This
short study of the budgerigar's relationship with eucalyptus
trees has been prompted by the lack of any other studies on
the subject.
Few studies have been conducted on Eucalyptus leaves in respect to Australian birds and their findings have so far been at best inconclusive. This small study is best read as a work in progress that the author intends to add to with more observational data from central Australia in 2004-5.
The studies thus far conducted on Eucalyptus bark and foliage have had as their focus the gaining of information on the digestive systems of Australian animals such as koalas, greater squirrel gliders, grey-headed fruit bats, possums etc. Yet other studies have focused on the usefulness, or otherwise, of eucalyptus trees as stock fodder in times of drought.
Firstly, there are approximately 600 species of Australian eucalypts and different creatures use them in different ways. The koala, a total folivore, typically uses the leaves from only 12 species with particularly high oil contents. In their case they have digestive systems that can unlock the tannin bound proteins in the leaves. Similarly, the greater glider eats only about seven Eucalyptus species although its digestive system is far removed in "design" from that of the koala.
Wild budgerigars are nomadic and therefore opportunistic feeders and they tend to use only five species of eucalypt foliage. It would perhaps be expected that these opportunistic feeders would use a higher number of species if only because they cover a much wider area in the hunt for sustenance.
Although budgerigars are mainly granivorous (seed-eaters), they regularly supplement their diets with small quantities of other items such as insects and green vegetable matter. They use a wide variety of the seeds of native weed species, as well as insect larvae, lerps, physillids, berries etc. They also take in succulent vegetable matter from the leaves, stalks, buds and immature nuts of eucalyptus trees from which they extract the saps and oils.
This diversity which, in a nomadic bird, is often determined by availability of foods, allows nutritional needs to be met and minimizes the risk of nutrient deficiency. The eucalyptus in its various forms represents by far the most predominant plant life in Australia, particularly in those areas inhabited by the wild budgerigar.
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The Budgerigar's Selection of Eucalyptus
trees:
The selection process that makes budgerigars choose one tree, or one species of tree, over another is not entirely clear.
Budgerigars certainly select faster growing tree species and, within them, the fastest growing, healthiest looking (to humans) trees. Faster growing eucalyptus trees produce fewer carbon-based chemical defense compounds such as tannins so the ratio of nutrients to tannins is more favorable in these trees.
The high levels of anti-nutrient compounds in Eucalyptus leaves ie phenols, toxins, terpenes etc. adversely affect nutrient levels in the leaves and so, in turn, affect the diet selection of the birds. I.e. the species that budgies prefer have lower levels of those phenols, toxins and terpenes.
These fast growing trees also exhibit the greatest phloem sap
volumes. The phloem1 sap contains
a low concentration of sugars but possesses the highest concentrations
of total amino acids.2
Most studies show that amino acids in phloem sap become progressively more concentrated the further up the tree they are sampled. The reverse is true of the sugar content of phloem sap, which becomes more concentrated at the lower levels of the tree.
Budgerigars prefer to graze the upper levels of Eucalyptus trees where amino acids concentrations are highest. This seems to indicate that although the birds would no doubt benefit from the sugars in the phloem sap, they select trees more on the basis of tree growth and nitrogen status.
The essential amino acid lysine is contained in the highest concentrations in the saps of the faster growing eucalypts. Lysine, whilst present in some granivorous diets, is usually not in an available form and the birds are unable to assimilate it. Lysine in an assimilable form, however, is found in high proportions in both insects and unripe seeds. The writer has observed that budgerigars feed more on Eucalyptus leaves and phloem sap in autumn and winter when insects and unripe seeds are not to be found. This observation would seem to indicate that the scarcity of lysine in these seasons is one reason that budgerigars choose the faster growing eucalypt species.
It seems that budgerigars may seek the phloem sap in vigorously
growing trees in the summer as a source of water, which explains
their preference for trees with a greater sap flow. Conversely,
in winter they may seek out the phloem sap and cambium3
of eucalypts because hi energy foods are scarce.
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Selection of Leaves & Stems & Stalks:
Budgerigars typically descend in great numbers on a particular tree and strip it of leaves without any apparent sampling for taste or any other perceived quality. How and why these birds discriminate between trees is unclear but it is possible that the tree's individual nutrient content shows up in variations in the leaf color that is detectable by the flock. The birds selection of the more nutritious stems and stalks may simply be a matter of recognizing differences in canopy density or tree height. This being said, there are times when the tree in question is the only tree, or the tallest tree, in the vicinity. In these latter cases it must be presumed that moisture content has been the deciding factor in tree selection.
Where there is a choice of trees, budgerigars go for those with high amino acid concentrations in their phloem sap and this may be a factor in discrimination between trees. It is possible that high amino acid concentrations coincide with distinguishable features in stems and stalks. The trees with faster growth rates that budgerigars prefer can be expected to have an immature bark that strips more easily. These trees would also have a thicker cambium layer containing greater protein concentrations.
Lorikeets, cockatoos and rosellas will all go from tree to tree testing the stalks and leaves but the budgerigar, in this writers experience, never does this. Numerous other Australian birds will feed on the sap excreted by trees that have been wounded by sap eating mammals but the budgerigar appears not to. The bird's gregarious habits seem to inhibit it from this kind of individualistic
behaviour.
To gain access to the soft cambial tissues and phloem sap budgerigars strip the bark of the younger stems and lead shoots. However, they will rarely strip stems measuring more than an eighth of an inch in diameter. The writer has been unable to determine whether this is because the stem is too thick for their beaks to handle or because the thinner stems higher up the tree contain greater concentrations of high nitrogen/low sugar amino acid sap. Unlike this kind of
behaviour in most parrots, bark stripping by budgies is not a 'learned'
behaviour. It is instinctive.
In all eucalyptus species the highest concentrations of foliate based sugars occur in the mid stem region where the leaf is at its widest. The lowest concentrations in the leaves occur nearest the stem apex. Where budgerigars have choice, they will invariably attack trees with adult leaves rather than those in the juvenile stage.
Ascorbates in various percentages have been measured in all Eucalypt foliage but it has been found by a number of studies that those eaten by Australian parrots in general all contain a relatively high percentage of vitamin C.
Composition of Phloem Sap:
Analyses of phloem saps in various eucalypt species varies greatly depending on the season, climate and the soil the tree is growing in.
Phloem sap contains high concentrations of total sugars, predominantly sucrose and raffinose. While glucose & fructose are indeed present they are in minute quantities.
Generally, amino acid percentages are difficult to pin down because they are seasonal and vary widely with the amount of water stress the tree is undergoing. As the tree undergoes more water stress, the amino acid content goes down and the sugar content goes up.
Amino acid concentrations vary, with glutamine accounting for the most variation. Typically methionine, lysine and argenine are to be found in varying quantities in eucalyptus leaves , fresh bark and stalks. These are found alongside other amino compounds, including aspartic acid, threonine, serine, glutamic acid, proline, glycine and alanine.
The ten essential amino acids for birds are: histidine, tryptophan, isoleucine, leucine, lysine, methionine, arginine, phenylalanine, threonine and valine. The amino acids are extracted out of the more complex proteins in the birds digestive system where they also combine to manufacture those AAs in which the bird is temporarily deficient.
Lysine and methionine have been found to be the principal available amino acids in Eucalyptus leaves. The former is said to promote breeding. The latter is a builder of beaks, feathers and claws.
The sugars contained in the phloem sap of Eucalyptus trees are rapidly metabolized by almost all the Australian birds that make use of them. Rainbow lorikeets, for example, are able to extract well over 85% of sucrose and fructose from both phloem sap and nectar. Whilst these birds derive most of their energy requirements from nectar and phloem sap it is probable that budgerigars can easily metabolize well over 60% of phloem sap bound sugars.
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Cambial Sap:
Many bird species in Australia and elsewhere feed on the cambium sap of various trees. In northern European and American forests woodpeckers feed on the cambium sap of a number of deciduous tree species. This, however, tends to be seasonal behavior as opposed to birds that feed on evergreens - such as the eucalypts- in which cambial activity is continuous.
In Australia galahs, sulfur-crested cockatoos and Major Mitchell Cockatoos all strip the mature bark of eucalypts to gain access to the cambium tissue and thus the cambium's phloem sap. The writer has observed some cockatoos returning regularly to wounds they have created in the cambium of eucalyptus trees to take advantage of the continually weeping sap that exudes from them for up to a fortnight before the tree heals.
Birds are not alone in this practice which is utilized by sugar gliders and various possums. These animals seldom ingest the bark they strip but, instead, remove it to inflict a wound that will continue to weep. In the budgerigar's case its small beak size relegates it to the smaller stems where it finds the cambial sap it seeks in the soft fresh bark.
Salt Tolerance:
Australia's ground water in semi arid environments is extremely salty. For the granivorous birds such as the budgerigar that inhabit the semi arid environments of a large part of Australia, an ability to tolerate saline water is critical to their survival. Maintaining their body-weight on water with a saline content of roughly half that of seawater is impossible for most land birds.
The tissue bound water contained in the leaves of Eucalyptus trees may offer an effective method of gaining moisture when surface water is unobtainable or too saline. Another point worthy of consideration is that budgerigars in flocks drinking groundwater predisposes them to predation.
Food that is rich in water content and poor in sodium would benefit budgies, which could theoretically survive without drinking on a food source such as Eucalyptus leaves which typically contains 45 - 70% water.
For Budgerigar nestlings succulent insect and plant food material obtained from Eucalyptus leaves are often the only source of water available.
Minerals:
The minerals contained in the bark and leaves of the eucalypt species most favored by budgerigars have yet, as far as the writer is able to determine, to be analyzed. Unlike the koala - which derives all of its nutritional requirements from foliage - the budgerigar spends time on the ground and takes in minerals from grit, from the water it drinks and possibly from the grass seeds that form the major part of its diet.
Xylem Sap:
The xylem sap has not been considered in this study as it is of little use to budgerigars except for the provision of water. The xylem serves to transport water and mineral ions upward from the roots to the leaves. The xylem is therefore very important to the tree but carries little in the way of nutrients for birds. The amino acid Glutamine is present in xylem sap but in such minute concentrations as to be virtually nutritionally useless to birds or animals.
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Uses Other Than Nutritional:
Anti parasitical properties.
Cineole (a terpene) found the leaves of most fast growing eucalypts
reduces populations of harmful micro-organisms that budgerigars
may have in their digestive tracts. Cineole has been shown to
have bioactivity against a range of microorganisms in lab experiments.
Unchanged cineole in budgerigars feces indicates that it passed
through their digestive tract unaltered (and as it passed through,
it acted on microorganisms). What these studies fundamentally
indicate is that the leaves act against internal (gut) parasites,
as they are bactericidal.
There are probably many other reasons for the bark-stripping behavior of budgerigars. Some of these include beak cleaning, preening and perching. The latter inference being that the germicidal oils contained within the bark are beneficial to the bird's feet. Budgerigars also use the oils contained in eucalyptus to prevent or rid themselves of mites.
Anecdotal evidence from central Australian Aborigines indicates that Eucalyptus maculata makes budgerigars aggressive and this same effect is well-documented in honey bees. According to Aborigines interviewed by the writer, budgerigars will often seek out E. maculata and feed on its leaves prior to breeding.
The observational data and book-based research in this study
suggests that budgerigars are seeking a food and moisture supplement
from the foliage, cambium and phloem of eucalyptus trees. These
nutritional supplements are contained within the area from the
upper stems through the leaf stalks to the leaf tips.
Why they do this would appear to be as follows:
1 Dietary supplementation (amino acids, sugars, vitamins, ascorbates).
2. Gaining access to tissue bound water.
3. Health of gastrointestinal tract & feet.
4. Cosmetic preening & mite repellent.
5. Gleaning for lerps and psyllids.
6. Possible stimulant to breeding by developing aggressive behavior.
Footnotes
1 Phloem Sap: organic materials including
sugars, amino acids and hormones are transported both up and
down the tree by the living cells of the phloem sap.
2 Amino acids: these are the basic
building blocks from which proteins are "configured". Protein
molecules are made of amino acid molecules, bonded end-to-end.
3 The cambium layer: is the first
ring just inside the bark of the tree. When grafting trees this
is the layer that must meet its opposing layer on the other
tree to ensure sap flow and the subsequent "striking" of the
new scion (e.g.branch).
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The above article is the work & copyright of Peter McLaren and may not be copied in part or in whole and used in publication without the express permission of the author. If you wish to use this information for your club magazines, please contact the writer at:
info@budgieworld.net
Permission is usually given as long as the author is credited.
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