CloserLook Nature Photography

Pardalotes: A Bird (Nest) in a Burrow is Worth Two in the Bush

Male Spotted Pardalote at entrance to tunnel to nest cavity, Langwarrin Flora and Fauna Reserve, Victoria, Australia

(Canon 1D Mark II with 500mm f/4 lens plus 1.4x extender, 1/100 sec at f/9, ISO 640, exposure compensation -1/3 eV, tripod)

With a beak full of eucalyptus bark strips, the tiny bird races down the underground tunnel that is barely 5-cm wide. The end of the half-meter long burrow opens into a nest cavity that is neatly lined with generous amounts of bark. Placing the new pieces upon the bed of bark, our bird finds a few unacceptable bits of nesting material, scoops them up and hurries back out the tunnel to the open air. This process of nest enhancement continues all day, with repeated trips by both male and female. Meet the Spotted Pardalote, a colorful passerine that nests in a cavity deep within the earth. What would lead such a delicate creature to choose a dark, damp and dirty hole in the ground to raise its young? This question takes on an even more curious context when we consider what pardalotes do when they're not busy tunneling through the earth.

The Spotted Pardalote (Pardalotus punctatus) is a tiny bird, at only 9-grams and barely 9-cm in length from beak to tail tip. With a short stout beak and stubby tail, it has a distinct compact appearance. The flashy yellow rump and throat, especially that of the male, combine with a dizzying pattern of white dots on a black head and primaries to produce a spectacular sight. This pardalote is found along the eastern coast and extreme southwest coast of Australia and is one of four species of pardalotes indigenous to the continent. The Straited Pardalote (P. straitus) has the widest distribution, covering almost every part of Australia. The Red-browed Pardalote (P. rubricatus) is found in the northern half of the continent, and the endangered Forty-spotted Pardalote (P. quadragintus) is found only in southeast Tasmania. All four species will nest in burrows, however, generally all but P. punctatus and P. rubricatus will also utilize hollows and knotholes in trees.

These pardalotes have a serious sweet tooth, for the favorite foods for adult and fledgling alike are the sugary substance called "lerp" and the tiny invertebrates called "psyllids" (lerp insects or simply "lerps") that produce it. Lerps secrete this honeydew substance as a protective coating while they suck the sap from eucalyptus leaves. One of the eucalyptus species utilized by pardalotes is the Manna Gum (Eucalyptus viminalis), so named for the sugary white sap (manna) that oozes from the insect bores in the leaves and was collected for food by Aboriginals. To find these tiny carbohydrate-rich insects and their exudate, adult pardalotes spend much of their day hovering and probing in and about the leafy treetops, far above the ground. Damage to eucalyptus by lerps can be severe, with the most conspicuous symptom being leaf discoloration, but defoliation and dieback can occur during outbreaks of the insect [1]. So by feeding on these insects, pardalotes may play an important role in controlling these parasites.

Returning to our original question, given that these pardalotes feed amongst the treetops, why would some of them favor a burrow in the ground to a hollow up high in a tree? Perhaps we can put the idea of a bird nesting in an underground cavity into a context that may shed some light on the puzzle. At first glance, building a 10-cm diameter cavity at the end of a long horizontal tunnel seems like an unlikely task for a tiny bird. The excavation time and energy expenditure alone must be considerable, not to mention that once the cavity is hollowed out, the bird still has to go out and collect bark and bits of vine to line the nest. Furthermore, these passerines have feet well adapted to perching and beaks designed to glean insects off of leaves. Imagine these implements put to use in excavating a tunnel and we have a further incongruity. Putting aside for the moment the image of this dainty bird dirtying its fine plumage with cave dust, if we want to investigate why they would choose this nesting environment we need to examine the extent that various environmental pressures exert on the selection process. What advantages could such a confined nesting quarters offer a growing family? There is protection from weather to consider: out of the chilling wind or blazing sun, the interior of an earthen cavity offers relatively stable temperatures, albeit generally cooler than that above ground. Protected from gusting wind and the swaying motions of the tree branches, there is no worry that eggs or fledglings can fall out of the nest when one is on (or in) solid ground. Sheltered below ground from the rain and elements above, a well-constructed burrow would seem a stable environment in which to raise a brood. Perhaps comparable in offering some of these advantages might be the choice of nesting in a hollow in a tree. The protection from weather is certainly attractive in this case, as is the relative inaccessibility by predators (more on this later). Here is where an element of competition may come into play. "Competition for nest sites could be a factor, as it is extremely high in the Aussie bushland" says Dr. Lainie Berrie [2] who is a native Aussie now presently a research biologist at the University of Washington and an expert on nest predation. She further comments that "a small bird like the spotted pardalote might have trouble defending a hollow against occupation by larger birds and mammals, so they may have resorted to building their own holes in the ground." But, as she points out "This doesn't explain why striated pardalotes nest in tree hollows. "

Female Spotted Pardalote with new bark awaiting her turn to enter the burrow, Langwarrin Flora and Fauna Reserve, Victoria, Australia

(Canon 1D Mark II with 500mm f/4 lens plus 1.4x extender, 1/50 sec at f/8, ISO 640, exposure compensation 0 eV, tripod)

However, there are other forces at work in the nest selection process. Certainly there is an issue of accessibility to consider. That is, accessibility of the family fortress to the various predators that share this neck of the woods. To quote Berry [3] "Nest predation is believed to be one of the most important influences on breeding biology, given its direct effect on reproductive output and thus fitness." Certainly, an open nest above ground or in a tree is easy pickings for scavengers such as ravens, butcherbirds, hawks and owls. This makes the four strong walls and roof of a cavity, be it underground or in a tree hollow, seem attractive from the predation risk standpoint. But given the accessibility of a hole in the ground to terrestrial predators, is this not a dangerous place to raise a clutch? Weighing the relative risks inherent in ground versus arboreal cavity nesting sites, Berry offers "Predation might seem to be more of a risk on [or in] the ground than in a tree, but perhaps it is not. Many of the nest predators (other birds, some snakes such as the tiger snake, some mammals especially arboreal ones), may be just as likely to access a nest in a tree as in the ground." Be it small predators that can easily traverse the tunnel entrance, or larger carnivores and omnivores such as the bandicoot and quoll that could excavate the subterranean sanctuary, our tiny pardalote is no match for the determined invader.

These various predatory issues are among the pressures present during the evolution of pardalotes. But times change and we now find many alien invasive species in search of an easy meal. In some areas of Australia, cane toads are already wrecking havoc on the tunnel-nesting Rainbow Bee-eater's eggs and fledglings, reducing fledgling populations by one third compared to that before the toad was introduced from Central and South America [4]. More species of mouse and rat accompanied settlers to this southern continent and they fare well in the absence of their former predators. So it would seem that the pardalote must learn to cope with a new set of predatory forces in order to survive.

Besides the risk of predation, there are chemical and physical factors involved in cavity nesting to which the pardalote must have adapted. One advantage that below ground cavity nesting often provides is near constant 100% relative humidity. As the eggshell serves as a semi-permeable barrier to gas exchange, the near-saturation levels of moisture eliminates the danger of dehydration. But within the confines of a cavity, respiration of adult and fledgling tends to reduce the oxygen and increase carbon dioxide concentrations. Thus cavity-nesting species must be able to cope with varying degrees of hypoxia (low blood oxygen) and hypercapnia (high blood carbon dioxide). Furthermore, unless feces and food particles are removed, ammonia (NH3) can be produced in these humid environments. Studies of bee-eater nests have found NH3 levels of up to 0.5 Torr (0.07% or 700 ppm), which can be harmful to the birds [4]. All this is to say that with a typical clutch size of three fledglings per nest and two attending adults, it could get pretty uncomfortable at the back of the burrow.

Spotted Pardalote returning to the nest with prey, Dandenong Ranges, Victoria, Australia

(Canon 1D Mark II with 500mm f/4 lens plus 1.4x extender, 1/500 sec at f/8, ISO 400, flash @ -1.7 eV, exposure compensation 0 eV, tripod)

Recent literature suggests that physiological adaptation to cavity environments has been studied more in depth in mammals than in birds [5]. Nonetheless, we can infer from the above discussion that there are important adaptations in birds that choose to nest in enclosed spaces, and that these are different from those forced upon open-air nesting species. For pardalotes, the disadvantages of predatory risk from those that can breach the burrow, competition for site availability and the stresses of subterranean living are apparently outweighed by the advantages of environmental protection and some degree of predator inaccessibility afforded by burrow nesting. The rearing of offspring is always challenging for any parent. One fascinating aspect of this problematic endeavor is how various animals find the right balance to be successful given the resources and risks in their chosen environment.

If you want to see pardalotes for yourself, an excellent location is Bruny Island in southeast Tasmania. Three of the pardalote species, the Striated, the Spotted and the Forty-spotted Pardalotes, are all present on this island, often sharing the same manna gum forests. Forty-spotted Pardalotes were deemed "one of the world's rarest birds" [6], but if you know where to look, you can find these charming birds in relative abundance. Good places to search for Forty-spotted pardalotes are where the ravine of a creek crosses a backcountry road. In the shelter of the ravine, the manna gums and the foraging birds are protected from the winds. It's a bit of a neck-strain to search the upper foliage of trees that can reach 50 m in height, and the fact that the pardalotes are not much bigger than the leaves upon which they forage makes them even harder to spot. Listen for their soft low-pitched call "where... where... where..." (Listen to a recording here) to help locate them. These are agile, ever-moving tiny birds and the light levels inside the eucalyptus canopy can make photography quite challenging. Visit during the southern spring and perhaps you'll be lucky enough to witness how these fascinating creatures make a living high above you in the treetops yet raise their young below your feet.

Forty-spotted Pardalote foraging for psyllids and lerp among the high branches of a manna gum tree, North Bruny Island, Tasmania, Australia

(Canon 1D Mark II with 500mm f/4 lens plus 1.4x extender, 1/160 sec at f/8, ISO 400, exposure compensation 0 eV, tripod)

References and Further Reading:

[1] Forestry South Australia, "Forest Health Fact Sheets - Lerp Insects", no. 6 (1992).

[2] Berry, L., personal comm.

[3] Berry, L., "Breeding biology and nesting success of the Eastern Yellow Robin and the New Holland Honeyeater in a southern Victorian woodland", Emu 101, 191–197 (2001) .

[4] Boland, C.R.J., "Introduced cane toads Bufo marinus are active nest predators and competitors of rainbow bee-eaters Merops ornatus: observational and experimental evidence", Biological Conservation 120, 53–62 (2004).

[5] Ar, A., Barneab, A., Yom-Tova, Y., and Mersten-Katza, C., "Woodpecker cavity aeration: a predictive model Respiratory", Physiology & Neurobiology 144, 237–249 (2004).

[6] Knowler, D., "Tasmania's Endemic Birds", Tasmania 40° South, 36, 30-32 (2005).


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Copyright Benjamin R Miller / CloserLook Photography 2009