Flight feather#Primary extension
The primaries projection is a term from the topography of birds. It measures the portion of the blade tip which protrudes in the state in the longest shield spring. It takes place in the differentiation of some another very similar species as an auxiliary criterion application.
The Marsh Warbler ( Acrocephalus palustris) has a comparatively long primaries projection
Comparison of the primaries projection at Chiffchaff (left; short) and Willow Warbler (right, long)