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The common gull is a widespread species, occurring in almost the whole of Europe that shows no sexual dimorphism in plumage and some dimorphism in size. To propose a method for sexing birds from this species by linear measurements, a discriminant function analysis was applied to a set of morphometric traits in birds captured in northern Poland during the non-breeding season. In total, 138 males and 76 females were measured and sexed molecularly. The averages of measurements of males were larger than the average measurements of females. All individuals with a total head length shorter than 90.6 mm were females. However, the occurrence of larger individuals of the subspecies heinei among common gulls of the nominate subspecies canus wintering in Europe makes separating males according to a single border value of this measurement less efficient. The proposed discriminant function containing the total head length allowed for correct sexing of 95.7 % of males and 90.9 % of females. This discriminant equation may be used for sexing common gulls when DNA sampling is not possible, and could be applied to data collected in the past, providing the opportunity to re-examine the results of previous analyses. However the higher misclassification rate than in other gull species should be taken into account. [ABSTRACT FROM AUTHOR]