Save the best for last: ID & age of a mega

Vincent
27 nov 2018
3 minuten om te lezen

In many respects autumn was remarkably slow. But November turned everything upside down!

Like in spring (with an Alpine Swift), the girls from De Wulp delivered the highlight of the season (year!).

On 5 November Lizzy sent me a WhatsApp message. She asked if I knew which species was just taken into care. Despite the pics showed a wet and scruffy bird, my pulse raised and I missed a heartbeat or two. Even I was left speechless for a bit. This was an American Catharus thrush!

A new pic of a dry bird confirmed it was a Grey-cheeked Thrush (or a theoretical Bicknell’s). A new species for the Netherlands!

Despite it was in a rather poor condition, birds are in good hands at De Wulp: it recovered well. In all I saw the bird four times. They are tiny for a thrush! The size of a Common Nightingale, but with remarkably longer wings (no less than 2cm longer). On 20 November I released the now fully recovered bird at a suitable site near where it was found. It immediately disappeared into the thickets, never to be seen again.

So how did I rule out the very similar Bicknell’s? And how did I age it?

Plumage features overlap, but Bicknell’s - a split from Grey-cheeked - e.g. has a slightly more rufous tail. The cheeks should be plain. Bicknell’s is said to have more yellow on the lower mandible – which could fit this bird. However, David Sibley questions this feature. In this bird the cheek is striped, and the tail is not slightly rufous. It has the same colour as the upperparts. So that's reason nr 1) to think about Grey-cheeked rather than Bicknell’s.

The key features, however, are in the wing structure.

First of all there’s the wing chord. I measured 102 mm. Based on other sources, Rimmer et al (2001) mention:

"Wing chord of adult Bicknell's 82-100 mm (n = 415; VINS), of Gray-cheeked 93-109 mm (n = 200; Pyle 1997)."

and

"Majority of Gray-cheeked Thrushes have wings >95 mm in length (Ouellet 1993); 85% of Bicknell's have wings <95 mm (VINS)."

So 2) the wing chord seems to diagnostically rule out Bicknell's.

But there's more.

The longest primary is p3 (counted from outside in). In Bicknell’s this should be P4.

Finally I measured the primary projection with a ruler. In general this is ≥ 100% in Grey-cheeked and ≤ 100% in Bicknell’s. In this bird I measured c 115% (yes, they have extremely long wings). That’s even larger than the extreme for Bicknell’s (110; Lane & Jaramillo 2000).

Based on this (not ideal) pic I measure a primary projection of 111%. With a ruler I measured 115%.

So 3) the wing diagnostically confirms the ID

Ageing this bird is straight forward.

There’s more to it, but note all greater coverts are still juvenile, with a pale tip (in this species it seems to be normal that no coverts have been moulted). In adults these are plain.