Our largest bird of prey, the wedge-tailed eagle, has seen good timesand bad. It has survived land

clearing, livestock grazing, disappearingnative prey, even bounty hunters.

In the process, rabbits have become adietary staple for the eagle, so the latestchallenge it faces is the advent of rabbithaemorrhagic disease (RHD), or rabbitcalicivirus disease.

Since 1996, Chris Davey and his coll-eagues at CSIRO Sustainable Ecosystemshave been investigating the response ofeagles to declining rabbit numbers, and theramifications for native animals. Theirstudies have centred on eagles nestingaround the foreshores of Lake Burrendongin central-western New South Wales.

Davey’s laboratory bench is covered withplastic bags full of bones, teeth, feathersand regurgitated eagle pellets – the remainsof unfortunate prey he has gathered fromunderneath numerous eagle nests in the90-square-kilometre study area. Thesesamples are undergoing forensic-likeanalysis to identify, where possible, thenumber, size and species of victims.

Davey and his colleagues have alsocollected data on aspects of eagle breedingsuccess and monitored prey abundance byregular spotlight counts.

What’s on the menu?

A pair of wedge-tailed eagles usually hasseveral nests in its territory and each winterone of these is lined with green leavesbefore eggs are laid. Sticks are added tonests year after year and they can besubstantial enough to support a person.‘Active’ nests have both a food preparationarea and a living area.

Davey found that, during the five yearsof the study following the arrival of RHDin 1996, each breeding pair successfullyraised an average of 3.5 young. Davey saysthis is quite high for a long-lived species,given that each bird only needs to replaceitself to maintain a stable population size.

Breeding success varied from year toyear, as did chick survival at different stagesof development, but no consistent trendallowed a simple interpretation of the effectof RHD. This was despite a 75% reductionin rabbit numbers after the disease hit. Thereproductive success of the Burrendongeagles in the aftermath of RHD hasremained similar to that for other wedge-tailed eagle populations before RHD.

Analysis of skeletal material from undernests revealed that rabbits were the mostfrequent prey with 76% of bone materialidentifiable as rabbit, followed by 12%

macropods (kangaroos and wallabies) and5% birds, with lesser proportions of otherprey groups, including reptiles and fish.

Rabbits seem to be the preferred prey.They are nutritious, abundant, pose littlerisk of injury to hunting eagles, and comein a convenient size for a take-away mealthat can be carried to the nest as a com-plete carcass.

The study found that while adult eaglescould switch between prey species, theytried to supply the highly favoured rabbitfare to their chicks for as long as possible,even when rabbit numbers were in decline.

A conceptual model

Davey has proposed the following four-stage relationship between wedge-tailedeagles and their prey.

High rabbit numbers: There is a surplusto the needs of the eagle chicks and rabbitsform the main diet of adults and chicks.

Medium density rabbits: Adults switchto a diet that is dominated by other preyspecies – in the case of Lake Burrendong,large mammals and carrion. Chicks are stillfed on rabbits.

Low rabbit numbers: At Burrendong,chicks are fed a diet of mostly birds and ‘at-heel’ kangaroos or wallabies, supplementedby items such as small pigs, foxes and the

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Chicken feedSteve Davidson explores the dynamicrelationship between the eagle andthe rabbit.

Rabbits are the preferred

diet for eagle chicks.

occasional rabbit. Adults still feed mainlyon larger mammals or carrion (which,unless dismantled by other scavengers,can’t be carried back to the nest).

Hardly a rabbit in sight: When rabbitnumbers crash to very low levels, the goinggets tough for eagles. Adults survive on avirtually rabbit-free diet and, in a landscapewhere many alternative native prey speciesno longer exist, breeding ceases.

Davey says, rabbit numbers at LakeBurrendong appear to be fluctuatingbetween low and medium density, whereasin the more arid regions of Australia,RHD, myxomatosis and predation havepushed rabbit numbers down further.

Eagles out west

A similar eagle study was undertaken nearBroken Hill, New South Wales, in a semi-arid environment. Andy Sharp, then withthe NSW National Parks and WildlifeService, and several colleagues, also foundthat eagles apparently switched more toother prey as rabbit numbers were reducedby RHD. But there was a difference.

In the more arid landscape, reptilescomprised a far greater proportion of theeagle diet than at Lake Burrendong. And,out west, eagles turned primarily to reptileswhen rabbits became scarce, rather thanbirds and carrion as at Lake Burrendong.

The researchers also found that eagleswere returning lesser quantities of prey tothe nest as rabbit abundance declined.

Sharp says the way eagles adapt their dietin response to the relative abundance ofprey species means that small to medium-sized native animals are likely to experienceincreased predation rates immediatelyfollowing the ravages of RHD.

How this will af fect populations ofnative fauna remains unknown.

It could be argued that any impact willprobably be short-lived given that eaglenumbers are likely to fall to levels that aresustainable at lower levels of total prey. ButDavey says the population density of adultwedge-tailed eagles largely depends onfood abundance during drought years.

His research suggests that, thanks toprey substitution, declines in eaglepopulation density due to lack of offspringshould only occur if rabbit numbers arekept at extremely low levels for many years.Eagles are adaptable, live for many years,and resume breeding as soon as theavailability of prey picks up.

The ramifications of this scenario fornative prey species, post-RHD, are notclear. Certainly, though, the familiarwedge-tailed eagle should continue tograce our skies.

More about eagles

Sharp A Gibson L Norton M Ryan B MarksA and Semeraro L (2002) The breeding-season diet of wedge-tailed eagles (Aquilaaudax) in western New South Wales andthe influence of Rabbit Calicivirus Disease.Wildlife Research, 29: 175–184.

Davey C and Pech R (2001) Effect of re-duced rabbit numbers on the reproductivesuccess of wedge-tailed eagles (Aquilaaudax) in central-western NSW. InProceedings of the 14th conference of theAustralasian Wildlife Management Society,Dubbo, NSW, 3–5 December 2001.

Right: Collecting the remains of prey from an

eagle’s nest for later investigation.

Below right: If rabbit numbers decline, adults,

and eventually chicks, must eat more of the

less-desirable species. If rabbit numbers crash

to very low levels, breeding ceases and adults

survive on other prey.

The wedge-tailed eagle has modified its diet

as the smorgasbord of prey on offer has

changed with time.

Rabbit abundancehigh

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