The following articles written by Dr Rob Marshall are off-shoots from the paper "A Relationship between Airsac Mite Infection and the Moult in the Gouldian Finch" he presented to the Australian Chapter of the Association of Avian Veterinarians at Adelaide, South Australia in October 2009.
The Decline of the Gouldian Finch in Nature
The Endangered Gouldian Finch: Current & Past Research Review
The Gouldian finch was listed as an endangered species in 1988 following a rapid decline in total and population numbers over the previous two decades. As a result of this finding, the Northern Territory Government initiated scientific research into the possible causes of the decline.
Research initially focused on documenting the decline, breeding biology, population trends, disease factors, and the impact of fire on seed resources. This work revealed that the Gouldian relied upon a restricted seed diet and that the key wet season grasses were patchily distributed and fire, grazing and rainfall significantly affected seed production in these grasses. Recent research investigates the foraging behaviour of Gouldian finches and the aggressive effect of red-headed birds on breeding outcomes.
Twenty years of scientific research have now passed revealing significant findings about the Gouldian finch but our knowledge remains meager.
Possible Causes of Decline
Several possible causes for the rapid decline of the wild populations over the past fourty years have been presented. These include commercial trapping for aviculture, habitat destruction associated with land clearance by fire for cattle grazing and destruction of some important perennial grasses by grazing cattle, feral pigs and buffalo. Death from airsac mite infection has also been viewed as an important cause of declining numbers but the exact reasons for an increased susceptibility to this infection have not been investigated.
Trapping was vigorous for almost three decades from 1960-1988 and provided many thousands of Gouldian finches for aviculture. Although trapping had a significant effect on Gouldian numbers and populations during this time, numbers should have rebounded over the ensuing twenty year long period because of the prolific breeding ability of Gouldian finches. This has not occurred and numbers in remaining populations now appear stable but remain low.
Altered fire regimes have had a serious effect on the seeding grasses available to Gouldian finches and is believed to be the single most important reason why Gouldian finch numbers have not rebounded since trapping became illegal in 1988.
It is the loss of traditional fire burning practices over the past forty years that is most responsible for the destruction of essential understorey grasslands. Traditional knowledge disappeared when cattle stations no longer employed aborigines following legislation in 1966 that gave aborigines the right to receive equal pay to white Australians as station hands. Before this time, aborigines worked for food and lodgings, and imparted their traditional knowledge of burning practices to the landowners. Traditional fire burning practices - patch burn and use low heat fire by burning during late wet season or early dry season and in the morning - were lost as Aborigines were no longer used as station hands following the legislative change.
Traditional fire methods have a positive effect on the environment and help regenerate several of the perennial wet season grasses - Cockatoo Grass (Allopteropsis semilata), Curly Spinifex (Triodia spp), Ribbon Grass (Chryysopogon fallax)- favoured by Gouldian finches.
Non-traditional fire practices were incorporated intentionally in order to produce large hot wildfires that cleared the land - including areas inhabited by the Gopuldian Finch - of undergrowth and native grasses. These destructive fires accelerated the establishment of drought resistant grasses such as Buffel grass favoured by cattle but at the same time destroyed important perennial grass tussocks (e.g. Cockatoo Grass, Curly Spinifex, Ribbon Grass) and nesting habitats.
Traditional fire practices have a positive influence on the quantity of seed produced by Cockatoo and Curly Spinifex grass so that excluding fire completely would not be beneficial.
Grazing destruction by feral pigs and buffalo of perennial grass plants (Cockatoo grass and Ribbon grass) reduce the amount of their seeds available to Gouldian finches during the wet season, which ultimately has a negative effect on breeding outcomes.
Additional Views on the Causes of Decline
Although reduced availability of critical wet season grass seed resources due to changes in land use and consequent changes in grazing and fire regimes, combined with natural fluctuations in seasonal rainfall is thought to be involved with the decline of the Gouldian Finch in its natural environment, as yet there is no clear link between resource scarcity and its endangered status (Dostine and Franklin 2002; Fraser 2000; Crowley and Garnett 1994). In other words, starvation due to lack of food supply is not believed to be the cause of the decline in Gouldian numbers.
There must therefore be other more complex reasons for the decline in the numbers of Gouldian finches in the wild. Recent research by Pryke and Griffith has discovered behavioural differences between the red and black-headed Gouldian finches (Pryke & Griffith, 2009).
In Nature, black-headed birds outnumber red headed birds by three to one. It has been suggested that in the distant past the red and black headed populations were separated geographically and existed as two distinct sub-species before coming back into contact again to co-exist together as a single population. The geographic separation may also explain the distinct behavioural differences displayed by the different head colour.
Both male and female red headed birds are more aggressive than the black-headed birds. The more competitive nature of the red headed birds may be explained by a limited availability of nest sites, water and food resources in the region they inhabited during their geographic separation. The passive nature of the black-headed birds may reflect habitation in a plentiful and less competitive environment.
Previous research has shown that there is a higher mortality of daughters compared to sons when interbreeding between different species, subspecies or races occurs. Pryke and Griffith found that fewer Gouldian offspring survived when red-headed and black-headed birds were paired together compared to when pairs of the same head colour were allowed to breed. This finding has not been observed in large populations of captive gouldians where black headed birds freely select red-headed mates although there is an opportunity to chose same head colored mates (personal communication with George Simpson from Tully, Northern Queensland, who has bred the same family of birds of original wild stock for more than 50 years with no introductions in very large aviaries).
However, this new work by Pryke and Griffith that reveals a behavioural and genetic incompatibility between black and red headed birds may provide the complex reason behind the sudden decline in populations during the trapping years and a failure of these populations to recover over the following 25 years.
Under normal circumstances Gouldians prefer to mate with birds of the same head colour with black headed pairs being able to produce red-headed offspring and red-headed pairs capable of producing all three head colours: black (three times more likely than red heads), red and yellow (one in a thousand birds).
Although red-headed birds gain a huge dominance advantage (i.e. occupy the best nesting hole, first to drink at water holes etc.) over the black-headed birds via there social aggression, the high stress levels and sex hormones associated with this behaviour predisposes them to health issues such as airsac mite infections, as they are unable to maintain normal levels of immunity.
When population numbers were dramatically reduced by trapping there is a likelihood that a greater proportion of the pair bonding occurred between red and black headed birds because when there is a lack of mate choice, females breed unwillingly with incompatible mates. Higher numbers of these incompatible pairs would have a negative impact on the ability of the local population to increase, due to a decreased likelihood of offspring survival and an increased number of males amongst surviving offspring (Pryke & Griffith, 2009). The end result is a decrease in females passing into the next and future generations (Pryke & Griffith, 2009). Behavioural differences between different head colour birds may also be responsible for a growing lack of genetic diversity in the wild populations and lead to a dominance of the highly aggressive red headed finch that is more susceptible to airsac mite infection.
Earlier research efforts attribute the decline in wild Gouldian populations to a restricted diet and seed shortages during the moult (Dostine et al 2002). This research revealed that Gouldian eat only certain seed types and with destruction by time and grazing, limit the availability of seeds.
Gouldian Airsac mite infections have also been thought to play a role in the decline of the species (Tidemann & Woinarski 1994). The research findings regarding the behavioral differences between black and red- headed birds adds to the complexity of the Gouldian problem but may be the missing piece that solves the puzzle.
It has been speculated that the impact of S. tracheacolum might be exacerbated during periods of physiological stress associated with the moult and food shortages at the onset of the wet season (Lane & Goodfellow 1989 cited in O'Malley 2006a; O'Malley 2006).
I have noted the rapid effect that airsac mite S. tracheacolum has on the health and survivability of captive Gouldian finches and believe that airsac mite infection, which often appears during the moult and breeding period, has played a part in the decline of the wild Gouldian Finch population. However, Gouldian finches enjoy a symbiotic relationship with airsac mites and any devastating effect on the wild population I believe requires extenuating stressful circumstances. Such a situation occurs in captive Gouldian finches when a compressed moult (compressed moult link) is abruptly interrupted by cold weather or another sudden onset environmental stress.
Refer to other sections on this page that explain my views regarding a link between airsac mite infections and the moult in captive Gouldian finches, and when linked to the impact of red-head black-head incompatibility theory may be an important factor in the demise of the wild populations.
Managaement of Wild Populations
The Gouldian finch is now considered rare in Western Australia and endangered in both Queensland and the Northern Territory (Tidemann et al., 1999; O'Malley, 2006). In 2000, wild population numbers were predicted at 2500 breeding birds, with a downward trend evident (Gelis, 2003).
To conserve the wild Gouldian Finch populations and save the species from endangerment, management strategies and recovery programs need to organised.
The most significant action taken to improve the current status of the Gouldian finch has been the development of a National Recovery Plan (O'Malley, 2006). This was initiated in 1994 in collaboration with the National Gouldian Finch Recovery Team, as a guide into research of the diet, reproductive biology, population dynamics and potential threats of the finch (O'Malley, 2005). The plan outlines actions such as land management, taking into account the impact of fire and grazing on the finch, restoration of habitat and reintroduction into the wild (O'Malley, 2005; Soucek, 2008).
Land management has important conservational implications with regards to the Gouldian Finch (O'Malley, 2006). Although precise habitat requirements are still unclear, persistence of populations of the Gouldian Finch at certain sites enables recognition of critical elements contributing to the success of such populations (Dostine et al., 2001). A number of landscape components appear to be important for the survival of Gouldian Finches. Large areas of rocky hills with a dense understorey of sorghum grasses characterise the finches' breeding habitat during the dry season (O'Malley, 2006; Soucek, 2008).
The topography of these sites, in addition to natural barriers such as rivers and creeks, restricts the spread of fire, reducing its impact on seed availability (Dostine et al., 2001). Presence of large numbers of gum trees in these areas, favourably salmon gums or northern white gums, is also important in providing nesting hollows for the finches (Dostine et al., 2001).
Gouldians need to drink every day, hence reliable water sources are essential, preferably in the form of shallow waterholes protected from predators (O'Malley, 2006). Patches of grassy woodlands within 10 kilometers of the Gouldian Finch breeding grounds in the lowlands provide a food source in times of seed shortage, such as throughout the wet season (Dostine et al., 2001). In habitat management, preservation of these areas of woodland is critical to ensure the finches have access to alternate feeding sources at times of food shortages that may occur towards the end of the dry season (Dostine et al., 2001; O'Malley, 2006).
Identification and preservation of key habitat areas is critical to conservation of the species (Dostine et al., 2001). It has facilitated monitoring of population trends and analysis of health in different finch populations, which is an essential in assessing the success of any management regime (Dostine et al., 2001; O'Malley, 2006). Knowledge of the landscape is also important in developing appropriate fire and grazing management strategies in major Gouldian habitat sites (Dostine et al., 2001; O'Malley, 2005). Finally, realising aspects of the habitat that contribute to the persistence of Gouldian Finch populations in these areas could also assist in strategising the reintroduction of additional populations into carefully managed habitats (O'Malley 2005).
As described earlier, fire and grazing processes are significant threats to the long-term survival of the Gouldian. Current management regimes regarding these two issues are being implemented at sites where significant Gouldian Finch populations have been identified in an effort to preserve the crucial habitat required by the finch for survival (O'Malley, 2006). Manipulating the distribution and timing of fire has also been described as a way of encouraging seed growth of the key wet season grasses, but also protecting nest trees and feeding areas from the detrimental effects of large, hot fires (Dostine et al., 2001; O'Malley, 2005). Fire management is based on forming mosaic patterns of burnt and unburnt patches of land, with the intervals between each burning varying (O'Malley, 2005). This mimics traditional practices carried out by Aboriginal people, prior to European settlement (O'Malley, 2006).
In terms of grazing management, fencing off important wet season grasses from production animals and feral pigs has been suggested to preserve feeding habitat (O'Malley, 2006). However, this could affect livestock productivity, thus cooperation with pastoral land managers may be difficult. Feral herbivore control is the current grazing management practice. These management regimes are still not ideal, so further progress needs to be made with regards to current knowledge on habitat, diet and foraging behaviour of the Gouldian Finch, and the precise impact of inappropriate fire and grazing practices on the survival of the species (Dostine et al., 2001; O'Malley, 2006).
The availability of cockatoo grass during the wet season is thought to have a positive impact on breeding outcomes for wild populations. Landowners in regions inhabited by Gouldians could benefit financially by replacing grazing land for a commercial plot of perennial grasses favoured by Gouldians and at the same time provide Gouldians with a reliable food supply during the wet season.
Commercial production of native grasses of the Northern savannas by Sam Crowder and Boronia Saggers of "Greening Australia NT" is underway near Katherine, Northern Territory. Amongst the grasses being cultivated are Cockatoo Grass (Alloteropsis semilata), Ribbon Grass (Chrysopogon latifollus) , Giant Spear Grass (Heteropogon triticeus) the seeds of which are favoured by Gouldian finches during the wet season.
Recently, the reintroduction of aviary-reared birds into protected habitats has been trialled as a conservation tool (O'Malley, 2005) in Mareeba, Queensland. Its success relies strongly on habitat enhancement, threat abatement in release areas, and continual monitoring of the reintroduced birds (O'Malley, 2006). Predation has hindered reintroduction efforts (Wildlife Conservancy of Tropical Queensland, 2009), and O'Malley (2005) describes plans for future trials where finches spend less time in captivity, in the notion that they will have retained predator avoidance behaviour by the time of their release.
One final management strategy to be considered is community involvement and increasing public awareness. Members of the community can facilitate the monitoring of Gouldian populations by reporting sightings or participating in annual waterhole counts (O'Malley, 2005). Additionally, encouraging pastoralists and Aboriginal landowners to test fire management regimes on their properties would be of significant benefit to the Gouldian recovery effort (O'Malley, 2006). Although this would involve some economic cost to the landholders, productivity losses would be reduced due to improved fire practices thus limiting large, hot wildfires that destroy property and pasture resources (O'Malley, 2006). The Jawoyn Aboriginal Corporation is actively involved in the recovery effort, participating in fire management and feral herbivore control on Jawoyn lands with significant Gouldian Finch populations. It is in the interest of the recovery team to increase the involvement of the Aboriginal co mmunity, as the finch
distribution covers much of their land (O'Malley, 2006).
There have been 3 successive recovery plans for this species (O'Malley, 2006). The current program runs from 2007 to 2016, with an estimated cost of $970, 000. This is being met by State and Territory governments, numerous organisations, including Aboriginal, pastoral and conservational groups, and the general public. However, it is likely that costs will continue past the length of the plan, as the recovery process of the finch is anticipated to exceed 5 years (O'Malley, 2006).
For the conservational status of the Gouldian to be changed, populations will need to show a sustained increase in numbers over several seasons (O'Malley, 2006). By adopting the management practices described above, such a goal may be achieved.
Relationship Between the Moult, Airsac Mite Infection & the Decline of the Gouldian Finch in Nature
Moult Period in Wild Populations: Special Features
The time taken for the Gouldian finch to complete the moult is rapid compared to co-occurring masked and long-tail finches (Franklin et al. 1998), and thought to reflect the more mobile and dispersive nature of the Gouldian finch (Tidemann and Woinarski 1994).
Gouldian finches have adapted to an environment under control of a tropical weather system that divides the year into two seasons - a wet and dry season - where they breed and moult at different times than other finches. A rapid moult appears to be an evolutionary adaptation to an unpredictable climate and tropical woodland breeding environment where the wing moult needs to be completed before the end of the dry season when seed shortages are frequent and prior to arrival of the wet season when torrential rains reduce foraging activity and curtail their ability to fly long distances in search of alternative food supplies.
The rapid nature of the Gouldian moult has been noted by scientific researchers (Franklin et al. 1998) and is its most notable feature. Conflicting scientific reports regarding the moult in wild Gouldian finches reflect a variable rate of progress rather than any observation failing. Milton Lewis (2001) has noted both adults and juveniles moult during September, October & November with most of the wing flight feathers being replaced during October and the body moult is completed by mid December. The moult of captive birds - housed in temperate climatic regions of Australia, that have finished breeding by June and have been provided with a perfect diet - progresses far more rapidly. Their wing moult is complete by mid October. The head moult is well underway by the second week of November and the moult is completed before December. The moult period in USA is said to be more prolonged than the ideal captive moult period mentioned here.
From Milton Lewis' research the early completion of the wing moult appears to be a significant event that guarantees strong flight by the onset of the wet season when heavy rains make foraging activities more demanding. Tidemann & Woinarski (1994) record that the moult period finishes in November. This finding compares favourable to the closure of the captive bird moult period. These two researchers also mention that seed shortage can occur near this time, a finding that supports the need for Gouldian finches to complete their wing moult as rapidly as possible.
The moult of Gouldian finches is an annual seasonal event with a starting time that may vary slightly depending upon local climatic conditions. In captive Gouldian finches I have observed the moult to start as early as July. A start time around July may also be plausible for wild birds following a good wet season.
A loss of the first (most proximal) primary flight feathers in each wing heralds the start of the moult period. The appearance of these - and the next three primary flight feathers feathers - on the floor of Gouldian aviaries often goes unnoticed because they are each replaced very gradually over a period of several weeks.
For wild Gouldian finches, there is a recovery period following the completion of the moult before breeding behaviour starts late in the wet season (March and April). Breeding activities (e.g. seeking out nesting hollows) begin in response to a drop in humidity that stimulates the germination of Sorghum grasses and sex hormones that initiate breeding condition.
The total rainfall and extent of the wet season varies from one year to another, so that Gouldian finches are both seasonal and opportunistic breeders. During drought periods the extent of the wet season is truncated whereas during good seasons the amount of rainfall and extent of the season may be prolonged resulting in up to three clutches of eggs to be produced during a breeding season.
The amount and timing of rainfall during the wet season influences not only breeding success (Dostine et al. 2001) but also has a direct impact on the ability of Gouldian finches to complete their moult as quickly as possible. The start and extent of their breeding activity coincides with a period of peak resource availability within their habitat. Following good wet seasons, plentiful supplies of native sorghum and other fallen seeds (Dostine & Franklin 2002; Dostine et al. 2001; Goodfellow 2005; Tidemann 1993b, 1996; Tidemann et al. 1993) provide nestlings and fledglings with a reliable food resource that remains into the moult period.
Woinarski and Tidemann (1992) noted that the Gouldian finch is more vulnerable to drought during a moult than other co-occurring finch species because it is moulting at a time when seed shortages may occur (figure 4). The moult is a time during which birds may experience physiological stress, as it is a highly energetic process.
The rapid moult of Gouldian finches renders them more vulnerable to the effects of stress., because there is a greater energetic cost involved with a rapid moult than a normal moult (Guillemette 2007).
Although Gouldian Finches have a more restricted diet compared to other co-occurring granivorous birds (Dostine and Franklin 2002; Fraser 2000; Crowley and Garnett 1994), the seeds of the annual grasses (e.g. Sorghum spp., Sarga spp., Vacoparis spp. Fire grass etc.) they seek and available to them for most of the moult period provide a higher quality of nutrient resource than early wet season perennial grasses such as Cockatoo grass and curly Spinifex grass.
It is thought that the critical period for physiological stress for wild Gouldian finches occurs at the end of the dry season and onset of the wet season when food supply may be very low during drought. This is a time when the flight feathers are also being replaced. The length of time food supply is scarce may vary according to the pattern of rainfall in the wet season and the dry season fire regime.
A potential to slow down or accelerate the growth of new feathers is a notable feature of the Gouldian moult that has been observed in captive but not recorded in wild birds. This adaptive feature - that must also occur in Nature - allows the rate of progress of moult to increase or decrease according to the availability of nutritional resources and changing climatic conditions.
Providing additional nutrition via a soft food helps to accelerate the progress of the moult in captive Gouldians. The moult may also be delayed (slow down) when nutrient resources are lacking, during excessively cold or hot weather, when breeding activities extend into the moult period or by disease.
In Nature, with an uncertain food supply, physiological stress in the Gouldian is avoided by its ability to accelerate or slow down the moult. In captive birds, a compressed moult is the visible sign of an accelerating moult, whereas a delayed moult refers to a moult that is progressing slowly. The concepts of a compressed and delayed moult have not been discussed in Gouldian finches before. This paper details my understanding of a delayed and compressed moult in Gouldian finches, their relationship to each other and airsac mite infection and links to the decline of wild populations.
The Compressed Moult
From my observations, captive Gouldian finches are capable of growing multiple primary flight feathers simultaneously. The goldfinch (Carduelis tristis) shares this ability (Middleton 1977). This moult pattern is known as a compressed moult (Storer & Jehl 1985). A compressed moult is the visible evidence of an accelerating moult (photo 7).
Theoretically a compressed moult in captive Gouldian finches may occur at any stage of the moult but mostly involves the 4th to 9th primary flight feathers during the peak moult period. Sometimes it is seen at the end of the moult period when many pin-feathers appear together on the head. There are a number of reasons why a compressed moult may occur.
A compressed moult in Gouldian finches occurs most frequently during the peak period of the moult (September - October in Australia and March - April in USA). For some seabirds a compressed intense moult is believed to be an adaptation for exploiting an abundant food source (Storer & Jehl 1985). In Gouldian finches a compressed moult occurs only when plentiful food resources are available as there is a great energetic cost for flight feather growth (Guillemette 2007, Murphy M.E. 1996) with daily energy expenditure increasing up to 20% during the peak period of the moult (Jenni & Winkler 1994). Protein requirements are also increased during the moult as feather mass comprises 20% of total body protein (Murphy, King et al. 1988).
Consequently a compressed moult will not occur when food resources are low or of poor nutritional quality. A compressed moult should be considered a natural and healthy event for Gouldian finches.
A compressed moult in captive Gouldian finches is prevalent during September and early October in Australia following a slow start to the moult. It appears to occur as a compensatory mechanism to ensure that the wing moult is completed as rapidly as possible.
Prolonged cold winter temperatures or exposure to cold spells at the beginning the moult period coupled with an inadequate diet is the most common causes of a delay in the start of the moult in captive birds. Food supply must be plentiful during September or early October if a compressed moult is to occur in wild birds. Theoretically, physiological stress associated with the growth of the flight feathers in wild Gouldian may start as early as August and continue until late October at the close of the wing moult (Milton Lewis, 2001). During this time seed resources are declining and by late September and early October may be at their lowest level. Often when there is drought, food supply may abruptly decline (Crowley and Garnett 1994) around this time and delay the progress of the moult.
For captive finches, a moult may be delayed by an extremely poor level of nutrition or by overlapping stress factors that may stop the moult completely. If this is to occur it will be seen after the first four primary flights have been replaced.
Theoretically, conditions for a compressed moult in wild Gouldians occur when premature rains falling in September break a drought and quickly provide a bountiful supply of the annual and nutrient rich Fire Grass, the seeds of which are highly nutritious and favoured by the Gouldian. A compressed moult may also appear following a drought period when heavy rains fall in early October and initiate rapid tussock growth of Cockatoo Grass that produces a very large nutritious seed also relished by Gouldian finches.
Compressed Moult: Survival Implications for Gouldian Finches
I believe a compressed moult is possible in wild birds although its presence has not been previously recorded. A compressed moult of the wing feathers is most significant for Gouldian finches, as these feathers require most nutrient and energetic support and when interrupted renders them vulnerable to acute onset Airsac mite and other infections. Interruption of a compressed moult of the head feathers is highly significant because it occurs at the end of the moult period when immunity is at its lowest level. Captive Gouldian finches become vulnerable to Airsac mite and other infections when this form of a compressed moult is interrupted by cold weather or other stressful episodes.
A compressed moult appears to be an evolutionary adaptation of the Gouldian finch to an unpredictable climate enabling it to accelerate the wing moult when conditions have delayed the start or interrupted the early progress of the moult.
A compressed moult becomes possible in wild birds when the moult is interrupted or delayed by cold September temperatures followed by warm weather and premature wet season rains that provide a good food supply during the peak period of the moult in October. When the progress of the moult is delayed or interrupted by inadequate food resources or cold temperatures, a compressed moult may occur if food supply is restored.
In this scenario, drought conditions and low rainfall during the previous wet season curtail the availability of seeds from Sorghum and other grasses and delay the progress of the moult. This lack of available food resource at the start of the moult combined with continuing cold temperatures into September delay the germination of annual grasses that are needed to support a rapid moult. This lag period can be overcome by a compressed moult, which occurs when warm temperatures and early rains create favourable germination conditions and a sustained supply of seeding grasses for the peak period of the moult in October.
Airsac mite infections associated with the moult are most likely to occur in wild populations of Gouldian finches in October when adverse conditions suppress the immune response during a compressed moult and in November at the conclusion of the moult (Martin L.B., A. Scheuerlein, and M. Wikelski. 2003).
The consequence of Airsac mite infections is rapid and severe because infra-populations may dramatically increase in size within a very short period of time. Infections decrease appetite and mobility and become rapidly life threatening because finches must eat and drink each day.
Gouldian finches, especially juveniles are thought to become vulnerable at the closing stages of the moult when the nutritional resources needed to support the moult are lacking. In Nature, Gouldian finches are most vulnerable at the end of the moult period when food resources are low or abruptly decline at this time. The high prevalence of Airsac mite infection seen in captive Gouldian finches at this time supports the view that airsac mite infections are a result of a depressed immune response.
Potentially devastating losses of wild populations from Airsac mite infections may be possible in October when a compressed moult is interrupted by a sudden decline in food supply because the energetic needs of a compressed moult are extremely high and override the energetic costs associated with maintaining a normally functioning immune system (Derting & Compton 2003)
Catastrophic losses are possible as a result of infection because Airsac mite numbers can rapidly explode when the immune response is severely compromised. Losses are likely to occur as a result of Airsac mite infection at the conclusion of the moult in November and when a compressed moult is interrupted by a sudden decline in available food resources during October.
Airsac Mite Infection, Compressed Moult & Gouldian Finch Survivability
Gouldian finches enjoys a symbiotic relationship with the Airsac Mite, infections occurring when they are experiencing physiological stress.
In wild birds, infections are most likely when there are overlapping stress factors. This circumstance exists during drought when food resources are low during their moult. By delaying (slowing down) the moult wild birds may reduce the level of stress and limit the effects of airsac mites. Under these circumstances airsac mites are unlikely to cause catastrophic losses because there will not be a sudden increase in their numbers. However, catastrophic losses may occur when there is a sudden interruption of a compressed moult.
Loss of protective immunity related to the stress associated with an increased level of aggression by red-headed Gouldians towards black-headed birds - at water holes and whilst foraging towards the end of the dry season during the moult - also has the potential for catastrophic losses of red-headed males and females.
Captive Gouldian finches become extremely vulnerable to airsac mite infection when cold spells interrupt a compressed moult involving the simultaneous growth of three or more primary flight feathers.
In wild Gouldian finches, a compressed moult may be interrupted by an abrupt decline in food supply.
In Nature, a compressed moult is likely to occur when rain follows a dry period that has delayed the onset of the main wing moult (i.e. the simultaneous growth of the primary and secondary flight feathers that starts in August). The sudden availability of a rich food resource following rain promotes a compressed moult but the Gouldian finch will experience a sudden and high level of physiological stress if this food supply abruptly stops. Food supply may abruptly decline during a compressed moult when no further rain follows early September (producing fire grass) or October downpours (germinating Cockatoo grass). A compressed moult is interrupted at this time because the life cycle of these grasses is very short-lived and seed availability abruptly declines.
Airsac Mite infection is likely because the acute onset of energetic and physiological stress occurs when immunity is already depressed as a result of the moult.
Sparrow studies reveal an increased energetic cost and a reduced immune response during a moult (Martin L.B., A. Scheuerlein, and M. Wikelski. 2003). Lowered immune responses were seen in sparrows during the heaviest part of their moult and greatest loss of immune function occurred immediately at the conclusion of the moult (Martin L.B., A. Scheuerlein, and M. Wikelski. 2003).
A critical reduction in immunity is to be expected when a sudden decline in food availability occurs during the height or at the conclusion of the moult (Franklin et al. 1998). This statement is even more relevant when there is a sudden decline of food during a compressed moult. In wild populations of Gouldian finches, it is during these instances of extreme physiological stress that immunity against airsac mites may be overcome (Lane & Goodfellow 1989 cited in O'Malley 2006a; O'Malley 2006).
Airsac mite infections are common in captive Gouldian finches during the peak period of the moult (September-October in Australia and April -May in USA) and at the conclusion of the moult (November-December in Australia and June-July in USA). Airsac mite infections during these months are often due to poor nutrition or fluctuating weather conditions. Fortified nutrition and repeat Airsac Mite treatments will prevent infections at these vulnerable times and help complete a timely moult.
Airsac mites are found naturally in the Gouldian finch (E. gouldiae) and 6 co-occurring species (long-tailed finches (Poephila acuticauda), masked finches (P. personata), pictorella manikins (Heteromunia pectoralis), zebra finches (Taeniopygia guttata), double-barred finches (T. bichenovii) and budgerigars (Melopsittacus undulatus) (P.J.Bell 1996). The prevalence and intensity of infection in Gouldian finches is significantly higher than in other species except Pictorella manikins (P.J.Bell 1996).
In the face of continuous threats from parasites, hosts have evolved an elaborate series of preventative and controlling measures - the immune system - in order to reduce the fitness costs of parasitism (Sheldon B.C. and S. Verhulst 1996). However, these measures do have associated costs (Sheldon B.C. and S. Verhulst 1996). In Gouldian finches, infections are capable of causing respiratory problems that can lead to death (Bell 1996; Tidemann et al. 1992c, 1993).
A symbiotic relationship between Gouldian finches and the Air-sac Mite (Sternostoma tracheacolum) is likely to exist, as this endoparasite is present in a high proportion of the wild population (Tidemann et al. 1992c, 1993).
In captive Gouldian finches, Airsac mite infection is a common cause of illness and death. Infections cause illness in captive birds that then interrupt their moult. Signs of infection may not be obvious in many birds other than the effect it has on the progress of the moult. Infection delays the moult the signs of which - mostly baldness - do not become apparent until the end of the moult when immunity is at its lowest ebb.
Acute infections often result in death as Airsac mite numbers can rapidly increase within a short time. Persistent infection often results in death from secondary infections (photo 8). Death from Airsac mite infection is a rare event when adequate nutrition is provided during a normal moult i.e. when flight feathers are being replaced one at a time. However, diets fed to captive Gouldian Finches during the moult are often inadequate, which predisposes them to subclinical Airsac mite infection, the signs of which are non-specific (i.e. fluffed up look, inactivity, ill-thrift and a delayed moult).
Acute illness and death associated with Airsac mite infections are most common when a delayed or compressed moult is interrupted by adverse weather conditions.
Immuno-protection during part of the life cycle of Sternostoma tracheacolum helps explain the symbiotic relationship. Transmission of infection between Gouldian finches is by non-gravid non-gorged females that mainly inhabit the upper respiratory tract, buccal and nasal cavity. These females may also move to the posterior abdominal airsac, where they are protected from the host's immune response (P.J.Bell 1996).
Disease caused by a sudden increase in gravid female numbers is controlled by conditions that maintain a healthy immune system. Non-gravid non-engorged female mites residing in the posterior airsacs being protected from any immune response remain a potential source of rapid re-infestation should immuno-suppression occur.
When immunity is compromised - by social aggression or when a compressed moult is suddenly interrupted - a rapid increase in gravid females may occur because unfertilised eggs in the lungs are capable of arrhenotokous parthogenesis (i.e. unfertilised eggs capable of developing into haploid males) and proportionally more male mites persist in the lungs with small infra-populations (Experimental and Applied Acarology 1996).
Gravid females tend to occupy the airsacs, syrinx and trachea and move to the lungs to lay their eggs. This form of the mite is most responsible for the sudden onset of severe respiratory symptoms that will end in death. The eggs quickly hatch with the nymphs and sub-adults feeding off the blood rich pulmonary tissue. This stage of infection causes asthmatic type symptoms leading to an inability to fly and disinterest in foraging. Adult males remain mostly in the lungs. The life cycle may be completed within 6 days (P.J.Bell 1996) so that many birds can become infected and die over a very short period of time.
It is the ability of Airsac mites to complete their life cycle rapidly under certain conditions and produce many mites that renders Gouldian finches extremely vulnerable during times of acute stress. Acute physiological stress during the moult period is most likely to occur when a compressed moult is interrupted, when a delayed moult is compromised by cold weather or poor nutrition towards the end of the moult period. Acute airsac mite infection of red-headed individuals is also possible when they exhibit social aggression at the beginning of the breeding season.