Tuesday, 14 April 2015

Mixed Foraging Flock

A Buff-rumped Thornbill: they glean food from the lower branches, trunks and ground litter

Now that autumn is well on the way here in Canberra, the woodland passerines are gathering into their mixed foraging flocks. These flocks are similar to those in northern-hemisphere woodlands where several species; such as coal tits, blue tits, great tits, goldcrests and treecreepers all join together and roam through a local patch of woodland. In and around Canberra, a typical mix of species would be like one I saw yesterday which consisted of; several buff-rumped thornbills, several yellow-rumped thornbills, perhaps ten grey fantails, I think three white-throated treecreepers, a family party of superb fairy-wrens, two scarlet robins, a pair of speckled warblers and a grey shrike-thrush.

A Yellow-rumped Thornbill: they flit down to the ground for their food

These mixed flocks are an efficient strategy for the birds to collect food within the safety of their group. Many eyes are better than one for locating food and for detecting any approach of potential predators. And each species has its own preferred type of food or place to gather it from. The buff-rumped thornbills pick away at the bark for insects beneath and they might not catch every insect they disturb. Then the fantails might zip in and snatch a insect trying to escape, or a speckled warbler following along on the ground can pick up any grubs that the thornbills dislodge. The yellow-rumped thornbills keep in their own flock within the mixed flock, dropping to forage on the ground then flying back up to the lower branches after a quick feed. The superb fairy-wrens probably only joined the party as the flock moved through their home patch, taking the advantage of the group foraging behaviour while they could.

A Grey Fantail: they fly-catch tiny insects, often those disturbed by the other birds in the flock

Two male Scarlet Robins dispute in the branches
Within each mixed foraging flock there are usually two scarlet robins, one male and one female. They seem to adopt the flock as their winter/non-breeding season territory and they defend it from all other scarlet robins. Yesterday I watched two such male robins displaying and threatening one another in such a dispute for ownership. The resource of a mixed foraging flock, with its advantages for survival, is clearly a valuable commodity for woodland birds.

That mixed flock seemed to consist of mainly lower branch- or ground-feeding birds. I saw another two like that later and one flock which was formed mostly of striated thornbills and striated pardalotes, working their way through the canopy, picking food from the leaves and twigs, never venturing near the ground. Each bird in each flock will know their local patch of the woodland and together, they will work their way through the habitat every day of autumn and the oncoming winter.

Then in spring, they will disband and settle into pairs on breeding territories.

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