THERE REALLY is no end to the fascination that can grip you once you start to examine the lives of birds. Of course not one single organism lives in a vacuum, everything is pressured in so many ways by the behaviour and activities of other species and all must find ways to respond to threats and opportunities or else face a significantly decreased life span and fewer opportunities to breed. It’s always been like this and so this unfathomably complex biosphere which we find ourselves in is constantly changing. The change brings one constant . . . more change. The changes begin not at the species level, not at the population level, but at the individual level: for its the individual that carries the agents of change – the genes which carry the blueprint for the character traits which help to shape the behaviour and morphology which will ensure success or failure.

Since Europeans visited Australia they have engaged in large land clearing projects both for farmland and to accommodate the urban sprawl. The result has been fragmentation of bush in urban areas and the influx of exotic plants and animals.

Because birds are mostly diurnal and many are relatively easily studied we are developing enough understanding about how they relate to their environment to help tell us about the health of urban bush reserves, making them an important management tool. Two species of Little Brown Bird which can be found in the Sydney region may be particularly valuable in this regard: The Brown Thornbill and the Striated Thornbill. If we examine their lifestyles, this becomes very clear.

IT’S AN interesting fact that the smaller the bush fragment, the less likely you are to find Striated Thornbills in it, though Brown Thornbills may still be present. Here on Sydney’s Northern Beaches, there are no Striated Thornbills in any of the council reserves. However, they are present in Garigal National Park (including Narrabeen Lagoon catchment) and around Ingleside in Kuringai National Park. I believe that some may be present in Ingleside Park on the escarpment overlooking Irrawong Reserve.

Fascinating Striated Thornbills

THE STRIATED THORNBILL has a very interesting lifestyle. Like many of the Australian thornbills, the Striated Thornbill is a cooperative breeder. However, they have a unique approach to this. Striated Thornbills spend most of the year in large extended family groups numbering between 20 and 50 individuals. During those months they remain for the most part in the forest canopy levels where they forage for small invertebrates.

From late August to mid October they form small family groups, comprised of a breeding pair and two or three helpers which may be their offspring from the previous breeding season. They construct a beautifully woven nest of bark, roots, lichen and grasses and raise mostly three young, with all the helpers pitching in to provide food for the ravenous chicks, which follow the adult birds relentlessly as they beg for sustenance.

During these months the Striated Thornbills use every level of the forest, from canopy to forest floor, as they seek to find the food to keep up to the hungry chicks – and themselves. Each family in the extended family group, or tribe, builds a nest roughly 50 metres from its neighbours and so when the Striated Thornbills are breeding in a forest you’re really going to know about it, as the bush becomes a hotbed of these gorgeous little birds.

If a family of Striated Thornbills fails in its attempt to raise its young, they simply move next door and help their neighbours raise their offspring. When the young are of a sufficient age to mostly care for themselves, the entire extended family joins up once more and moves back into the canopy where, if any young should still beg, any bird in the tribe will offer it a morsel.

The Odd Couples

UNLIKE MOST thornbills, the Brown Thornbill is a monogamous pair breeder (keeps one partner, perhaps for life). It is generally found in pairs in the shrub layer, or mid story of forests where it holds a territory which comprises of a given amount of shrubbery.

If the shrubbery is dense, it holds a smaller territory and if it’s sparse, the territory is larger. Both arrangements have their pros and cons. It has been observed that if the size of the reserve is small, that the Brown Thornbill will expand its territory to include the canopy and ground cover layers.

Thornbill wars

So it is clear that at certain times of the year Brown and Striated Thornbills are going to come into conflict and I have witnessed many pitched battles between these birds, with the Striated Thornbills making all manner of unique war cries as they move through the shrub layer and gang up on the Brown Thornbills, sometimes with considerable success.

The average Brown Thornbill is 2 grams heavier than the average Striated Thornbill, For birds this small, that means that the “browns” have a significant advantage over their striated cousins when it comes to birdie pugilism. In smaller bush fragments, where Brown Thornbills move into the canopy – the regular domain of the Striated Thornbills – that they are simply too difficult to overcome for the Striated Thornbills, even when they have the weight of numbers.

Dispersal and gene flow

THERE IS STILL much to be learned about the dispersal behaviour and patterns of thornbills – at what age do they disperse? Do both sexes disperse at the same age? Do both sexes disperse? How far do they disperse? We do know that gene flow – the movement of individuals carrying genetic diversity from population to population – needs to be adequate to maintain robust health for all organisms and this includes thornbills. So how would a family group of Striated Thornbills deal with being trapped in a fragment of bush where it couldn’t receive an influx of fresh genes from others of its kind from further afield? There is a strong chance that inbreeding would occur, threatening the tribe with weaknesses which would make it less capable of dealing with disease and other ardours of life in the treetops. As the population in the fragment shrinks, the problem worsens.

Larger groups require larger resources to feed themselves, and if they’re constantly clashing with Brown Thornbills as they attempt to forage in shrinking reserves they’re likely to struggle to meet their needs.

Indicators of bushland health

IT SEEMS TO ME that if a reserve is not capable of supporting Striated Thornbills, it might not be capable of supporting other species – further compomising the viability and quality of the urban bushland reserve, so we can understand that Striated Thornbills may be an important indicator of environmental health. For this reason, wildlife corridors and urban plantings take on an even greater importance. There are many bird species trapped in reserves which are not capable of crossing the human barriers to neighbouring bush. However, with appropriate urban planning and community education this problem can be overcome and also introduce many more birds into suburban gardens: a lifestyle boost for the birds and we humans – and our property values . . . now that’s an argument that should be easy to win!

In the next few weeks we’re going to look at the lives of other groups of birds with unique and interesting lifestyles which can tell us more about environmental health. These include fairy-wrens and bark foragers, like shrike-tits, tree creepers and Varied Sitellas.

Until next time . . . happy birding!

Filed under:
Birding, Conservation, Evolution by Ricki