Wednesday, 25 September 2013

Tree-creeper bill shape

Adult male White-throated Treecreeper
One of the advantages of mist-netting birds for banding is the opportunity to see them as we seldom have a chance to do so when they are flitting about the woodlands or wherever they live. While we were banding, aging, sexing and measuring this White-throated Treecreeper Cormobates leucophaeus during the same study as the previous blog article below, I noticed just how fine its bill was. There is a graceful curve when seen side on, but when viewed face-on, the bill can be seen to be remarkably tapered to a point. This is an adaptation for probing into cracks in tree bark, where they hunt for their food; tiny invertebrates.

The sleek lines of the treecreeper's bill - superbly adapted for reaching and grabbing tiny insects under bark
In comparison, other birds we caught that day included the Western Gerygone Gerygone fusca with a fine pencil-pointed bill for feeding on the tiniest of insects. Very fine, but straight and simple in line as it has no need for any further specialist shape like the treecreeper.

The tip of  this Western Gerygone's bill is polished by contact with the surfaces it has lifted insects from
There were also honeyeaters, which have slightly curved bills for probing into flower-heads to sup nectar, but their bills are mostly sturdy like that of this White-plumed Honeyeater Lichenostomus penicillatus as they also eat insects. The bill is very much thicker than that of the treecreeper.

The White-plumed Honeyeater has a slightly curved bill for accessing nectar in flower heads
And we caught a the generalist insect-eating Rufous Whistler Pachycephala rufiventris.  It has a broad, thick bill with a curved tip on the upper mandible, which enables it to catch insects on the ground or in the air and hold them firmly. Only when we held these birds successively in the hand did I appreciate just how finely tuned the treecreeper's bill is. Evolution is marvelous. 

An adult female Rufous Whistler: the hooked tip to its bill helps it hold prey tight

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