Many well-meaning households across the UK could be feeding the seeds of change to Britain’s garden birds.
Putting out food for the birds that visit our gardens, be it purpose-bought seeds or leftover scraps, is a popular way of interacting with wildlife for many people across the world. In some areas of the UK it is thought that up to 75% of households partake in what is known as ‘supplementary feeding’. And why shouldn’t they? In an increasingly urbanised world, attracting such charismatic birds as blue tits, goldfinches and robins to a garden can provide a lifeline to the natural world, and a feast for the eyes as they feast on the feeders.
As the breadth and significance of this pastime becomes apparent, so researchers have grown increasingly interested in the potential effects, both positive and negative, on the ecology of these garden guests. (Although technically not guests as they’ve existed long before gardens have!)
Concerns have been raised that feeding garden birds could encourage the spread of avian diseases such as trichomoniasis – after working in a wildlife hospital for the last four summers I know first-hand how easily this disease spreads between individuals and species! Questions as to the quality and suitability of supplementary food compared with natural foods have also been asked, and whether or not wild birds will become dependent on the food provided.
It has even been suggested that garden bird feeding could create an ‘ecological trap’ for birds, as the food provided could belie the quality of a habitat. What on the surface may appear a bountiful territory may turn out to be unsuitable for rearing young in terms of food quantity and quality. Like moving to an area with a Waitrose, only to find out it had been taken over by Aldi.
First-world problems aside, researchers at the University of Birmingham found concerning results in their study populations of blue and great tits in a wood in north Worcestershire. It seemed that providing supplementary food reduced the number of eggs laid and the number of offspring that hatched, compared with an area in the same woodland where no supplementary food was provided.
To avoid the risk of over-sensationalising, it must be said that most supplementary feeding studies report either positive findings or no significant effect at all. Impacts include a change in egg quality, clutch size (number of eggs laid), chick survival, and adult overwinter survival. It has also been found that the date on which eggs are laid is often brought forward, which has the potential to desynchronise birds with their food source, contrary to the old idiom, the early bird is too early to catch the worm (or caterpillar in this case). The extent to which these effects are positive or negative depends on the species in question, highlighting the difficulties faced by wildlife organisations and researchers in providing garden bird feeding advice to the public.
A further quandary facing researchers is the uncertainty surrounding the quantity of supplementary food actually being consumed by birds. Short of direct observation which is time-consuming and impractical to implement twenty-four hours a day, there is no simple way of estimating this. New research however, conducted by Gillian Robb and colleagues, has shown that by assessing ratios of stable isotopes in birds’ tissues, it is possible to infer the source from which food has come. Tissues such as claws can be clipped in a relatively uninvasive manner and by looking at from where the constituent nutrients have come, the relative importance of supplementary food to the bird’s diet can be determined.
Research into the affects of supplementary feeding is ongoing and innovations in the field such as this are invaluable additions to the armoury of methods. However the myriad effects of supplementary feeding on different species makes progress extremely difficult. We are still a long way off from finding out the full effects of supplementary feeding on the ecology of garden birds, and whilst we are blissfully ignorant, we might as well sit back, continue providing food, and enjoy the avian attendees to our gardens.
As a brief coda, my final-year undergraduate project at the University of Birmingham aimed to identify key demographic groups to which garden bird feeding advice should be directed and determine the public’s motives for feeding birds. It was based on a questionnaire launched last summer through collaboration between BBC Midlands Today and the University of Birmingham. David Gregory’s (BBC Science Correspondent for the West Midlands) coverage of the results can be found here.