The Iberian Chiffchaff Phylloscopus ibericus has recently been accepted as a full species, but is still very difficult to distinguish from the Common Chiffchaff P. collybita when molecular and acoustic evidence is lacking. Distinguishing these cryptospecies is essential for obtaining accurate population estimates for each, which is especially important in the case of the Iberian Chiffchaff due to its restricted distribution. We re-assessed the discriminant methods available in the literature and propose some alternative traits to distinguish both species. We used information from 24 morphological traits measured in c. 6,700 individuals of the two species trapped during the autumn migration period in Doñana National Park, Spain, between 2004 and 2015. Discriminatory methods available in the literature were unable to distinguish the two species efficiently. Despite some biometric differences, morphological measurements showed a high degree of overlap and so could not distinguish between Iberian and Common Chiffchaffs. The best discriminatory factor at our study site was the passage date, since 90% of Iberian Chiffchaffs have already departed when the first Common Chiffchaffs arrive. Those traits associated with migration, such as wing size and shape and fat deposits, were also able to discriminate the species, albeit weakly. This result fully concurs with the trans-Saharan migration of the Iberian Chiffchaff in contrast to the Common Chiffchaff, which chiefly winters in Mediterranean latitudes. In conclusion, biometric traits are useless for discriminating the two species and we do not recommend their use. A comprehensive examination of colouration and plumage seems to be the only reliable way of guaranteeing the correct identification of these species in the hand.