Tuesday, 3 June 2014

  Inland Thornbills
- Differences between plumages of old and young birds

Adult Inland Thornbill
- showing a smoky grey upper plumage, very fine white scalloping in its forehead, and a pure black bill
When catching birds on a recent trip to Charcoal Tank Nature Reserve as described in the previous two posts we caught several Inland Thornbills Acanthiza apicalis albiventris. The bird illustrated above was caught on an earlier trip during the breeding season and was confidently aged as an adult bird. Those illustrated below are young birds, hatched in the previous spring, i.e. about October/November 2013.

Face of 1st year immature Inland Thornbill (older?)
Face of 1st year Inland Thornbill (younger?)

The bird on the left is developing a dark grey crown, similar to the adult bird, but the scalloping on the forehead is still buff-coloured. The narrow buff tips creating contrast between the scalloping and background colour, although not as strong as on the adult bird.

The second young bird, left, has a pale green/grey crown and forehead, with broad buff tips to scalloping, creating less contrast between the scalloping and background colour.

Profile of 1st year Inland Thornbill (older?)

In profile, the developing grey on the crown of the first young bird shows the dark head with fine buff scalloping, concentrated on the lower forehead. It also has a dark bill, but not fully dark as in an adult, especially at the base of lower mandible

Profile of 1st year Inland Thornbill (younger?)

In profile, the second young bird has a paler head, with less contrast between the background colour and the buff-coloured forehead scalloping, which extends well above the eye. The base of bill is yellow/buff, and the rest of the bill is dark grey, not black as on an adult

Wing of 1st year Inland Thornbill (older?)
Although these birds were not of exact known-age, as they had not been trapped and marked before, it would seem that young Inland Thornbill plumage progresses from broad buff-coloured scalloping on the forehead, through fine buff-tips, to fine white-tips by the time they reach adulthood. And the crown changes gradually from greenish-grey to smoky grey over the first year.

The first-year birds also retained some of their first-grown flight feathers. The first (older?) bird had three old outer primaries, and the inner primaries were recently moulted and freshly grown.

Wing of 1st year Inland Thornbill (younger?)

The second of the young birds still had all of its first-grown primary feathers, from its juvenile plumage.

The combination of all these features support that the second young bird was younger than the first, but only the recapture of known-age birds will help to confirm these ideas.

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