Saturday, 8 November 2014

White-browed Scrubwren nest

A pair of White-browed Scrubwrens Sericornis frontalis, male behind
A pair of White-browed Scrubwrens nested successfully in our garden this spring and I took a look at the nest once the young had fledged. Two chicks were reared from three eggs, the third did not hatch, as often happens with birds. So all was well and the birds are all still foraging about the garden three weeks later.

The domed nest in situ, hidden at the base of a clump of irises
The birds took over a week to build the nest, the female doing all the work while the male escorted her as she collected material from the garden. She would only go into the nest once there was no danger of predators watching her. Yet she must have made the journey thousands of times.

The nest laid on top of a hat to show the scale of the nest
The nest was large and oval, about the size of a hat crown, with a side entrance well hidden from above by an overhang of grasses. The nest was predominantly made of grasses, including bamboo leaves, and most of the outside was covered with lawn-cuttings mixed with leaves. It looked just like a clump of leaf litter lying in the garden.

The dark interior of the nest betrayed by a single Eastern Rosella feather.  The nest had a well-defined doorstep, seen here laid out to the left of the entrance, and a pronounced overhang above the door which was opened up by the fledglings on the last day when they sat at the door before fledging
Once the outer covering was removed the internal dome could be clearly seen as a separate tightly-knit dome of finer grasses. This was the true nest-chamber, and it felt warm when a finger was placed inside to check the contents.

The inner dome is seen here with the cryptic covering laid open
When opened up, the inner dome was found to be densely lined with feathers and the single unhatched egg
lay there, pale against the dark background. These feathers were wrapped all around the inside of the dome, not only in the nest cup.

The inside of the inner dome was lined with feathers and held a single unhatched egg. The feathers seem pale in the sunlight, but in the complete shaded nest they were dark.
The feathers of the lining were well-flattened into a mat by the large nestlings trampling on them - they were in the nest for two weeks. This and the feather scales in the nest were a sign that the brood had likely fledged successfully.

The fine white feather scales on the nest lining are an indication that the nest held well-grown chicks - for this is the shreds of feather sheaths cast off as the young birds' feathers developed
I then pulled the nest further apart and counted the feathers used in the lining. There 230 feathers; 7 (3%) from Red Wattlebird, 25 (11%) from our backyard chickens, 33 (14%) from Eastern Rosella, 34 (15%) from Crested Pigeon, and the majority 131 (57%) from Australian Magpie. (This might seem like a lot of feathers, but in Europe, the Long-tailed Tit can add 2500 feathers to its nest lining).

Most (222, 97%) of the feathers were dark (dark grey or black) or had a dark base, as with those of the Rosella and Pigeon feathers which were otherwise light-coloured. The few white feathers (8, 3%) were from chickens. Yet there were four white chickens in the flock and only one black, and magpies have light feathers as well as dark ones, so why did the Scrubwrens use so many dark feathers and so few light ones? It might be that the dark background aids the Scrubwrens to see the contrast of their pale eggs and or the light yellow of their nestlings' gapes. Or it might help the adult birds distinguish any cuckoo eggs or young and evict them early. I shall study this further.

All the feathers from the lining laid out

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