July 10th, 2011

Lessons from bark foragers

ANYBODY WHO regularly visits the Australian bush and has an eye for birds will tell you that many Australian birds will spend a little time foraging on the bark of trees and some will spend a fair bit of time foraging on the bark of trees. However, there are only three groups of birds which are totally dedicated to making a living off the bark of trees. Interestingly, although they are only distantly related, they have all devised similar social strategies which can probably tell us a thing or two about the profitability of foraging on bark for a living but may also have some important lessons for helping us understand the viability of remnant bush plots, appropriate management practices and forming better planning guidelines. I think that this is an area crying out for further study . . .

Let’s begin by meeting the bark foraging specialists: they are the Varied Sitellas, the Tree Creepers and the Shrike-tits.

Varied Sitellas

The Varied Sitella Daphoenositta chrysoptera is a very small, highly active bark foraging bird which, as its name implies, comes in a great variety of colour and pattern combinations according to the part of the continent where they live. There’s at least 5 races of this species, every one of which has obvious yellow bare parts (bill, feet and eye rings). They are regularly seen in large, extended family groups and there have been frequent suggestions of cooperative breeding among these birds.

White-throated Treecreeper

Treecreepers are medium-small passerines that spend their lives working up the trunks of trees taking invertebrates from on and beneath their bark. Unlike the woodpeckers found on other continents and which use stiffened tail feathers as props to lean on, treecreepers appear to get their ability to stay propped out from branches and trunks from their long hallux, or rear toe, nail. There are six species of treecreepers found across the continent, with some showing more commitment to life on tree trunks than others: the Brown Treecreeper Climacteris picumnus, Rufous Treecreeper C rufa and Black-tailed Treecreeper C melanura all spend varying percentages of their day also foraging on the ground. We’ll be looking at the White-browed Treecreeper C leucophaeus here, which lives on the eastern seaboard and is highly dedicated to life on tree trunks.

Crested Shrike-tit

The Shrike-tits are a medium-small passerine which, like the sitellas, are highly acrobatic bark foragers. The black and white banding of their heads, erectile crest, radiant golden underbodies and a call which sounds like the artificial “Flipper” the dolphin call from the old 1960′s TV show all make for one of the more charismatic birds that one could hope to meet on a day’s outing in the forests which they frequent. There are three species, with the most widespread being the Crested Shrike-tit Falcunculus frontatus, which is found over much of the South East Coast of Australia and a good way inland. Crested Shrike-tits spend much of the year in pairs, with the remainder of the year raising their two or three young.

Resource-sharing strategies

ALL THREE of these species have, to varying degrees, adopted a social strategy called “intraspecific resource partitioning”. This is a system where each sex forages practically side by side but in a slightly different manner or for slightly different prey, so that they enjoy the benefits of each other’s companionship but without directly competing with each other. In the case of the bark foragers, the males tend to forage on the trunks and main branches of the tree and the females tend to forage on the outer branches and sometimes the leaves. Of course, opportunism rules, so either sex may occasionally forage on the other’s turf. There are certain times of year when this strategy is more strictly followed, but as a general rule, each sex tends to play by the rule. Generally we see that the males are the enforcers in this strategy: That if the female strays from the leaves and outer branches and forages on the trunk, the male will frequently chase her back onto the outer branches.

Why this is so may be a mechanism for pairs or social groups to spend less time in vigilance protecting larger territories. It could be so that more of their kind can be crammed into smaller patches or territories, or it could be that it is not very profitable to make a living foraging purely on bark unless certain sharing strategies are adopted to reduce competition. It could be all of the above. I think that when we examine the presence and absence of these species in remnant woodlands we find much to suggest that it is the latter of the three which is the main driving factor.

IT’S OF INTEREST that each of our bark foraging species exhibit varying degrees of meticulousness when it comes to resource partitioning: the Crested Shrike-tit tends to be very strict for most of the year – so much so that the male and female have remarkably different bill sizes and structures; the White-throated Treecreeper tends to be fairly strict for a good part of the year and; the Varied Sitella tends to be not so strict all year. Also of interest is that in remnant forests, as the patches get smaller the species begin to disappear. i.e. in very small patches we don’t ever get bark foragers, in slightly larger patches we may get Varied Sitellas, in larger patches again we get White-throated Treecreepers and Varied Sitellas and in still larger patches we begin to get all three species. It appears that there is a correlation between the presence or absence of these species, the strictness to which they adhere to resource partitioning or require its practice and the size of remnant forests or patches where they are found.

Lessons for environmental management

To me, this suggests that bark foraging is not very profitable and this is the reason for this social strategy. It also tells us that there are important management principles required if we want to have functional urban bushland remnants which support all the species required to keep them humming along in a viable and sustainable manner. The first of these is to keep urban bushland remnant plots as large as possible. Failing this, linking remnant plots with appropriately structured bush corridors (more on structure in a future post) is the next best strategy as this effectively turns small patches into large ones, enabling species which require larger patches a better chance of holding on. Bush corridors also permit the movement of individuals which in turn permits “gene flow”, the spread of more individuals’ genes which keeps species’ genetic diversity high and therefore more robust.

IT ALL ADDS UP to understanding the needs of our environment and being aware of the indicators to help us respond to those needs. The outcome is not only sustainable remnant forests where diversity is maintained, but happier communities of humans who enjoy the benefits of investment in the care of our natural world. Humans are naturally drawn to nature, because we are intimately connected to it in many ways and dysfunctional when we live lives where we are disconnected. Just watch property values plummet in any locality where there are fewer well managed bush reserves and therefore fewer birds and mammals and reptiles and amphibians and butterflies and cicadas present. Have a chat to the Real Estate Agents around Avalon, on Sydney’s Northern Beaches, who are offering a bounty for anybody who can confirm the presence of Koalas in the locality.

The morphology, behaviour and social structures of these birds have much to tell us about the environment and the strategies that birds use to carve a niche for themselves in their respective habitats and I’ll be visiting the bark foragers frequently in these notes to share, explore and celebrate their diverse and wonderful ways.

Until next time . . . happy birding!

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