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Gardens for Birds - The Keith Duckworth Project
By Hugh Hanmer, BSc (Hons), MRes

Paper Now Published

Nest predation in urban gardens
Does feeding birds increase local nest predation?

Hugh J. Hanmer
People and Wildlife Research Group, University of Reading, UK

LINKED PAPER
Provision of supplementary food for wild birds may increase the risk of local nest predation. Hanmer, H.J., Thomas, R.L. & Fellowes, M.D.E. 2016. IBIS. DOI: 10.1111/ibi.12432 

Across the world, providing food for garden birds is a huge source of pleasure for hundreds of millions of people. In more urbanised countries, bird feeding is the main way that city dwellers are able to actively engage with wildlife. It is difficult to exaggerate just how widespread this is; in our local area (Greater Reading, UK) over 55% of households feed wild birds (Orros & Fellowes 2015) and nationally the figure approaches 50%. The UK may well be at one extreme, but in the USA some 53M people feed birds in their gardens (U.S. Fish and Wildlife Service 2014). The result is a huge perturbation experiment, where we in the UK as a nation add over 50,000 tonnes of supplementary food to our urban ecosystems every year, with a highly conservative estimate suggesting that we are providing enough food to maintain over 31M of our most common garden birds (Orros & Fellowes 2015a).
 
This is simply staggering, but the influence of such a common and widespread practice has received much less attention from researchers than it deserves. Evidence suggests that our urban avifauna has greatly changed in direct response to the provision of food. The poster child (bird?!) of this change is the European Goldfinch, Carduelis carduelis. In the 1970s, only 1% of gardens were visited by goldfinches, and now over 60% of gardens in the British Trust for Ornithology’s Garden Bird Watch survey record their presence. This is closely aligned with the enormous increase provision of specialist food, such as nyger seed. It is not only passerines which benefit from changes in the provision of supplementary food. In Reading, particularly during the breeding season, the town is visited by several hundred Red Kites, Milvus milvus, every day, where more than 1 in 25 households regularly provide meat for these magnificent raptors (Orros & Fellowes 2014, 2015b; see Mel’s excellent blog post here).
 
But is the provision of supplementary food always a good thing? While research has considered the effects of supplementary food on the breeding success and over-winter survival of urban birds, my supervisors (Professor Mark Fellowes and Dr Becky Thomas) and I were interested in asking about some indirect effects of attracting high densities of birds to predictable point sources of food. Our research group started by showing that numbers of insects were reduced near bird feeding stations (Orros & Fellowes 2012; Orros et al. 2014). A chance observation of a Great Spotted Woodpecker, Dendrocopos major, visiting a peanut feeder, and then flying to a nest box where it proceeded to hammer through the wood and then devour the Eurasian Blue Tit, Cyanistes caeruleus, chicks inside set us thinking as to whether, as with the insects, nesting close to bird feeders would result in increased predation. Feeders are frequently visited by magpies and grey squirrels, both common nest predators in the UK.
 
This is tricky to study! We had understandable ethical qualms about placing feeders near wild bird nests, even if logistically we could find enough for meaningful statistical testing. Seeking correlations between nest success and feeder density is possible, but given the ubiquity of feeding in the UK it would be difficult to isolate variance that was not entangled with socioeconomics and differences in habitat quality, but this approach could work in areas where feeding is less common. Instead we took an experimental approach using artificial nests, each continually monitored by camera traps to record every predation event.
 
Using our suburban parkland campus at Reading, we set up over 100 wire and grass ‘nests’ in sites mimicking where Eurasian Blackbirds, Turdus merula, would nest. Each was baited with two quail eggs. The nests were 5 and 10m away from the feeding station, to replicate typical suburban garden size. The feeders were either empty or filled with peanuts, and the filled feeders were either of a standard open design, or surrounded by a guard designed to prevent access by larger species such as squirrels and corvids. The feeders were also monitored by camera traps, so we could see what species visited them.

Nest Set up
A trial experimental nest set up with a trail camera on the left and an artificial nest baited with quail eggs on the right © Hugh Hanmer


Not surprisingly, almost no visits were made to empty feeders. Unguarded feeders were frequently visited by Eurasian Magpies, Pica pica, but Grey Squirrels, Sciurus carolinensis, dominated. Guarded feeders received relatively few visits from magpies, and Grey Squirrel visits were also much reduced, but small passerines were now able to access food without being excluded.
Did nesting adjacent to a feeder matter? Around 50% of nests placed near empty feeders survived a week’s placement, but only around 10% of those near filled feeders were not predated, a five-fold difference in predation rate. Unexpectedly, the presence of a feeder guard did not make a significant difference. This is a substantial increase in predation, and magpies and Grey Squirrels to a lesser extent were common nest predators. What was less expected was that Eurasian Jays, Garrulus glandarius, were also important nest predators, even though they didn’t visit the feeders. Our suspicion is that the magpies and Grey Squirrels are attracted to the food, and then forage locally, increasing the chances of finding local nests, and that the Jays respond to the presence of the other species, rather than the presence of the food itself. Irrespective of the behavioural underpinning, the result of providing food was increased nest predation.

Predicted survival rates
Predicted Cox’s proportional hazard survival distribution by feeder nest type over the course of a mean experimental week (bold lines) with individual 95% confidence intervals (shaded). Final predicted survival rates were 0.49, 0.12 and 0.08 for empty, guarded and unguarded feeder nests, respectively.


People who feed birds do so because they wish to help them, but may inadvertently be causing harm. So, should we stop feeding birds during the breeding season? We would certainly not advise that given the evidence for a net positive effect on bird populations, and the benefits of feeding go beyond the direct effects on bird populations, but it also helps bring people closer to birds and nature. The amount of pleasure hundreds of millions of people get from feeding backyard birds is an enormous enhancement to our well-being and connection to wildlife on a global scale. Instead, we’d suggest that feeders are placed as far away as possible from potential nests sites during that key time in late spring, that we offer food which is less attractive to nest predators and more suitable for the birds we want to support, and we would still advocate the use of feeder guards, even though this didn’t reduce predation. This is simply because the use of guards means that more food goes to the target beneficiaries, rather than helping grow local populations of squirrels and magpies!

Feeder visitors

Common suburban feeder visitors in south-east England, ring-necked parakeets and grey squirrel © Mark Fellowes


References


Orros, M.E., and Fellowes, M.D.E. 2012. Supplementary feeding of wild birds indirectly affects the local abundance of arthropod prey. Basic and Applied Ecology13: 286-293. VIEW
 
Orros, M.E., and Fellowes, M.D.E. 2014. Supplementary feeding of the reintroduced Red Kite Milvus milvus in UK gardens. Bird Study 61: 260-263. VIEW
 
Orros, M.E., and Fellowes, M.D.E. 2015a. Wild bird feeding in a large UK urban area: characteristics and estimates of energy input and individuals supported. Acta Ornithologica 50: 43-58. VIEW
 
Orros, M.E., and Fellowes, M.D.E. 2015b. Widespread supplementary feeding in domestic gardens explains the return of reintroduced Red Kites Milvus milvus to an urban area. Ibis 157: 230-238. VIEW
 
Orros, M.E., Thomas, R.L., Holloway, G.J., and Fellowes, M.D.E. 2015. Supplementary feeding of wild birds indirectly affects ground beetle populations in suburban gardens. Urban Ecosystems 18: 465-475. VIEW

About the Author



Hugh Hanmer
 is a final year PhD student at the University of Reading working on urban bird ecology and conservation with Professor Mark Fellowes and Dr Rebecca Thomas. The primary focus of his PhD has been on the indirect effects of garden bird feeding though other work has taken in nest boxes and domestic cat ranging across the urbanisation gradient. In his rare spare time Hugh is a keen birdwatcher, bird ringer and kayaker

Over 90% of us live in urban areas, with most having access to a garden. Gardens cover more land area in the UK than our National Parks, and although they are made up of many small patches, they are varied and most importantly they are under our control. It is only recently that we have begun to realise that gardens are important havens for biodiversity, but how do we make sure that we provide the right helping hand to our closest wildlife neighbours?

Around half of garden owners regularly feed wild birds, and as a nation we spend over £200M each year on seed and peanuts. We provide nest-boxes for a select group of species, and this has morphed into an increasingly hi-tech form of wildlife watching, with cameras allowing us to watch our birds on our TV screens. We know that we can make a real difference to the abundance and survival of garden birds. The recent and rapid expansion of goldfinches has been a welcome result of the provision of nyger seed in specialised feeders. Blackcaps are increasingly frequent winter visitors to gardens, with research suggesting that our summer blackcaps travel south in winter, and they in turn are replaced by migrating central European birds, attracted by the predictable and abundant food provided in gardens. Nuthatches are now common visitors to gardens, where once they were scarce, and they have been joined by huge increases in woodpigeons, magpies and exotic additions to our avifauna such as the ring-necked parakeet.

Fledgling House SparrowFledgling House Sparrow
By Mark Fellows
Parakeets & Squirrel Rose-ringed Parakeets & Grey Squirrels By Mark Fellowes

But of course it is not all good news. Some of our most common garden visitors have declined at a startling rate. Within a couple of decades we’ve lost huge numbers of house sparrows, starlings and song thrushes from across the country. We do have some understanding of why this is happening, but as a nation of bird-supporting gardeners are we doing enough and are we doing the right things to help support wild bird populations? This is where SongBird Survival has stepped in, funding a new research project which we hope will begin to answer these critically important questions. I have just started my PhD research at the University of Reading, where I will work to provide insights into what we can do to help our garden birds.
Garden birds face a huge number of challenges, many of which are unique to their urban ecosystem. The most important difference is that birds in urban areas receive a relatively large volume, but limited range, of supplementary food. This favours some species, such as finches and tits, over others, such as warblers and thrushes. But supplementary feeding is surprisingly complex in its potential effect on the assemblage of species found in gardens. For example, our previous work has shown that the numbers of insects found in gardens is lower near feeders which may in turn affect nestling survival. Building on this, we speculate that feeders also provide resources for potential predators of eggs and nestlings, such as corvids, grey squirrels and great spotted woodpeckers. Just how much food these species take is not well understood. Are we inadvertently providing resources for the species who may act as significant predators of urban songbirds? If birds nest near feeders are they more likely to be predated? Just how effective are feeders which exclude such species, and does their use affect local nest predation? Each of these are important questions for urban birds and my first job will be to test this using remote cameras and artificial nests, studying how the structure and location of feeders affects nest predation rate.

But garden bird success goes far beyond this. Not only do birds need to avoid nest predation, raising chicks demands suitable food and there is thought to be a shortage of insects and other invertebrates in suburban and urban areas compared to more natural habitats. That shortage may help our garden plants, but nestlings need a diet high in protein and suitable vitamins that invertebrates provide and typical supplementary foods lack.  This particularly hurts birds that only have single broods such as blue tits and great tits, which may as a result produce fewer fledglings. Will the planting of insect-friendly vegetation or the provision of more protein-rich food such as mealworms overcome this? By using specialised equipment I will be able to study how often parent birds use feeders, and to consider the success rates of their broods.
But supplementary feeding may also play a more hidden role in the survival of our garden birds. The role of disease is poorly understood, but the gathering of birds at bird feeders could potentially increase the risk of disease spread. The spread of Trichomonosis in Greenfinches suggests just that, but there is a lack of scientific evidence to see how serious this is. This provides a final thread to my initial research. 
With SongBird Survival’s support I hope to cast new light on these issues and others to seriously improve how we look after our gardens for the betterment of wild birds in all our lives.

Squirrel of a feederGrey Squirrel by Mark Fellowes

Diseased GreenfinchDiseased Greenfinch by Keith Cowieson
Great Tits on a feederGreat Tits by Keith Cowieson
Rose Ringed ParakeetRose-ringed Parakeet by Keith Cowieson

Related Research
Parakeet on feederExperimental evidence of impacts of an invasive parakeet on foraging behaviour of native birds .... Click here to find out more
Squirrel on feederAssessing the potential for Grey Squirrels Sciurus carolinensis to compete with birds at supplementary feeding stations .... Click here to find out more

 




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SongBird Survival is a charity commissioning research into the decline of Britain’s songbirds. With your support and membership we hope to reverse their decline. 
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