Kristen Beyer
April 20, 2005
Biology 394
Red Tailed Black Cockatoo
Abstract
Calyptorhynchus banksii, or red-tailed black-cockatoo, has coped with extreme
variations in its environment. Much like the plant species of Australia, the red-tailed
black-cockatoo has evolved physiological and anatomical adaptations that have allowed it
to survive in changing surroundings. One of the most prevalent adaptations was that seen
in the beak apparatus. Changes in the beak allowed the cockatoos to consume the newly
evolved sources of food. The metabolic requirements of the cockatoos and the
availability of food are also forces that have influenced the beak apparatus and the
distribution of red-tailed black-cockatoos throughout Australia.
Introduction
Calyptorhynchus banksii, commonly known as the red-tailed black-cockatoo, is of
the Family Cacatuidae (Cockatoos), which is a branch of the Order Psittaciformes. Redtailed black-cockatoos are indigenous to Australia and can be found throughout the entire
continent. Calyptorhynchus banksii is distinct in that there are five different subspecies:
Calyptorhynchus banksii( C.b.) banksii, C.b. macrorohynchus, C.b. naso, C.b.
graptogyne, and C.b. samueli (Del Hoyo et al., 1997). Differences in beak apparatus are
one of the most prevalent variations observed within the subspecies. Throughout the
paper an examination of evolutionary forces will explain the changes that have occurred
in the beak apparatus of the red-tailed black-cockatoo.
Background
Male red-tailed black-cockatoos are black with areas of red on the underside of
their tails except on the two central feathers. They have an upright, backward-sloping
crest and dark grey bills and feet. Females are brownish black with areas of yellow on
their head, shoulders, and the underside of their tales. Their bills are cream colored, and
their feet are dark grey. The actual size of the cockatoo ranges from 50-65 cm and 570870 g (Del Hoyo et al., 1997). Calls of the cockatoo are loud, harsh, and can be heard
from a distance (Higgins, 1997).
Red-tailed black-cockatoos usually form a lifelong bond with their mating partner
(Del Hoyo et al., 1997). They typically nest in large hollows of eucalyptus trees where
they rear their young, generally one egg per clutch. Parental care is shared between the
two mates; however, the female does most of the incubating and the male does most of
the feeding (Higgins, 1997).
Subspecies Variations
Subspecies of red-tailed black-cockatoos differ in regards to their body size, beak
structure and size, and the coloration of females. The variation in beaks is due to the
presence or absence of a groove at the tip of the upper maxilla and the shape of the
cutting edge on the lower mandible (Higgins, 1997). Figure 1 illustrates the variations
observed in four of the five subspecies beaks. The significance of these variations will be
addressed in later sections.
Figure 1 (Higgins, 1997) (a) C.b. banksii (b) C.b. macrorhynchus (c) C.b. naso (d)
C.b. graptogyne.
C.b. banksii is the largest of the subspecies. The groove in the upper maxilla is
usually very small or absent, and the cutting edge of the lower mandible is slightly
rounded and concave. C.b. macrorohynchus has the largest beak in the subspecies and a
relatively large body size. Characteristics of the beak include a large distinct groove in
the upper maxilla with a rounded and concave cutting edge on the lower mandible. C.b.
naso is smaller with a more spherical beak. The groove in the upper maxilla may be
large or completely absent, and the cutting edge on the lower mandible is concave. C.b.
graptogyne is the smallest of the subspecies. The upper maxilla may have a small or
absent groove, and the cutting edge of the lower mandible is sharply defined. C.b.
samueli is close in size to C.b. naso. The cutting edge of the lower mandible is flat, and
the groove in the upper maxilla is small or absent (Higgins, 1997).
Different characteristics in the bill and morphology, as well as diet and use of
foraging substrates, suggest that regional differences in food sources have had an
evolutionary impact on the species and subspecies of the red-tailed black-cockatoo
(Franklin et al., 2000).
Species Distribution
Australasia is a region that has a limited number of total bird species probably as
a result of the following factors: (1) The large number of islands which tend to have
fewer species than continental regions; (2) The relatively small land mass as opposed to
other avifaunal regions; and (3) The arid climate (Podulka et al., 2004).
Granivores, or seed eaters, account for 20 percent of the land bird species in
Australia including red tailed black-cockatoos (Franklin et al., 2000). The granivorous
red-tailed black-cockatoos can be further divided into 1 of 2 groups: terrestrial, those that
feed on the ground; and arboreal, those that feed in trees. Terrestrial granivores are most
abundant in inland northern Australia and the semiarid zone, particularly the north-west.
Arboreal granivores are most prominent in areas of higher rainfall like the coastal and sub
coastal regions (Franklin et al., 2000).
Consequently, the subspecies of red-tailed black-cockatoos are distributed
throughout the continent of Australia according to their foraging mode. C. b. banksii is
found predominately in forests of tropical northern Australia. C.b. macrorhynchus
inhabit woodland of eastern Australia. C.b. naso and C.b. graptogyne are found in
eucalypt forests of southwest and southeast Australia. C.b. samueli, the terrestrial forager
of the five, inhabits the semi-arid inland of central Australia (Del Hoyo et al., 1997).
Evolution of Available Food Sources
The separation of Australia from Gondwana had a significant impact on the food
sources of the red-tailed black-cockatoo. Australia’s original biota consisted of forests
dominated by gymnosperms: conifers, araucarias, and podocarps whose seeds are not
enclosed in an ovary (Pyne, 1991). However, the continental shift is perhaps responsible
for the stimulation of angiosperms, plants whose seeds are enclosed in an ovary (Pyne,
1991). Retrieving seeds from gymnosperms did not require red-tailed black-cockatoos to
have specialized morphological traits but would require changes for feeding on
angiosperms.
The northern movement of Australia into the tropics stimulated a dramatic change
in climate. Increased aridity and high temperatures had an extreme impact on the plant
species present. As a result of the change in climate, the continent began to dry and fire
became much more prevalent (White, 1994). Those plants that survived the climate
change evolved into sclerophylls, plants with small tough leaves that resist transpiration
of water. In order to protect their seeds from drying and burning, plant species developed
thick-walled woody fruits characteristic of fire-adapted serotinous shrubs and trees
(Higgins, 1997). Sclerophylls eventually permeated every ecological niche in Australia
(Pyne, 1991). Consequently, the feeding apparatus of the red-tailed black-cockatoo was
affected by the change in food sources resulting in two distinct styles of beaks.
Beak Apparatus
The conventional configuration of a bird’s beak allows for two major motions: the
lowering of the mandible and the raising of the maxilla (Homberger, 2003). By utilizing
particular muscles Psittaciformes are able to bring together the tips of the upper and
lower beak. They meet when the maxilla is depressed and the mandible is lowered
allowing Psittaciformes to grasp objects in their beak (Homberger, 2003). Detailed
descriptions of the beak movements are illustrated in Figure 2.
Figure 2 (Higgins, 1997) (A) At rest with beak closed. (B) Fully opened with raised
maxilla and lowered mandible. (C) Grasping position.
The design of the illustrated beak prevents caudo-rostral, back and forth
movement, and medio-lateral, side-to-side movement (Homberger, 2003).
Evolution of the Beak Apparatus
The beak apparatus of certain subspecies of the red-tailed black-cockatoo has
diverged from the conventional configuration of Figure 2 allowing for side-to-side
movement. This observed change is thought to be an adaptation for eating seeds
contained in hard, woody nuts that must be removed via cutting instead of by
compression (Homberger, 2003). Choreographed movements of muscles result in
transverse movement when the beak is closing which simulates a cutting motion. The
motion of the jaw is very similar to that seen in ruminants (Homberger, 2003).
There are two basic types of beaks within the Psittaciformes, the psittacid and
calyptorhynchid. Psittacid type beaks are generally found in ground feeders, i.e. C.b
samueli, and do not allow transverse motions. The inside of the maxilla has a
characteristic notch and rough texture used for manipulating food, and the mandible has a
broad transverse cutting edge (Homberger, 2003). Psittacid type beaks are efficient at
shelling seeds intra-orally without use of a foot. Seeds are shelled by positioning them
against the rough notch of the maxilla and cornering them there with the transverse edge
of the mandible. Before shelling the seed the weakest area of the seed is located
decreasing the amount of force that needs to be exerted (Homberger, 2003). The use of
the tongue in positioning food is illustrated in Figure 3. Since there is no lateral motion
of the beak, the entire force generated by the jaw muscles is focused on biting force. This
makes the psittacid type beak extremely effective at removing seeds from shells with
weak points, but ineffective at removing seeds from within lignified thick shells
(Homberger, 2003).
Figure 3 (Homberger, 2003) Seed shelling process of a cockatoo with a psittacid type
beak.
The calyptorhynchid type beak is usually found in arboreal feeding species and
does allow for transverse movement of the jaw. The internal surface of the maxilla is
smooth and typically has no apparent notch. The transverse cutting edge of the mandible
is emarginated and V-shaped creating sharp corners on the lower jaw. It is necessary for
cockatoos with calyptorhynchid type beaks to use their foot when positioning food.
Seeds are held against the transverse cutting edge of the lower beak which aligns with the
maxilla during transverse movement creating a cutting motion (Homberger, 2003).
Instead of using sheer force to shell seeds, cockatoos with calyptorhynchid type beaks cut
open shells by using side-to-side motions and their transverse cutting edge.
Diet
Red-tailed black-cockatoos with a calyptorhynchid type beak feed on seeds
enclosed in fibrous-woody, thick-walled shells such as Eucalyptus, Allocasuarina,
Banksia, and Hakea (Cooper, 2000). Eucalyptus is typically favored among the
subspecies of arboreal feeding red-tailed black-cockatoos.
Red-tailed black-cockatoos that have a psittacid type beak typically feed on seeds
from shrubs, grasses, and fruits of trees (Homberger, 2003). The terrestrial feeding
subspecies of red-tailed black-cockatoo, C.b. samueli, favors a diet of burrs and hard
seeds, i.e. Emex and Erodium.
Metabolism
The metabolic requirements of red-tailed black-cockatoos may also influence
their distribution, choice of diet, and the evolution of their bills. A study conducted by
Cooper et al. (2002) determined the basal metabolic rate (BMR) at a thermo neutral
temperature for inland and forest red-tailed black-cockatoos. The evaporative water loss
(EWL) was also determined due to its implications in their distribution and habitat.
Programs were used to record O2, CO2, and dew point data collected during the
experiment and to calculate the rates of O2 consumption, CO2 production and EWL
(Cooper et al., 2002).
The basal metabolic rate of inland red-tailed black-cockatoos was found to be
0.62 +/- 0.13 mL O2 g -1 h-1 and 1.11+/- 0.13 mL O2 g -1 h-1 for the forest red-tailed
black-cockatoo. The EWL of the forest red-tailed black-cockatoo was 0.44+/-0.07 mg g1 h-1 and 0.89+/-0.16 mg g-1 h-1 for the inland red-tailed black-cockatoo (Cooper et al.,
2002). It is beneficial for species that inhabit arid habitats to minimize energetic
requirements and metabolic heat production (Cooper et al., 2002). Since the inland redtailed black-cockatoo live in an arid environment it is logical that they have a lower BMR
than forest red-tailed black-cockatoos, which are found in less arid environments.
Energy Requirements
In order to meet their energy requirements the cockatoos must create a balance
between how much energy they spend obtaining food and how much energy they gain.
Often a greater amount of energy is expended on a certain food than would be on another,
but the energy return is greater. For example, forest red-tailed black-cockatoos can
recover a seed from Eucalyptus marginata nut in almost 12 seconds while it can take 2.45
minutes to recover a seed from Corymbia calophylla, but the energy return is greater for
C. calophylla which justifies the extra time spent to retrieve the seed (Johnstone and
Kirkby, 1999).
Table 1 (Cooper et al., 2002) The number of nuts, cones, and/or seeds needed to fulfill
the energy demands of inland and forest red-tailed black cockatoo.
The relationship between available energy and the amount of time it takes to
acquire the energy is derived from Tables 1 and 2. Fewer nuts, cones, and/or seeds of the
Eucalyptus marginata, Corymbia calophylla, and Banksi attenuata are needed to meet
the energy requirement of the forest red-tailed black-cockatoo (Table 1). It takes slightly
longer to acquire the energy from a Corymbia calophylla nut than it does from the
Eucalyptus marginata, yet the energy return is much faster than the other nuts listed
(Table 2).
Table 2 (Cooper et al., 2002) Amount of time needed to acquire 1 kJ of energy from food
sources by inland and forest red-tailed black-cockatoos.
These results show that the longer time required to open Corymbia calophylla
nuts by forest red-tailed black-cockatoos is compensated for by their high energy return
(Cooper et al., 2002). Similarly, inland red-tailed black-cockatoos are able to meet their
energy requirements by feeding on E. australis seeds.
Conclusion
Considering the variations observed amid the five subspecies of red-tailed blackcockatoo, the differences in beak apparatus is perhaps the most significant. The exact
sequence of events that led to the differentiation in beaks is not precisely known, nor is it
definite how all of the previously discussed issues relate to one another. However, it is
clear that the combination of changing food sources, availability of food sources,
metabolism, and energy requirements are of the main reason for variations of the beak
apparatus in red-tailed black-cockatoos.
Literature Cited
Cooper, C. 2000. Food manipulation by Southwest Australian cockatoos. Eclectus. 8:3
Cooper, C.E., Wither, P.C., Mawson, P.R., Bradshaw, S.D., Prince, J., Robertson, H.
2002. Metabolic ecology of the cockatoos in the south-west of Western Australia.
Australian Journal of Zoology. 50: 67-76
Del Hoyo, J., Elliot, A., Sargatal, J. 1997. Handbook of the Birds of the World. Vol. 4.
Sandgrouse to Cuckoos. Lynx Edicions, Barcelona.
Franklin, D.C., Woinarski, J.C.Z., Noske, R.A. 2000. Geographical patterning of species
richness among granivorous birds in Australia. Journal of Biogeography 27: 820-842.
Higgins, P.J. 1997. Handbook of Australia, New Zealand, and Antarctic Birds.
4: 40-51.
Homberger, D.G. 2003. The comparative biomechanics of a predator-prey relationship:
The adaptive morphologies of the feeding apparatus of Australian Black-Cockatoos and
their foods as a basis for the reconstruction of the evolutionary history of the
Psittaciformes. In Vertebrate Biomechanics and Evolution, eds. Bels, V.L., Gasc, J.P.,
Casinos, A, pp. 203-228. BIOS Scientific Publishers Ltd, Oxford.
Johnstone, R.E., and Kirkby, T. 1999. Food of the forest red-tailed black cockatoo.
Calyptorhnchus banksii naso in southwest western Australia. Western Australian
Naturalist. 22: 167-177.
Podulka, S., Rohrbaugh, R.W., Bonney, R. 2004. Handbook of Bird Biology.
Princenton University Press.
Pyne, S.J. 1991. Burning bush: a fire history of Australia. New York: Henry Holt and
Company.
White, M.E. 1994. After the greening: the browning of Australia. Kangaroo Press.