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A method of sexing Marsh Harrier chicks in the nest in order to determine the correct ring size

The Sex of Marsh Harrier chicks can be determined in the nest
using the foot span measurement and the tables in the book
Handleiding veldonderzoek Roofvogels, Guidelines for fieldstudies on Raptors
KNNV Uitgeverij,- by Rob Bijlsma.
The relevant pages are contained in the embedded pdf document double click on it to access .


Footspan is measured from the tip of the toe of the front middle talon to the tip of the toe of the hind talon excluding the claws. See photos below.

If two people are not available then the tarsus measurement taken laterally ie from front of foot to back of foot is an absolute must as females will have a tarsus of over 9mm.
Two people are required to take the footspan measurement.


Occasionally it will be found that if both footspan and tarsus are taken although the footspan measurement = 80mm or more, the tarsus does not exceed 9mm. In these cases if the footspan measurement had only been used on its own it will never result in using a ring which is too small!

Page 138 in the tables shows the relationship between number of days old and winglength
Page 139 
in the tables shows the relationship between number of days old, weight and footspan

Translation of key words in the tables:

Dag= Day
Mannetje= male
Vrouwtje= female
Totaal= total
Gewicht= weight
Klauw zonder nagels (mm)= Claw without nails (mm) = Foot span
Vleugellengte = wing length

The measurement is only valid from day 19 or if day of hatching is unknown, when the wing length is 160mm or more then:

Footspan > or = 80 mm is female
Footspan = or < 78 mm is male
From day 19 or winglength 160 mm or more there is no overlap any more in the footspan between male and female and the chicks can be sexed in the nest and the appropriate ring size used.

F rings internal diameter 9mm for male chicks
G rings internal diameter 11mm for female chicks.
If there is any doubt also use the weight,
birds > or = 700g always female




North West Norfolk Ringing Group

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Last updated January 2017