SOME HAVE BILLS for probing and gleaning, whilst others are designed for hooking, tearing and prising. However their bills come, it appears that the Australian birds which make bark foraging their speciality have one thing in common: “Intraspecific Resource Partitioning”.

Many birds will opportunistically take an insect from a tree’s bark or perhaps even tear off a considerable amount of bark in search of a boring grub, as is the case with Yellow-tailed Black Cockatoos. I’ve frequently observed Grey-crowned Babblers in north-west Australia as they’ve torn open masses of bark to get at termite colonies. However, these birds are not specialist bark foragers. Those who have made searching amid or under bark as their almost sole means of finding a meal is quite an exclusive club in Australia. It consists of the Treecreepers (mostly), Varied Sitellas and the shrike-tits. Here in the Sydney region, it is the White-throated Treecreeper, Varied Sitella and Eastern Shrike-tit which we are likely to encounter.

We’ll begin our exploration of this fascinating topic by getting to know a little more about these birds which enjoy its benefits.

VARIED SITELLAS are cooperative breeders, with the young of previous seasons frequently assisting their parents in raising the young of subsequent seasons. Thus they are mostly found in foraging parties of at least 4 birds. It is generally concluded that the primary parents are monogamistic – staying together and probably not straying into the realms of cuckoldry or divorce too much.

This species is a remarkably acrobatic one which can be observed working their way around trunks and branches in a spiral fashion as they poke and prise with their specialised bill, which is quite upturned and very laterally compressed. It’s always a pure delight to hear the chittering calls of approaching sitellas and observe them flying from tree to tree with their conspicuous orange wing stripes giving them an almost butterfly-like appearance.

EASTERN SHRIKE-TITS, with their gorgeous colours, jaunty crests and industrious foraging style are a favourite with birdwatchers on the eastern seaboard. Whilst not being cooperative breeders, they are also monogamists and their young will often remain with them for extended periods as they learn the gentle art of ripping, tearing and prising at bark. Indeed, they are generally first detected by the sound of ripping bark in trees overhead.

Shrike-tits have two obvious styles of song, but each one differs from the other so greatly that it’s hard to believe they are made by the same species. One call sounds a little like the Brush Cuckoo: a series of high pitched but descending notes – “peer, peer, peer, peer, peer”. Their other song is what I call the “Flipper Call”, as it sounds like a dolphin or a weird chuckle: “eee-oooh-eee”. Shrike-tits are quite a confiding species and so will frequently forage in very close quarters to human observers.

WHITE-THROATED TREE-CREEPERS are our third of the bark foragers found around Sydney. Usually the first sign that one of these birds is in your presence is their loud, repetitive call which has quite a ringing quality to it.

Spending your entire life hanging off the side of a tree may appear to be a prodigious feet of strength. However, the treecreepers have come up with a simple system which allows them to achieve this: very long claws on their back toe, or “hallux”. This serves as a prop for these birds as they work their way up the trunks of trees. It can be seen quite clearly on the image to the right. Because of their specialised adaptation for climbing, they always work upwards in a somewhat jerky, almost skipping, motion.

A curious feature of the White-throated Treecreeper is that they spend a great deal of time listening for their prey. You will frequently see them with their ear against trunks and branches as they listen for borers and other prey hidden in crevices or beneath the bark. I’ve noted that in the southern part of their range (Albury & Chiltern, Victoria) that they don’t listen so much, but rather engage in a great deal of wing flashing, perhaps to startle prey off the surface of the bark or from within tight crevices (some cool papers in that lot for an industrious student).

Resource Partitioning

IT MAKES good sense to remain with the one partner for life as it does away with investing too much energy in courtship. However, another problem arises when remaining in close proximity, whilst sharing limited resources: How to share these limited resources without competing with each other? This is especially important, because if a territory is good for bark foraging, then there’s going to be plenty of other birds looking to get in on the action too.

One way of achieving this is for some family members to specialise in foraging in one part of a tree and for other family members to specialise in other parts of the tree. It means that families and partners can forage in close proximity but also find enough prey to maintain themselves. This is called “Intraspecific Resource Partitioning”. If their needs or behaviour diverges significantly, each generation of birds might begin to develop bills which reflect their slightly different lifestyles. Sometimes we see many species of birds sharing the same feeding resources but in different ways, such as in the case of waders on mudflats, where bill differences among sandpipers are very marked. This is known as “Interspecific Resource Partitioning” (more on this here) .

Resource Partitioning can be a great way of reducing the territory size required for a pair to survive, meaning that more individuals can make a living across the species’ distribution and all individuals waste less time defending the borders of large territories. This is exactly what our Aussie bark foraging specialists have done. Each of these species of birds will mostly be found foraging on different parts of the tree according to their sex. Generally it is the males on the trunks and larger branches and the females on the twigs, leaves and smaller branches. Of course there are frequent cross-overs, but the general rule becomes very clear to astute observers. On more than one occasion I have also observed Eastern Shrike-tits teaching their young to forage and it would appear that the adult males take the young males with them and the female adults take the young female fledgelings out with them. This practice is known as “Brood Divisioning”.

Eastern Shrike-tit bills tell the story

White-throated Treecreepers often don’t forage as a tight family unit, but if you ever see a male and female encounter each other on the same tree, the male will rigorously enforce his “trunk foraging rights”, chasing the female bird off to her special place on the tree. Varied Sitellas are even less strict with their Resource Patitioning, but it does take place. However, Eastern Shrike-tits are very determined Resource Partitioners.

I wanted to see how much the different foraging practices of the male and female shrike-tits had shaped their bill morphology. To investigate this, I paid a visit to the Australian Museum’s Bird Department. The Australian Museum has a fantastic storehouse of “skins”, “pickles”, skeletons, wings, eggs and feather specimens which scientists from all over may visit and study in order to build our understanding of birds. Gruesome as it appears to be, “collecting” these birds plays an important part in our understanding of each species and assists us with the kind of knowledge required that helps us work towards conserving the world of nature. A well prepared “skin”, or specially stuffed bird, can last for well over a century and contribute much to our knowledge.

I met with Curator of Birds and well known Ornithologist, Walter Boles, who availed me of his Eastern Shrike-tit collection for a little bill comparing.

Sure enough, an obvious difference between the bills of the males and females of this species does exist. The fact that they forage in differing parts of trees and in different ways has shaped the appearance of their bills accordingly. This demonstrates a solid dedication to resource partitioning. Another scientist, Dr Richard Noske of Charles Darwin University in Darwin, has studied Eastern Shrike-tits and found that there was a seasonal aspect to their foraging strategies and that this could also vary from location to location¹. All this points to the best use of resources and ensuring that various nutritional needs are met by the birds during times of feather moulting, courtship, feeding young and recovering from their arduous breeding season.

Next time you encounter any of these fantastic birds when you’re out and about, spend a little time observing their foraging behaviour. It makes you wonder: Is there anything these amazing creatures haven’t come up with?

Until next time . . . happy birding!

References:

1. Noske, RA, Sexual differences in the foraging behaviour of Crested Shrike-tits, Falcunculus frontatus, during winter, Emu 103(3) 271 – 277

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Ornithology, physiology by Ricki