As a result of finding several dead Greenfinches in his garden, Malcolm Watson went investigating, and passed on the following interesting article.
Outbreaks of mortality in wild birds in gardens in the U.K. were first reported in the mid 1960s, when members of the general public began to put out bags of peanuts to feed the wild birds. In these first outbreaks most deaths were due to infection with the bacterium Salmonella typhimurium and occurred in greenfinches (Carduelis chloris) and house sparrows (Passer domesticus). Mortality incidents have continued, and since 1994 post mortem examinations have been carried out by the Veterinary Science Division of S.A.C. on over 200 finches or sparrows found dead in Scotland. The results from these investigations have shown that, in addition to Salmonella typhimurium, another bacterium is responsible for many of the deaths in wild birds, a strain of the bacterium Escherichia coli referred to as E. coli O86.
During these seven years a seasonal pattern has emerged. Salmonella typhimurium (often a particular type referred to S. typhimurium DT40) typically causes mortality in the months December to March, mostly in greenfinches and to a lesser extent in house sparrows. E. coli O86 usually causes mortality in the months March to June, especially in greenfinches and siskins (Carduelis spinus). Both bacteria have also caused deaths in goldfinches (Carduelis carduelis) and chaffinches (Fringilla coelebs), although in smaller numbers, and occasionally Salmonella spills over into other species such as the great tit (Parus major). Dead birds or sick birds are usually found in the vicinity of the bird feeders. If seen alive the birds appear fluffed up, reluctant to fly, appear to be breathing heavily and may look as if they are having difficulty in swallowing.
The post mortem examination of birds dying from Salmonella often reveals substantial yellow/orange areas of damage to internal organs such as the gullet, liver, spleen (an organ that tries to fight off diseases) and sometimes the lungs and lower part of the digestive tract. The damage to the gullet can be so severe that it causes a partial blockage, preventing food getting to the bird's stomach even if it continues to eat. The post mortem findings in birds dying from E. coli O86 are different. This bacterium doesn't cause such obvious damage to the internal organs but has the ability to produce toxins (poisons) that prevent the digestive tract from working properly. Birds dying from E. coli O86 typically have much food in the gullet but little further down the digestive tract. Confirmation of the cause of death requires the culture of the bacteria from the carcase on special laboratory media and identifying the bacteria that grow.
Deaths in garden birds in the U.K. have become a regular occurrence each winter, and the year 2001 looks like being the same. On January 12th 2001 The Royal Society for the Protection of Birds reported unprecedented numbers of telephone calls from the public reporting sick and dead birds in gardens. Some dead birds have been examined by the Zoological Society of London, some by the Veterinary Science Division of SAC in Scotland, and the results confirm that Salmonella is again the major cause of the mortality. So far in 2001, salmonellosis has been confirmed by SAC in the greenfinch, chaffinch, goldfinch, house sparrow, and also the tree sparrow (Passer montanus), another bird whose population in the U.K. is in dramatic decline. Deaths in siskins in the second week of February 2001 were the result not of salmonellosis but of E coli O86 infection, which in past years has usually occurred later in the year, from March onwards.
Deaths from salmonellosis have not been confined to sparrows and finches at bird tables in the U.K. Since 1988, many finches have been found dead around garden feeders in the United States and Canada, mostly pine siskins (Carduelis pinus) and common redpolls (Carduelis flammea), but also evening grosbeaks (Coccothraustes vespertinus), house sparrows, and American goldfinches (Carduelis tristis). As in the U.K. incidents, the type of Salmonella involved was Salmonella typhimurium phage type 40. Cases of salmonellosis were also reported in domestic cats that preyed on sick birds around bird feeders.
A different strain of Salmonella typhimurium described as S. typhimurium DT160 caused outbreaks of mortality in house sparrows in New Zealand in 1999/2000. At the same time there was evidence of spread of disease to humans (including one death), and to young farmed ducks and quail, dogs and cats, deer and horses. In one incident more than 400 dead birds were found at one location on one day. This strain of Salmonella typhimurium also caused a small outbreak of mortality in house sparrows in Central Newfoundland, Canada, in February/March 1999.
Although the mortality incidents in the U.K. usually occur at sites providing supplementary feeding for the wild birds, the food is not believed to be the initial source of the bacteria but rather the cause of the congregation of large flocks of birds in a small space. It is likely that some birds carry small numbers of Salmonella typhimurium and E. coli O86 in their intestines, and when the birds congregate at the bird tables and feeding stations a build up of these bacteria occurs, contaminating the food and water, the feeders and drinkers, and the surrounding environment. Under these conditions, the bacteria then have the chance to overwhelm the birds, damaging their intestines and causing their deaths.
Clearly prevention is very important, and is based on preventing a build up of these potentially lethal bacteria. Regular cleaning and disinfection of bird tables, feeders and drinkers will help, as will periodically moving the feeding sites. If the birds can be spread out by using several different feeding sites, so much the better. The areas beneath the feeders can also quickly become contaminated, and should be kept as clean as possible, with any uneaten food removed.
This greater awareness of the causes of mortality in finches comes at a time when the populations of some of the UK's wild birds are in decline, especially woodland species (down by 20% since the mid 1970s) and farmland species (down by 40% in the same period). In the report The State of the UK's Birds 1999, published by The Royal Society for the Protection of Birds and The British Trust for Ornithology, attention is drawn to the continued and alarming decline of once-common species such as the redpoll (down by 92% since the 1970s), the house sparrow (down by 58%) and the tree sparrow (down by 87%), three bird species known to be susceptible to salmonellosis. Indeed, the Report suggests that, such is the severity of the decline in house sparrow and redpoll numbers, they should be considered for inclusion on the Birds of Conservation Concern (BoCC) red list of endangered species.
Mr. Tom W. Pennycott
Senior Veterinary Investigation Officer, Avian Health Unit
SAC Veterinary Science Division
Ayr KA6 5AE
February 12th 2001