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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.
Calyptorhynchus banksii, commonly known as the red-tailed black-cockatoo, is of the Family Cacatuidae (Cockatoos), which is a branch of the Order Psittaciformes. Red-tailed 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.
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 570-870 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 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).
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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).
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.
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.
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).
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.
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.
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 g-1 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 red-tailed 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.
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).
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).
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.
Considering the variations observed amid the five subspecies of red-tailed black-cockatoo, 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.
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