiation occurred among the Gramineae and certain other plant
families (Stebbins, 1947). Although three genera of grasses are known
from the early Tertiary (LaMotte, 1952), grasslands did not become
widespread or ecologically important until the Miocene (Elias, 1942;
Chaney, 1947). The expansion of the grasslands is almost universally
correlated with the development of hypsodonty in mammalian evolu-
tion, a condition that appears to have been initiated in more groups
during the Miocene than in any other epoch (White, 1959). It is thus
reasonable to believe that grassland-inhabiting and grass seed-eating
birds formed an insignificant part of the avifauna prior to the Miocene.
The granivorous birds may be divided into two sections, the pri-
mary granivores, which seem to have developed in response to the
new hiotype, and secondary granivores belonging to pre-Miocene
families which underwent a secondary adaptive radiation as some of
their members developed the habit of feeding on grass seeds.
The Fringillidae (Emberizidae of certain authors) form the most
important family of primary granivores. Apparently this family was
among the latest to develop and to undergo adaptive radiation, for
it is structurally advanced, both externally and in the skeleton (Ash-
ley, 1941; Wetmore, 1957; Storer, 1959). It appears to have arisen
from New World insect-eating ancestors (Tordoff, 1954). In the mod-
ern fauna the family Fringillidae contains about 289 species, most
of them confined to the New World.






BULLETIN FLORIDA STATE MUSEUM


Other primary granivorous families include the Turnicidae (14
species), Pedionomidae (1 species), Thinocoridae (4 species), Pterocli-
dae (16 species), and Alaudidae (75 species). These too probably did
not arise or at least did not exhibit much speciation until the Mio-
cene, with the possible exception of the Pteroclidae, which are re-
corded from the Oligocene.
The secondary granivores include members of seven other fami-
lies of earlier origin, some of whose species adopted the grain-eating
habit and thus underwent a second, mid-Tertiary radiation within
each family. The leading family is the Ploceidae, which seems to
have arisen in the Old World, where it is now more or less the eco-
logical counterpart of the Fringillidae in America. No less than 342
of its 385 species have become granivorous, the exceptions lying in
the subfamily Carduelinae, the only ploceid subfamily that spread
to America.
The secondary group also includes representatives of six other
families, most of whose members still feed largely on food other than
grain. These families, with the estimated number of grain-eating
species in the modem fauna, are as follows: Tinamidae (16 of 43
species), Phasianidac (71 of 149 species), Columbidae (115 of 307 spe-
cies), Psittacidae (37 of 326 species), Icteridae (40 of 88 species), and
Thraupidae (7 of 232 species).
Primary and secondary granivores accordingly total about 1027
species at present, or about 11.9 percent of the Recent avifauna. Prior
to the Miocene, therefore, the late Tertiary number of coeval bird
species should be reduced by 11.9 percent, for an estimated total of
10,200 species of birds living simultaneously during a given portion
of the early Tertiary.
MESOZOIc AVIFAUNAS
The angiosperms control many of the biotypes in which land birds
live, and the invasion of birds into these environments necessarily fol-
lowed the development of the habitats. The angiosperms underwent
their first burst of radiation in the Cretaceous, with the evolution of
woody plants adapted to subtropical or tropical forest conditions
(Stebbins, 1947).
A few land birds probably existed during the Cretaceous, but the
species must have been limited in number before the radiation of the
flowering plants and while the reptiles still dominated. All known
Cretaceous birds are water birds, as Caenagnathus Sternberg (1940)
appears to be a reptile. The water birds comprise about 1014 species


Vol. 5






BRODKORB: HOW MANY SPECIES OF BIRDS


in the Recent fauna. Although the land birds may have begun their
expansion before the close of the Cretaceous, the average number of
bird species living contemporaneously during different parts of the
period is estimated as approximately 1000.
The oldest undoubted birds are of late Jurassic age, and there is
little reason to suspect that the Class Aves arose much earlier, cer-
tainly not before middle Triassic time (Gregory, 1955). The number
of species of birds living during the late Jurassic must have been
negligible, and one-tenth the Cretaceous number, or 100 contempo-
raneous species, is arbitrarily assigned to the Jurassic.

LONGEVITY OF AVIAN SPECIES
Students of other groups of animals have estimated the length of
life of a species without, however, indicating the basis of the esti-
mates. Teichert (1956) gives the mean length of an epoch, 12 million
years, as the top limit of longevity for any animal species. Simpson
(1952) estimates the average length of life of an animal species as from
500,000 to 5,000,000 years. With birds the available evidence indi-
cates that both the top limit of longevity and the average longevity
of a species are shorter than in many other groups, as might be sus-
pected from their high rate of metabolism.
Almost no avian species are known to cross epochal lines. The
few so recorded fall into two groups, those occurring on both sides
of the still equivocal Pliocene-Pleistocene boundary, and a few Terti-
ary forms inadequately known and thus perhaps incorrectly identi-
fied at the specific level. These moot cases do not invalidate the con-
clusion that the top limit of longevity is less than the duration of
an epoch.
Living species of birds first appear in deposits of so-called "Blan-
can" provincial age, assigned by different authors to the late Plio-
cene or to the early Pleistocene (cf. Hibbard, 1958). Records of living
S species of birds in older Tertiary (Merriam, 1916) or even Cretaceous
deposits (Shufeldt, 1915: 25) have been shown to be misidentifications
(Lambrecht, 1933: 583; Brodkorb, 1956). As the beginning of the
"Blancan" is variously estimated as from 1,000,000 to 4,000,000 years
ago, the top limit of longevity of avian species must fall within those
limits.
Specific longevity is a variable, as evolutionary rates differ in sep-
arate phyletic lines. One line may give rise to several distinct lineal
species while another line remains unchanged. Although evolution-
ary rates have been little studied in birds, enough data are available






BULLETIN FLORIDA STATE MUSEUM


to indicate that the rates do differ between lines. For example, the
quail Colinus hibbardi occurred in the latest Pliocene (Wetmore,
1944). By the third glaciation of the Pleistocene it had been replaced
by Colinus suilium, which in turn was supplanted by the living C. vir-
ginianus by the fourth glaciation (Brodkorb, 1959a). Yet the living
mourning dove, Zenaidura macroura, has persisted apparently un-
changed since the late Pliocene. Because of such different rates of
evolution it is thought that the average longevity, rather than the top
limit of longevity, is the datum needed in calculating evolutionary
rates.
When the longevities of the various phyletic lines represented in
a fauna are normally distributed, the fauna will be composed of a
few species of short longevity, a few species of great longevity, and
a majority of species of intermediate longevity. For example, assume
that a fauna consists of 12 phyletic lines whose longevities are as
follows: n years for species A; 2n years for species B and C; 3n
years for species D, E, and F; 4n years for species G, H, and I;
5n years for species J and K; and 6n years for species L. The
average longevity is 3.5n years; in this time half the original species
will have been replaced. Thus the average longevity is equal to the
time needed to replace half the fauna when longevities are normally
distributed. When the longevity curve is somewhat skewed, the
time needed to replace half the fauna should still be roughly equiva-
lent to average specific longevity.
In table 1 several Pleistocene avifaunas are analyzed on the basis
of the time required to replace half the fauna. It is obvious that
extinction rates from IV glacial time to the present are not typical
of the whole Pleistocene. If these rates were applicable to earlier
portions of the Pleistocene, then complete replacement of the avi-
fauna should have occurred between III glacial time and the present,
whereas about 78 percent of the III glacial birds represent living spe-
cies. Complete replacement of the avifauna does not even occur
during the interval between the I interglacial and the present, during
which interval, on IV glacial rates, the fauna should have been en-
tirely replaced several times. As other factors are obviously distort-
ing the extinction rate in the IV glacial stage, data from that stage
alone cannot be used to determine the extinction or evolutionary rates
for longer periods.
Fifty percent extinction must have been reached between the III
glacial and I interglacial stage, as extinct species comprise approxi-
mately 22 and 68 percent of their faunas, respectively. The times


Vol. 5







BRODKORB: HOW MANY SPECIES OF BIRDS


estimated as necessary to reach 50 percent extinction on the data from
these two stages are similar, 420,091 and 518,366 years, respectively.
Thus it is concluded that the average longevity of Pleistocene avian
species is approximately one-half million years, and the top limit of
longevity approximately one million years.

TABLE 1
LONGEvITY OF AVIAN SPECIES

Estimated
Years average
Locality and stage before Total Extinct Percent longevity
present species species extinct in years

Florida, mid-IV glacial' 17,000 63 6 9.5 89,474
California, IV glacial 25,000 121 22 18.2 68,681
Florida, III glacial' 184,000 70 16 21.9 420,091
Idaho, I interglacial 691,500 9 6 66.7 518,366


'Combined faunas of Seminole Field, Melbourne, and Hock Spring, Florida
(Wctmore, 1931; Woolfenden, 1959). Years for stage based on radiocarbon aver-
ages (Horsbcrg, 1955).
SCombined faunas of Rancho La Brea, McKittrick, and Carpinteria, Cali-
fornia (Miller and DeMay, 1942; with additions by A. H. Miller, 1947, and Daw-
son, 1948). Years for stage based on radiocarbon averages (Horsberg, 1955).
SCombined faunas of Rcddick, Arredondo, Haile, and Williston, Florida
(Brodkorb, 1953, 1957a. 1959a; IIolman, 1959). Years for stage based on geo-
logical evidence (C. F. Kay, 1931).
Hagerman, Idaho, fauna (Brodkorb, 1958). Years for stage based on geo-
logical evidence (G. F. Kay, 1931).

It is probable that the rate of evolution is rapid following eco-
logical access to a new habitat, slower and more steady when physi-
cal and biological environments are more stable. In view of the en-
vironmental changes outlined previously, two phases of rapid evolu-
tion might be expected, one near the Mesozoic-Cenozoic boundary
when the angiosperms were expanding, and one in the Miocene fol-
lowing the spread of grasslands. As data to test this hypothesis are
not yet available, the Pleistocene rate is here applied to other epochs.

TOTAL SPECIES EVOLVED

As explained in the section on Methods, the number of species
that evolved during an epoch may be determined by multiplying the


1959







BULLETIN FLORIDA STATE MUSEUM


number of contemporaneous species by the duration of the epoch
and dividing by the average species longevity, according to the

SD
formula = N. Addition of the species evolved during each
L

epoch gives the total number of species of birds which have existed
since the origin of the class. The duration of the epochs are taken
from M. Kay (1955) for the Mesozoic and Tertiary, from G. F. Kay
(1931) for the Pleistocene.
The total number of birds, past and present, is estimated at ap-
proximately 1,634,000 species (table 2).

TABLE 2
ESTIMATED NUMBER OF AVIAN SPECIES

Span in millions Contemporaneous Species
Epoch of years Species evolved

Pleistocene 1 11,600 23,000
Pliocene 10 11,600 232,000
Miocene 15 11,600 348,000
Oligocene 10 10,200 204,000
Eocene 25 10,200 510,000
Paleocene 10 10,200 204,000
Upper Cretaceous 25 1,000 50,000
Lower Cretaceous 30 1,000 60,000
Upper Jurassic 15 100 3,000

Total 1,634,000



It should be emphasized that all the data used in arriving at the
estimate are inexact, and future refinements in both data and methods
may radically alter the total. Wherever a variable exists the more
conservative choice has been selected, and future revision may result
in an increase over the current estimate. For example, the land bird
fauna may actually have expanded in the late Cretaceous rather than
in the early Tertiary as here assumed. This would increase the total
by several hundred thousand species. In any case, the 9500 living
and fossil birds now known represent an infinitesimal fraction, about
one-half of one percent, of the species potentially knowable.


Vol. 5







BRODKORB: HOW MANY SPECIES OF BIRDS


LTRLATURE CITED
Ashley, James F.
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52 BULLETIN FLORIDA STATE MUSEUM Vol. 5

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BRODKORB: HOW MANY SPECIES OF BIRDS


Teichert, Curt
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1959








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