An article in a recent issue of the British Trust for Ornithology (BTO) Garden Birdwatch magazine Bird Table reports that a sudden decline in greenfinch numbers noted at garden bird feeders in late summer 2006 was the result of a disease called trichomonosis (Robinson R. & Lawson B. [2009] Bird Table Issue 59, 12-13). The disease, which is caused by infection with a protozoan parasite called Trichomonas gallinae, had been seen before in pigeons and doves but not in small garden birds.
The disease produces lesions in the throat and gullet, and infected birds appear lethargic and often fluffed-up, and may drool saliva and have difficulty swallowing. Although the initial major outbreak, which killed thousands of birds, occurred mainly in the West Midlands and South-west and North-west England, a further outbreak in 2007 included areas of the south-east.
Research into the disease is ongoing, and it is too early to say yet where it originated or what the long-term impacts on greenfinch populations will be. However, the parasite cannot survive for long outside its host, and it therefore seems likely that the main route of transmission is via food or water contaminated with saliva. It is not possible to treat wild birds effectively in the field, so the BTO stresses the importance of good hygiene at bird feeding stations.
Typical lesions of avian pox (© BTO).
Another worrying recent trend is a possible increase in the number of cases of avian pox. Cases used to be reported sporadically in a number of different species including woodpigeon and dunnock, but a number of infected great tits have been reported in late summer for the last couple of years. Avian pox typically causes lesions on the head, around the eyes and the beak (see photograph), and sometimes on other areas of the body. If anyone finds a dead bird with such lesions, the BTO would like to hear about it, ideally with a photograph. Please contact Katie Colvile at the Institute of Zoology (Tel. 0207 449 6685) in the first instance.
Lynn Whitfield