Fuertes-Recuero M., Longhin D., Chiesurin L., Tusini S., Fontanillas Pérez J.C. & Cantarero A. (2023) First report of polymelia in a passerine bird, the Pied Flycatcher. ARDEA 111 (2): 554-557

Polymelia is a congenital defect characterized by an excessive number of limbs. Leg malformations are unusual and have rarely been reported in birds. The ultimate mechanism that regulates this type of abnormal development is not yet well understood. Here we report polymelia in a nestling of the European Pied Flycatcher Ficedula hypoleuca. We have ringed more than 12,100 Pied Flycatcher nestlings since 1991 and have never observed this malformation before. To our knowledge, there is no study indicating polymelia in any bird of the order Passeriformes. We observed and studied a Pied Flycatcher nestling that had two extra limbs fused at the pelvis. We used X-rays, Computerized Tomography (CT) scan and Magnetic Resonance Imaging (MRI). We describe the observed malformations and discuss potential causes. The malformation could be due to an embryonic cause, such as a parasitic twin (pygopagus parasitic) or a genetic mutation.