Some of these articles and short notes have been published before, others were made for this site. Some contain (partly) new information, others are to present described features more clearly, e.g. like how to use characters for ringers in field identification.  

Blue Ross's Goose (with Fred Visscher)

Markers for Snow Goose genes in a blue "Ross's" Goose in The Netherlands

Ageing Spotted Crakes

The best markers for ageing, including a (possibly) undescribed feature.

Hybrid redstarts vs. Eastern Black pages (with Nicolas Martinez)

citroenkwik Teylingen nov 2010.jpg

Citrine wagtail calls
Can ssp. be identified based on calls?
by Maarten Wielstra & friends

The Chiffchaff ID files
Iberian and Siberian (looks, calls, genetics)

Blyth's Reed Warbler
How wing structure can be used  for field identification

Crossbill flight calls (by Thijs Fijen)
How to separate Parrot Crossbill calls from several Common Crossbill types

Coues's Arctic and Mealy Redpolls
A note on the undertail coverts