The doubled clutch in the Chiffchaff's nest. - N.N. Balatsky. - Berkut. 3 (2). 1994. P. 144.
In environs of the town of Noyabrsk (Tyumen region, West Siberia) a nest with 9 eggs was discovered on 26.06.1994. The eggs had different measurements and the shell mass. This is the evidence, that the eggs were laid by the different females. [Russian].