SEEN - SE European Bird Migration Network
Fig. 1. Technique of tail-length measurement

Tail-length measurement
("to the back" method* after Busse 1983, 1990)
Measurement of the tail with the pygostyl: a simple, very qui ck and safe method for the bird.
The ruler with the stop cannot be used.
The body of the bird is held vertically with the tail directed at right angle to the back (Fig.    1). The ruler lies at the tail with the butt pressed firmly to the back (controlled with the fourth and the fifth finger under the belly). The rectrices should touch the ruler along their whole length. The longest tail feather measurement is read.
Proper measurements can be obtained only if bolded remarks in the description above are followed.
Precision of measurement - 1 mm.
The most common mistakes in measurement -
Tail-length underestimated:
- an acute angle made between the tail and the back (the bird body too close to the ruler),
- the ruler end not firmly located (the fourth and the fifth fingers do not press the belly),
- rectrices do not touch the ruler along their whole length (not pressed to  the ruler by the fingers of the left hand),
- false reading of some mm will result if the butt of the ruler is at the tip of the pygostyl instead of at the bird's back.
Tail-length overestimated:
- an obtuse angle made between the tail and the back (the bird body too far from the ruler),
- the butt of the ruler touches the bird's back well above the pygostyl instead of laying on it.
Other techniques - see APPENDIX I.

*Note that a description of this method (and drawing) given in "Identification Guide to European Passerines" (Svensson 1992) is incorrect.
Created by Pronetix 2006