Friday, August 3, 2007

The Wolli Valley's changing birdlife - by Gavin Gatenby

The Wolli Valley’s changing birdlife

By Gavin Gatenby

Early one morn, alone on the fence,

A little birdie sat, his head bowed in suspense.

He was dreaming of days,

And bags of hays, when horses were all the rage.

Now big trolley cars and automobiles

Have taken the place of the horse.

Since they moved out the racecourses,

The birdies have no place to go.

What would a poor sparrow do

When there’s nothing left for him to chew?

You can’t get a meal from an automobile.

That’s what the bird sang to me.

Fats Waller

My partner and I moved to the Wolli Valley in 1980. In the quarter-century since, I’ve witnessed big changes in the local birdlife. A few species have disappeared locally. Although some – mostly feral – are seen less often, some species have become much more common. Some species, never, or only rarely, seen in the 1980s, have become common. A few species once present only in the remnant natural habitats of the valley have begun to move into nearby suburban gardens.

The late Neil Rankin, who compiled fifty years of bird observations in the Wolli Valley – the work of himself and others – in the 1989 edition of Birds of Wolli Creek, witnessed and recorded many of these changes before his death in 2001. Others have occurred since.

In some cases these changes are part of much wider movements seen across the Sydney Basin and down the east coast, but others probably occur for local reasons. Ecosystems are made up of infinitely complex interactions, and in urban environments all manner of human actions can have unintended impacts, for good or ill, on our non-human fellow-travellers. What follows is what I like to think of as informed speculation, based on my own observation and reading, about the reasons for these changes. I hope it will stimulate discussion and observaton.

The “island biogeography” effect

Ecologists have spent a great deal of time studying the fauna of islands, large and small, and have discovered a set of laws that govern how many species are likely to be found on an island, based on its size and relative isolation. The concept of “island biogeography” is also useful in understanding the occurrence of species in isolated fragments of native vegetation like the Wolli Valley bushland.

Think of the valley as a small island of viable habitat in a sea of suburbia, unsuitable for most native birds. Birds that are highly mobile (such as ducks), can reach this island, and survive there for some time, but it may not be big enough, or suitable enough, to sustain them for long. Even if they manage to breed, and establish a small colony, the group is vulnerable to local extinction through predation, disease or infertility. On the other hand, the amount of habitat may be sufficient to support a viable colony.

We have also to factor into the equation the fact that European settlement severely modified the original bush. Trees were cut down, some important plant species (such as cabbage-tree palm) disappeared, and cattle, goats and pigs trampled and grazed the vegetation. By the early 20th Century, Wolli was, in ecological terms, almost a desert island – completely unsuitable for many native birds that had once thrived there. But here and there, in inaccessible places, small patches of bush remained and after agriculture declined, communities of native plants began, once again, to spread over the area, re-creating lost habitat. It has taken some decades for some of the vanished species to find their way back – a process that continues.

Decline of the ferals

In my observation, the ubiquitous feral birds of the Sydney I grew up in – the House Sparrow, Starling, and Spotted Turtle-dove – have declined during the last quarter of a century.

The reasons aren’t too hard to find. All three species were highly dependent on humans for food, and the Sparrow and Starling on man-made structures for nesting places. Mostly, Sparrows and Starlings nested in the rooves of old, poorly-maintained, often weatherboard, houses. When we moved into our old cottage in Turrella, there were Sparrows or Starlings nesting in all four corners of the ceiling. Structures like these have mostly been pulled down or gentrified and the renovations have blocked entry holes, forcing these birds to nest in sub-optimum places (such as palm trees) where they’re more vulnerable to predation by Currawongs and Ravens.

The decline of poultry-keeping, cage birds and horses

In his justly celebrated song about urban wildlife ecology, quoted at the start of this essay, Fats Waller puts his big finger on the fact that technological change is an important factor in the rise and decline of avian species within our cities.

An important reason for the decline of the European House Sparrow, the Starling, and the Indian Turtle-dove has been a marked decline in the practice of keeping poultry and cage birds in suburbia. If you’ve ever kept backyard chooks, you’ll know that feral birds show great enterprise in raiding chook runs for food.

The disappearance of the few remaining horses from the Wolli Valley has eliminated another source of spilled wheat and oats – a rich source of food for feral birds. This completes a process that began when the railways, the electric tram, and finally cars, buses and trucks drove horse-drawn transport off our streets.

Urban reforestation

Another impetus to change has been the steady “reforestation” of Sydney’s suburbs. In my youth, eucalypts, wattles and casuarinas (let alone native shrubs like banksias and grevillias) weren’t a common feature of the suburban landscape. People mostly planted exotics like roses, wisteria, camellias, fuschias, non-native deciduous trees and exotic pines in their gardens. With the exception of the exotic pines (conifers), this flora is almost useless to native birds. In the case of the exotic conifersthere was, in fact, a food windfall, but the native birds hadn’t yet learned to exploit it, and it would be some decades before they did. There were plenty of big, mature, Pinus radiata near where I grew up in Strathfield, but I never saw a Yellow-tailed Black Cockatoo until decades later. Nowadays, these cockatoos are a regular visitor wherever these conifers exist.

From the 1960s on, growing affluence and the movement towards respect for our native flora encouraged more householders to plant native trees and shrubs with high value for honeyeaters, lorikeets, cockatoos and insectivores, and to create an artificial vegetation structure that mimicked their natural environment. Urban reforestation has probably also accellerated the process of recolonisation controlled by the island biogeography effect I discussed above by helping birds cross suburbia to the Wolli bushland. Gardens and small parks landscaped with natives have become stepping-stones for the recolonisation of the valley.

To illustrate how these factors (and others) have affected our native species let’s look at some individual species.

Australian Pipit

Neil Rankin rated Pipits as “very common” up to 1988, but this meant that he was seeing the same pair, in the same place, month after month. For the first years I lived in the valley this pair could almost always be seen on the open grass of Turrella Reserve near Nannygoat Hill. They disappeared many years ago and have not returned. I put this case down to island biogeography. Perhaps one or both the birds were killed by a cat or a raptor. Perhaps they died of old age. Perhaps they just decided to move to greener pastures. At the time of writing, no other pair of Pipits has recolonised this piece of habitat. There was only ever sufficient habitat for one pair of pipits. This is a very clear example of the island biogeography effect. The disappearance of that pair removed the species from the Wolli habitat “island” – a fact that will remain until another pair of Pipits happens upon the valley.

Yellow-tailed Black Cockatoo

This species had never been recorded in the valley when Rankin wrote the first edition of Birds of Wolli Creek. Their return is an exciting event and they’re now a very common visitor, with flocks of up to 25 feeding on wood-boring grubs and banksia and casuarina seeds. The Yellow-tailed Black Cockatoo has benefited from the mass planting of casuarinas over the last 30 years, from old plantings of exotic pines, and from the regeneration of banksias in the valley itself.

Superb Fairy Wren (“Blue Wren”)

Blue Wrens remain very common in the valley and can always be seen where scrubby habitat interfaces with mown grassland. Technically this sort of abrupt transition is known as an “ecotone”. Many people feel Blue Wrens declining in suburban gardens, which may be correct. In my own garden they were much bolder and more visible in the past. They still occur but are rather secretive, a fact that may reflect the increased presence of currawongs. I believe their decline might also be related to the fact that nowadays fewer households are keeping dogs. Dogs exclude cats from backyards and Blue Wrens are reluctant to nest where cats occur.

Red Wattlebird

Neil Rankin rated this bird as “uncommon” in the period 1982–88, seeing them on less than half of his transects. They are now a ubiquitous feature of the valley’s bushland (for example on the slopes of Nannygoat Hill) and are common in streets and gardens. Their rise is almost certainly related to the vastly increased food supply provided by the planting of red bottlebrushes as street trees, the popularity of grevilleas as garden plants, and the gradual regrowth of banksias in the bushland. Wattlebirds pugnaciously defend their nests against nest-robbers like currawongs. To survive on these mean streets it helps a lot to be pugnacious.

White-browed Scrub Wren

Neil Rankin rated these small wrens as “scarce”. They are now frequently encountered in the valley and in early 2005, a family of six, including two juveniles, began to visit our garden which is 400 metres from the nearest typical habitat. I have never previously seen them outside bushland or gardens on the edge of the bush. Perhaps they’ve benefited from a couple of decades of bush restoration in the valley, supplemented by nearby suburban native gardens.

Red-browed Finch

Once confined to the Wolli bushland these beautiful little native finches are now frequent visitors to gardens around the valley. In ours they spend days on the ground and the roof, under our casuarinas, looking for fallen seed, and when these run out they scour the trees, pulling the remaining seeds out of the fruits. Even outside the casuarina seed season small family parties sometimes visit in company with Australian White-Eyes (Silvereyes?) or Blue Wrens. Their rise is clearly related to increased food sources, such as casuarinas.

Brown Goshawk

In my experience the Brown Goshawk is becoming much more common. It preys on smaller birds, notably feral or domestic pigeons (rock doves), as well as mice and rats. Eucalypts planted in the 1960s and 70s are now tall enough to provide nesting opportunities. The recent proliferation of compost bins has brought with it new opportunities for the house mouse and the black rat, and may be partly responsible for the increase in this species.

Boobook Owl

After only a handful of records over a 50-year period the boobook was rediscovered by Neil Rankin in the late 1990s. They are now regularly sighted roosting during the day and heard calling at night. I have photographed one in the valley and heard them at night in our garden, where I suspect they’re hunting the mice that make a living out of our compost bin. Owls need quiet secluded places to roost during the day and sizable tree hollows in which to nest. One regularly roosts in an exotic conifer in a Bexley North garden. I think the Boobook’s return reflects the steady regrowth of woodland in the valley.

Spangled Drongo

Until 1988, Rankin rated the Drongo as scarce. Of recent years this migrant has become a seasonal fixture in suitably wooded gardens and along streets, as well as in bushland. Its rise may be due to our gradually warming climate as well as revegetation.

Sacred Ibis

Becoming more common in parks and along creek flats, the Sacred Ibis is now something of a nuisance species throughout Sydney. It has few natural predators and it has found that its long, sensitive bill, which evolved for probing for beetles, worms, and insect larvae in soft soils, is beautifully “pre-adapted” for probing in rubbish bins for discarded sandwiches.

Common Koel

The Koel is a cuckoo. Most cuckoos are parasitic – laying their eggs in the nest of another species and allowing the foster parents to raise their offspring. Koels migrate each year from New Guinea and the Lesser Sunda Islands and they parasitise Orioles, Magpie-Larks and Red Wattlebirds, all of which are present in the valley. Neil Rankin rated them as “scarce”, and they were certainly not a regular feature of the valley when I first moved here. They are now an inescapable feature of spring and summer, even if most folk only hear their persistent call. Adult Koels mostly eat native figs, supplies of which are increasing, but the rise in Koel numbers is probably due to the increased number of Red Wattlebirds whose nests can be parasitised.

Channel-billed Cuckoo

People who worry (in my opinion excessively) about the depradations wrought on the small native bird population by Currawongs can take heart from the relentless rise of this very large parasitic cuckoo. Until about 2001 they were almost never seen in the Wolli Valley and none were recorded by Neil Rankin or earlier observers.

During the winter, Channel-bills migrate south from New Guinea arriving here in the spring. They mostly parasitise Currawongs, but at first, those reaching the valley were a few weeks too late to parasitise the Currawongs’ first breeding attempt (Currawongs often try for two broods in a season). The Channel-bills now strike with unerring accuracy. Since the 2003 season, the Currawongs that nest in our garden have raised at least three Channel-bill chicks and only a couple of their own. As this phenomena reproduces itself across Sydney, the failure to recruit to their own population will certainly bring about a collapse in Currawong numbers.

The downside is that living with a growing Channel-bill is a great deal more stressful for humans than living with baby Currawongs. They’re incredibly noisy and persistent and they stay with their hapless foster parents a lot longer than Currawongs.

Topknot Pigeons

With the regrowth of the Wolli bushland and the planting of more native trees in local gardens, the Topknot Pigeon has been seen more often in the valley and in larger numbers. Neil Rankin’s only records were of 7 birds in 1984 and 15 in November 1986. These big, grey, rainforest pigeons with their floppy brown topknot are using the valley as a welcome migration route through the hostile Sydney landscape. I have seen about 40 feeding on soft green Angophora fruits, but the valley also provides magenta lillipilli fruit. Once, I found a dead, emaciated, juvenile Topknot in my front garden. The Cabbage Tree Palms recently found regenerating in the Wolli Creek and Bardwell valleys may be from seeds carried in by rainforest pigeons like the Topknot.

Crested Pigeons

Neil Rankin first observed one of these charming native pigeons in 1982 but they’re now very common. They feed extensively on the tiny seeds of native figs and casuarinas, a resource not favoured by the introduced Spotted Turtle-dove, which is declining in numbers as the supply of human waste and domesticated bird food diminishes.

Rainbow Lorikeet

“In the main, birds overfly the valley”, Rankin noted of the Rainbow Lorikeet, and rated them as uncommon. Rainbow Lorikeets are one of those species that, in retrospect, can be seen as excellently pre-adapted to the mean streets of the big city – provided only that a sufficient supply of abundantly pollen- or fig-bearing native trees are on hand. The species is pugnacious, intelligent, very social, and unafraid of humans and can rapidly travel long distances to exploit a temporary food supply. Consequently, numbers have increased spectacularly throughout Sydney. There’s now a breeding colony in the palms lining Mawson Street, Turrella. The local increase of this bird simply reflects a much wider trend.