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Comment on British Trust for Ornithology (BTO) research

Thursday, 20th June 2019

The recent report by staff of the British Trust for Ornithology (BTO), "The composition of British bird communities is associated with long-term garden bird feeding", published in Nature Communications 10, 21 May 2019, provides evidence that providing supplementary diets to garden birds in Britain both supports and enhances the numbers of our native avifauna.
This is excellent and very welcome news. For years there has been debate, sometimes a degree of acrimony, regarding the pros and cons of “garden bird feeding”. Proponents have contended that it must be helpful to garden birds if they receive extra food; after all, survival and reproductive success of all species is closely linked to adequate nutrition. Some members of the public have argued vociferously that, because birds readily take the food they are offered – and, indeed, often come back for more - it must be beneficial to them. Antagonists, often including professional biologists, have retorted by pointing out that eating habits are not always based on an appreciation of what is good for you, giving as examples obesity in humans and mineral/vitamin deficiencies in zoo animals that are allowed to choose what they ingest. Some critics of the practice even suggested that garden bird feeding was a mere indulgence, by persons who were disinclined to do “proper bird watching”.


Growing reports of outbreaks of infectious diseases in garden birds, including salmonellosis, provided further ammunition for these and other sceptics, who were then able to claim that by “putting out food” the public were encouraging birds to congregate in larger than usual numbers and thereby facilitate the spread of pathogens.  In the absence of proper scientific evidence, people had to make up their own mind – and most, boosted by the greater availability of bird-feeding products of high quality, opted to continue. The authors of this Comment (John and Margaret Cooper) have always supported and encouraged the concept of feeding wild birds, mindful of how the pioneer of “bird gardening”, the naturalist Maxwell Knight (see remarks of Simon King and viewed the practice. Maxwell Knight and his wife Susi fed the birds in their garden for many years and came to the conclusion that if done sensibly and hygienically, this exercise appeared to cause no harm (individual birds were seen to return time and time again, sometimes in succeeding years), it probably did a lot to help birds in the winter and at other times of hardship, and (to the Knights perhaps the strongest argument) it brought ordinary people, including those unable to venture further afield, into closer contact with birds and the natural world. Now, thanks to the work of the BTO researchers, the need for ambivalence is over. We have convincing evidence of the value of garden bird feeding and the fact that it emanates from the BTO (British Trust for Ornithology), an internationally respected body that has been carrying out scientific studies on British birds since 1932, adds weight and credibility to the conclusions.

Blue Tit feeding

We have corresponded with two of the authors of the BTO report. Back in 2013 John first met and had interesting discussions with Dr Gavin Siriwardena, at a time when Haith’s was expressing concerns about the supplementary feeding of farmland birds – in particular, the possible adverse effects on avian health if the cereals being used were not of the highest quality. On this recent occasion, however, the topic of discussion was more positive – namely, how and why British bird populations are benefiting from long-term garden bird feeding. In our correspondence with Gavin we welcomed the findings of the recent BTO report but commented that there was no mention in it of the potential danger of spreading of pathogens (for instance, if the food being offered was not of the highest quality, or if presentation and hygiene were poor) nor of the potential danger to wild birds of inducing other (largely metabolic) diseases by overfeeding. Gavin responded


“You're right that we didn't directly discuss pathogen effects in the paper, but they are there in the patterns for greenfinch, at least. Kate's previous work has considered negative effects of "the wrong feeding" and this work is about changes in community structure that take multiple forms, including increases and decreases. This will probably include interspecific interactions and sub-lethal disease or diet effects could well be a part of these, although it would be very difficult to separate different types of interaction from survey data alone”

A follow-up letter from Dr Kate Plummer, the senior author of the report expanded on this as follows:
“As Gavin says, we were looking to describe the broad patterns that have occurred over time and across whole communities, so we didn't focus on the specific mechanisms by which food can influence bird numbers. But of course you are right, both positive and negative impacts of feeding have been described and all of these are encompassed by the overall community responses we present. 
None of this is to suggest that the findings in the BTO study are anything but welcome. As Simon King points out, they substantiate Haith's long-held belief that it is correct to promote the importance and potential value of feeding garden birds and to couple this with the provision of food items of high quality.
To us, the important “take home message” resulting from the findings reported by the BTO researchers is:
1) there is now published scientific evidence to support the long-held belief that there is a correlation between the provision of supplementary food to garden birds and increases in numbers of some species – especially, but not exclusively, passerines. Species formerly rarely seen in gardens appear to have exploited the growing variety of foods that are being provided. Urban areas of Britain are permitting the populations of feeder-using bird species to grow while the numbers of species that do not use feeders remain unchanged. 
2) The population increase seen has taken place over forty years, coinciding with the growth and increased sophistication of bird-feeding practices. However, like Britain’s membership of the European Union, one cannot assume that a period of four decades means that further changes will not take place. Attracting birds to gardens, where they congregate on or around feeders, may be having other deleterious effects, of which at present we have little knowledge.  For instance, as mentioned earlier, there has been an increase in recent years of reports of certain infectious diseases, such as salmonellosis and trichomonosis, but we do not have unequivocal evidence as to whether this might be in any way attributable or related to garden feeding and closer contact between birds.  We know even less about the possible effects of supplementary feeding on non-infectious/metabolic diseases. For example, do some forms of garden feeding encourage the ingestion of too much food of the wrong quality – excessive fat, for example, which can lead to liver and kidney disorders in both free-living and captive birds? These and other possible unexpected side-effects of garden feeding on birds and other species need qualitative multidisciplinary research, to include veterinarians and wildlife pathologists as well as ecologists. 
It is interesting that the official journal of the British Veterinary Association, the “Veterinary Record” (184 7 June, 2019) picked up on this. In a feature for vets commenting on the BTO report, entitled “The Big Picture: How feeding garden birds has changed the pecking order” ( production manager Georgina Mills wrote:
The message is clear; we cannot afford to be complacent.  
3) Providing food for garden birds can clearly be a good thing – both for the birds themselves and for the people who get so much pleasure from feeding them. But that is only one part of the picture. The TYPES of food that are offered and, as intimated in Georgina Mills’ piece above, the QUALITY of that food are vital concomitants. As far as the first of these is concerned, Haith’s has long taken a leading role in and promoting novel products, for instance – as pointed out by Simon King – by introducing its Native Finch mix nearly twenty years ago.  
“Quality” has always been a watchword at Haith’s, brought to a head in 2012 when the company introduced a quality control (QC/QA) programme aimed at ensuring that its products had been checked, using laboratory tests, before sale. That QC/QA programme, possibly the first ever by a bird food company, was described and advocated as good practice in one of the world’s leading textbooks on the health and welfare of birds – see the sections on diet in Dr Jaime Samour’s “Avian Medicine” (2016), published by Elsevier (   
Originally set up and supervised by Haith’s veterinary advisor, the QC/QA programme is now largely in the hands of trained staff at the Grimsby Bird Food Centre. The techniques that the Haith’s QC Team use are not confined to the company’s products. They continue to be taught elsewhere by Haith’s veterinary advisor to students and keepers who are interested in raising the standards of bird foods and of feeding practices and applying sound scientific rigour to these.
4) There are legal and ethical reasons for ensuring that food being sold for wild birds is visibly clean, dust-free, not sharp or abrasive, and not contaminated with unwanted bacteria, fungi or other micro-organisms.  Members of the public need to feel confident that there is no significant health risk to them, their families, or visitors when handling such products.  
And attention is still needed even after the bird food has been put out on bird tables or feeding stations. Other animals that may be attracted by the spread can include rodents such as rats and mice and these may both introduce pathogenic organisms and help spread them. The hygienic precautions advocated by Haith’s and other responsible bodies are therefore very important - see
Feeding station

What of the future? It is clearly most important that the findings in the BTO study are carried forward and that the mechanisms involved in the increase in numbers of some species are properly studied so that we understand them better. Population dynamics (“the study of the size and age composition of populations and the biological and environmental processes that influence them”) is, however, a remarkably complex discipline. An increase in bird numbers may be associated with, or due to, various interacting factors. The growth may in turn lead to other ecological changes, affecting diverse species (not just birds – insects, for example) and the wider environment. Future research will benefit from the input of members of the public, field naturalists and the bird food industry as well as ornithologists, ecologists, veterinarians and statisticians. All have a part to play if the encouraging trend in our garden birds reported by the BTO is to be sustained and the long-term health and welfare of our native avifauna is to be adequately protected and enhanced.  

Professor John Cooper and Margaret Cooper

JEC/MEC 19th June 2019

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