With photos by Angi Kim
What’s causing the decline of the Tasmanian forty-spotted pardalote?
Forty-spotted pardalotes don’t have the flash of a wedge-tailed eagle, or the danger of a cassowary, but they gain a certain mystique from their rarity and knack for passing undetected. As one of the smallest songbirds in Tasmania, they use drab colouration and quiet calls to avoid the attention of aggressive competitors, like the fearsome wattle bird. By staying out of the fray, they are free to spend their days high in the canopy, systematically gleaning insects, leaf by leaf. Although they often go unnoticed, they are essential to keeping forests healthy.
At one time, forty-spotted pardalotes (Pardalotus quadragintus) ranged throughout eastern Tasmania, but as forests were cleared for agriculture, they retreated into smaller, more isolated patches of habitat. By the 1960s they were limited to Maria Island, Bruny Island, and small mainland pockets in the southeast, as well as Flinders Island in the northeast. In 1993, forty-spots were listed as endangered, and much of their remaining habitat was protected in parks and reserves.
Because their habitat was well-protected, biologists thought the species was safe. By 2008, however, the soft “where, where” call of forty-spots was missing in many forests. A survey in 2009/10 revealed that the population had declined by 60% since the previous survey in 1997: a worrying rate decline over just 12 years.
Despite their rarity and endangered status, we don’t know much about forty-spots. Why have they declined so sharply within intact habitat? What are the current threats to their populations? And how can we protect the species into the future?
In 2012, I became interested in these questions. I had just finished a study of hollow-nesting birds in British Columbia, Canada, and like many biologists, was intrigued with the unique fauna of Australia (though in many ways, the grassland-aspen systems of interior British Columbia are similar to the dry eucalypt woodlands of eastern Tasmania). I started a PhD project investigating threats to forty-spotted pardalotes, which has allowed me to focus exclusively on this species for three years.
I recruited a crew of plaid-clothed and binocular-wielding field assistants, and we spend eight months of each year climbing to nest hollows, catching birds in mist-nets, and surveying habitat. We have research bases at Bruny Island and Maria Island, where we drive, cycle, hike, and kayak to field sites. We brave rain, wind, and tiger snakes (including a bite to the ankle and a helicopter ride to the Royal Hobart Hospital on one unfortunate occasion).
And, most importantly,we carefully document patterns of reproduction, survival and mortality in forty-spotted pardalote nestlings and adults. Although this research is ongoing, we have found that threats to forty-spotted pardalotes are more complex than initially thought.
Nest hollows are rare in second growth forests
The first step in each field season is to find all of the pardalote nests within the study sites before the nestlings fly away or become sweet snacks for predators. Forty-spots often reuse hollows from year to year. But if they don’t have an established nest site they inspect a variety of hollows, looking for a home with a small entrance to keep out predators and a large chamber with space for up to five nestlings. Incredibly, the expand existing holes with their tiny bills, in a ecological role reminiscent of Northern Hemisphere woodpeckers.
Typically, breeding pairs find a hollow and start laying eggs by September, but some of the pairs we observed were still searching for hollows into October and November. Several pairs never found a suitable hollow and were unable to breed. This points to our first threat to forty-spotted pardalotes: a lack of suitable nest hollows within existing habitat.
All forty-spot habitat has been logged in the past and the regenerating forest is dominated by young, healthy trees, which have very few hollows. In old growth forests, large trees with dying sections provide a large number and variety of tree hollows, but these veteran trees are rare or non-existent in second-growth forests.
In some cases when forty-spots started building a nest in hollow, we saw the arrival of their sister species, the much more common striated pardalote, displaying and fighting in an attempt to gain ownership. Forty-spots are thought to be an aggressive species, and dominant in when found in high numbers. Our initial impressions supported this: forty-spots fought off striated pardalote approaches, often chasing away intruders every few minutes over a period of days.
However, we would inevitably find striated pardalotes residing in the disputed hollow at the end of these altercations. Striated pardalotes are larger and have greater capacity for sustained flight, which likely give them the advantage in a war of attrition. Most takeovers occur while forty-spots are building nests, but occasionally striateds will toss eggs out of active nests.
Competition between striated and forty-spotted pardalotes is a natural phenomenon—both are native species—but it may be intensified by the scarcity of nest hollows.
The problem of nest hollow limitation is a common one for hollow-nesting birds and mammals globally. Supplemental programs to provide nest boxes, such as the Bluebird Trail in North America, have been vital in restoring higher numbers of nesting sites and increasing populations of many threatened species.
In 2009, a Tasmanian biologist, Matt Webb, installed 100 nest boxes at Bruny Island. When he checked them a year later, the only occupants were huntsman spiders. But in 2012, I checked the boxes again and found 27 forty-spotted pardalote pairs sitting on eggs or feeding nestlings. Like many other species of hollow-nesting birds, the forty-spots needed a few years to become acclimatised to boxes. Once they started using boxes, there were immediate benefits: they fledged more nestlings from boxes than from natural hollows and sites with boxes had higher densities of breeding pairs.
Excited by the potential of nest boxes as a conservation tool for forty-spotted pardalotes, the Bruny Island Men’s Shed built 240 bird boxes and students at the ANU Furniture Workshop developed an experimental design, created to mimic natural hollows and provide better protection from predators and the weather. With the help of sponsorship from Bruny Island Environmental Network and local residents, boxes were installed across the full length of Bruny Island, as well as on Maria Island and mainland Tasmania.
Nest boxes provide the opportunity to restore the availability of nest hollows in conjunction with protection and expansion of forests. It will take decades or centuries to return to old-growth conditions in forest reserves; in the intervening time, nest boxes will be an important conservation tool for protecting forty-spotted pardalote populations.
Blood-sucking fly parasite is the principal cause of nestling mortality
After we locate nests, we monitor the breeding success of each pair. In 2012, we found that nestlings were disappearing, often within days of hatching. We installed motion-sensor cameras at nests in order to identify predators. But when looking through the thousands of pictures taken at failed nests, we did not find any images of predators. I suspected a small, fast marsupial might be dodging the camera trap, or possibly a cold-blooded snake slipping past the heat-sensor.
Over the same period, I noticed odd growths or deformities showing up all over the nestlings: over their eyes, on their wings, legs, and bodies. Annie Phillips, a wildlife veterinarian from Hobart, came out to investigate. While she was to attempting to extract a fluid sample from one of the nestlings, a maggot squirmed out of the nestling and dropped into Phillips’s hand. Another great eureka moment in science. We took another look and realised that there were dozens of these maggots living just under the skin of the nestlings . They were causing the lumps that had initially looked like deformities.
The maggots were larvae of a parasitic fly, feeding on nestling blood. Phillips packed that first maggot in sawdust and sent it to a Launceston lab, where it was reared to adulthood. A taxonomist identified the adult fly as Passeromyia longicornis, a species found only in Tasmania. The only previous record of their presence in nestling birds was in house sparrows. Little is known about these flies.
As we continued to monitor nests, we found that P. longicornis larvae were the main source of nestling mortality, causing about 70% of nestlings to die. It is unusual for a parasite to have such a major impact on a bird population, especially when the species involved are both native to the same area. However, recent studies in Argentina and Canada have shown that changing climate and forest conditions can result in increased parasite loads in nestling birds. There is no historical information about parasitism in forty-spotted pardalotes, so we don’t know whether parasite levels have changed. And like the problem of competition with striated pardalotes, fly parasitism is a natural phenomenon, though possibly thrown out of balance by human activities.
I am currently trialling methods of parasite reduction and have found that a veterinary product, Avian Insect Liquidator, is effective in eliminating almost all parasites when applied to nest material. This strategy could be useful for protecting particularly vulnerable populations of forty-spotted pardalotes.
Habitat and food supply
Forty-spotted pardalotes are only found in forests containing white gum trees (Eucalyptus viminalis), which are their primary forage tree. Because they are so specialised, I was interested in how forty-spots use white gum trees and the relationship of habitat quality with breeding success. Lerps, a sugary shield produced by leaf invertebrates, are commonly thought to be the primary food source for forty-spots. But an American ecologist on my crew, Sam Case, was sceptical; there were very few lerps to be found in the foliage. So he used video cameras to record food items that adults brought to nestlings, and found almost no lerps in their diet. Instead, more than 90% of items brought to nestlings were manna—not the variety fallen from heaven—but grains of crystallised sugar exuded by white gum trees.
Little is known about manna as a food resource, and we are currently researching how manna is generated and potential factors affecting its abundance. We do know that manna is primarily, if not exclusively, produced by white gum trees (Eucalyptus viminalis), which explains the forty-spotted pardalote’s reliance on these trees. Its production may be effected by tree health and rainfall. During the recent drought, the canopy of many white gum trees became thin, and many died. Manna production likely suffered during this period and might have been the cause of widespread forty-spot decline. As white gum woodlands have become isolated by land clearing and attenuated by the drought, forty-spot populations have declined and become isolated themselves.
Conservation and the future of forty-spotted pardalotes
Although there is still much uncertainty about forty-spot ecology and conservation, we don’t have to wait to understand everything about the species before starting conservation efforts. Their reliance on white gum trees for foraging and tree hollows for nesting makes forty-spotted pardalotes vulnerable to environmental change, but it also means their habitat requirements are relatively clear. Conservation should focus on 1) protecting habitat and planting white gum seedlings, 2) restoring hollow abundance by installing nest boxes and retaining hollow-bearing trees, and 3) limiting fly infestation in vulnerable populations (e.g., patches where only one or two pairs exist, and flies prevent nests from fledging young year after year).
Bruny Islanders have been enthusiastically planting seedlings and creating new corridors. For example, Dennes Point residents Marlene and Brendan Schmidt have created a corridor of white gum trees which will extend existing habitat on North Bruny to the tip of Dennes Point—an important connection to the mainland Tasmania population at Tinderbox Peninsula. The Indigenous Land Corporation has expanded habitat patches at their Murrayfield Farm property through extensive fencing and planting programs. The Bruny Island Environmental network and local residents created a nest-box sponsorship program, resulting in nest boxes placed across the full length of Bruny Island. These actions all help to boost forty-spotted pardalote populations, expand their distribution, and maintain the genetic diversity required for healthy populations.
There is still uncertainty about long term population trends and how changes in forest structure, climate, and parasitism interact to create new threats to forty-spotted pardalotes (and many other species). Because there are no systematic annual surveys for Tasmanian birds (as exists in North America), long-term and detailed population trends are unknown for forty-spotted pardalotes and for most Tasmanian birds. However, Tasmania has many skilled birders, who as citizen scientists have a great ability- and opportunity – to contribute to bird conservation research.
The case of forty-spotted pardalotes shows that each species is unique and we cannot make assumptions about population trends and threats and simply hope they will survive. Research allows us to understand the mechanisms behind declines in endangered species, and to protect them effectively. We hope that with ongoing conservation efforts forty-spotted pardalotes will be a much more common sight in the future.