Quantifying a popular but understudied human–wildlife interaction
Wild bird feeding in an urban area: intensity, economics and numbers of individuals supported. Orros M.E. and Fellowes M.D.E. 2015. Acta Ornithol. 50: 43–58. DOI: 10.3161/00016454AO2015.50.1.006. VIEW
One of the most surprising things that I discovered during my PhD research on garden bird feeding is just how little we know about this incredibly widespread activity. Various studies have shown that around half of households in the UK, Australia and North America feed wild birds, yet my supervisor (Mark Fellowes, University of Reading) and I could find no year-round quantitative data on what is being given. When I started to think about the immense numbers of gardens/backyards, birds and calories that the participation figures represent worldwide I found it hard to believe how few researchers are working in this field. I came away from the BOU Urban Birds conference earlier this year with the sense that this is beginning to change but also that there is still so much more to learn.
Figure 1 Great Spotted Woodpecker on a garden bird feeder © Mark Fellowes
In the UK the intended targets of garden bird feeding are typically small to medium sized granivorous/insectivorous passerines (although the Red Kite is a notable recent exception (read the BOU blog: Orros, 2015) and elsewhere in the world, e.g. Australia, the provision of meat is common). However, as the bird feeders amongst you will no doubt be aware, by putting out food in gardens for your ‘preferred’ species, others are also being provisioned. Prominent examples in the UK include the grey squirrel, brown rat, mice and pigeons.
Here I share the results of a study that Mark and I conducted as a first step to getting a handle on just how much energy we are adding to ecosystems in the form of food intended for, but not always taken by, wild birds. In our study area of Greater Reading, a large urban area (roughly 72 km2 and 100,000 households) lying about 30 km west of London, we set out to characterise garden bird feeding in as much detail as possible. We used two approaches, a one-off questionnaire of a representative cross-section of residents and a two-year study in which volunteers recorded the quantities of all food that they provided for wild birds (see the linked paper above for further details). These two approaches allowed us to gain both a broad picture of the extent of the activity in the study area and a detailed, quantitative view of the amounts of food being given. They also allowed us to compare the estimates of provisioning activity given by those answering our questionnaire with the actual levels recorded by those in the two-year study.
The one-off questionnaire found that 55% of Greater Reading residents feed birds, fairly similar to recent national and international estimates, with roughly 2/3 of those feeding year-round. Surprisingly, prior to our work, the most recent published seasonal data on bird feeding activity in the UK were those of Cowie & Hinsley (1988). In their study, only 40% of feeders fed in the summer months, reflecting the original status of wild bird feeding as a predominantly cold-weather activity in the UK. In the intervening period, the advice of the RSPB and British Trust for Ornithology (BTO) has shifted to recommend year-round feeding.
Moving on to our two-year study, the median amount of food provided per garden per day weighed 127 g, cost 35 UK pence and represented 628 kcal of energy (the latter values calculated using the relative proportions of different food types given). The calorific value allowed us to account for the different energy contents of foods (e.g. fresh fruit vs seed and nuts) and thus avoid over-estimating the numbers of birds supported by houses giving lots of relatively low-energy foodstuffs.
Figure 2 Blue Tit on a garden bird feeder © Mark Fellowes
We used these results to estimate the numbers of 10 common feeder-using bird species that could be supported (those most often seen feeding in gardens in the BTO’s long-running Garden Bird Feeding Survey in winter 2012/13). Over 500 million Robins and 185 million Blackbirds could be fully provisioned across the UK assuming 100% uptake by a single species alone (see paper for other species). In order to gain a picture of the mixed-species provisioning typical of domestic gardens, we also calculated the energy requirement of an ‘average’ garden bird, based on the same 10 species (weighted by the percentage of gardens in which each was seen). Just over 195 million of these birds could be supported. For comparison, the total number of breeding individuals of the 10 species was estimated as 71.1 million in 2009 (Eaton et al. 2012).
As many readers will have noted, the median level of food provided by our dedicated study participants is unlikely to represent the wider bird feeding community in the UK – after all, measuring every gram of food you put out for two years is quite a task and likely to attract only the keenest feeders/committed volunteers. We therefore estimated support using the data for the study household providing the least food (who fed once a month or less). Even at this level, over 31 million of our ‘average’ bird individuals could be supported. Looking to other taxa, just 10% of the resources provided by this lower estimate would feed ~1 million grey squirrels.
It should of course be remembered that bird (and other) species, and individuals within species, differ greatly in the extent to which they use gardens and/or the food resources within them, and so this ‘support’ is not evenly distributed. Thus, whether or not the end consumers are the intended targets, they are receiving considerable additional energy and are thus being directly influenced in ways that species that do not take supplementary food, or do so to a lesser extent, are not.
I stress the approximate nature of our estimates – they are intended only to give some indication of the level of support deliberately made available to wild animals (target and non-target) by domestic households. However, they are a useful step towards more precise estimates at regional and international scales that we feel are necessary in order to investigate further the implications of garden bird feeding.
Figure 3 Nuthatch on a garden bird feeder
© Mark Fellowes
I mentioned above that our two study methods allowed us to compare bird-feeding activities between the two sets of participants. This provided, in my opinion, the most surprising and unexpected result of our study. The participants in the one-off questionnaire can be taken as representing the wider bird-feeding community, as we spoke to a representative cross-section of Greater Reading residents who were unaware of the questionnaire topic in advance. Conversely, as mentioned above, our dedicated citizen scientist volunteers who took the time to measure bird food for so long are more likely to represent ‘keen’ bird feeders, an assumption supported by the greater number of different types of foods that they provided compared with the questionnaire participants. Yet, when we compared the frequency at which these two groups fed birds, we found significantly less frequent feeding in the ‘keen’ group, suggesting that one-off questionnaires may over-estimate feeding frequency and thus sounding a note of caution in the use of such methods to assess provisioning levels.
Overall I hope that, by providing some preliminary numbers, our study highlights just how vast the quantity of energy being added to garden ecosystems via wild bird feeding is, and demonstrates the need for further research into the consequences. This is particularly important in the light of recent evidence of negative effects on birds, including reduced productivity and impaired egg production (e.g. Harrison et al. 2010, Plummer et al. 2013a, b) and indirect negative effects on other taxa in the form of depletion of arthropod prey around garden feeders (Orros & Fellowes 2012, Orros et al. 2015).
Eaton M.A., Cuthbert R., Dunn E., Grice P.V., et al. 2012. The state of the UK’s birds 2012. RSPB, BTO, WWT, CCW, NE, NlEA, SNH, JNCC. View
Cowie R.J., Hinsley S.A. 1988. The provision of food and the use of bird feeders in suburban gardens. Bird Study 35: 163–168. View
Harrison T., Smith J., Martin G., Chamberlain D., Bearhop S., Robb G., Reynolds S. 2010. Does food supplementation really enhance productivity of breeding birds? Oecologia 164: 311–320. View
Orros, M.E. 2015. A twist in the tale: Red Kites are back in town. BOU Blog, 28 Jan 2015. View
Orros M.E., Fellowes M.D.E. 2012. Supplementary feeding of wild birds indirectly affects the local abundance of arthropod prey. Basic Appl. Ecol. 13: 286–293. View
Orros M.E., Thomas R.L., Holloway G.J., Fellowes M.D.E. 2015. Supplementary feeding of wild birds indirectly affects ground beetle populations in suburban gardens. Urb. Ecosyst. 18: 465–475. View
Plummer K.E., Bearhop S., Leech D.I., Chamberlain D.E., Blount J.D. 2013a. Fat provisioning in winter impairs egg production during the following spring: a landscape-scale study of blue tits. J. Anim. Ecol. 82: 673–682. View
Plummer K.E., Bearhop S., Leech D.I., Chamberlain D.E., Blount J.D. 2013b. Winter food provisioning reduces future breeding performance in a wild bird. Sci. Reports 3: 2002; DOI:10.1038/srep02002. View
Featured image: Goldfinches (Carduelis carduelis) © Jans Canon | CC-BY-2.0 | Wikimedia Commons
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