It has been said that sometimes a bird can be sexed by the shape of its cloaca. Cloaca’s can be controlled by ”belly-blowing”. From 1968-1988 some Dutch ringers checked the type of the cloaca’s of 5799 birds, which could be sexed by other marks, e.g. by plumage (the group ”sex known before”). The conclusions are: As a distinguishing mark, the shape of the cloaca is too unreliable to be used, without any restriction, to sex birds. 8.4% would be sexed wrongly. Some birds have a ”heavily-formed” cloaca, type C. Of the 3212 birds of the group ”sex known before” Mr. Schaap found that 447 (= 14%) had such a ”heavily-formed” cloaca. All these 447 birds were males. Mr. Schaap also checked the cloaca’s of 2249 birds which could not be sexed by plumage. In this group he found 326 (= 14%) birds with a ”heavily formed” cloaca. It must also be true that of ALL birds 14% have a ”heavilyformed” cloaca and Birds with a ”HEAVILY-FORMED” CLOACA are MALES. This distinguishing mark is unreliable on birds bigger than a Mistle Thrush (Turdus viscivorus). This mark is very useful on Robins (Erithacus rubecula). A ringer must check many, many birds before he/she has enough experience to decide if a cloaca is a ”heavily-formed” one, type C. So be careful!!!