Great Tit with large tumour

Great Tit with large tumour

Observation - Great Tit with large tumour. Description: Great Tit with large tumour on leg. The bird was caught during a ringing session and despite, the huge tumour, it appeared to be in good health overall without any apparent distress. When released, it flew off well but did appear to be rather weighed do

Great Tit with large tumour on leg. The bird was caught during a ringing session and despite, the huge tumour, it appeared to be in good health overall without any apparent distress. When released, it flew off well but did appear to be rather weighed down. Any suggestions as to what might have caused this are most welcome.