Feeding birds is the most common way that people interact with wildlife in Britain. Up to two-thirds of all households do it, spending a total £250m on 150,000 tonnes of birdfood annually, mostly seeds (such as sunflower), peanuts and fats in various forms. Estimates suggest there are an average of 100 bird feeders per square kilometre in Britain or, to put it another way, one feeder for every nine feeder-using birds, such as Blue Tits Cyanistes caeruleus, Great Tits Parus major, House Sparrows Passer domesticus and GreenfinchesChloris chloris (Davies et al. 2009). Collectively, we provide enough food to feed the entire populations of the ten commonest feeder-using species all year round, three times over, even if they ate nothing else.
Those species that are best able to exploit this food bonanza can benefit from increased survival rates, breeding success and breeding densities. Consequently, species such as Blue Tit, Great Tit and even Great Spotted Woodpecker Dendrocopos major have increased their populations in no small part due to the growth of bird feeding over the past 50 years (Plummer et al. 2019; Woodward et al. 2020). Bird feeding is also beneficial to us humans, by improving our mental wellbeing, lowering stress and giving us a greater connection with nature. Indeed, bird feeding has become so important as a cultural phenomenon that it is developing into its own branch of ornithological study (Toms 2019).
Increasingly, however, there are also concerns over the negative impacts of bird feeding operating on such a vast scale. Mike Toms’ (2019) comprehensive account of our relationship with garden birds lists the grim toll of disease epidemics that continue to affect many species, with most of the disease transmission happening around bird feeders. A well-known example is the Greenfinch, whose abundance rocketed during the 1990s as they exploited garden bird food, only to dramatically collapse from 2005 onwards as they were infected by the protozoan parasite Trichomonas gallinae (which causes the disease trichomonosis) while congregating at feeders.
In addition to the direct effects on birds that use feeders, there is also growing interest in the knock-on effects that bird feeding is likely to have on the wider bird community. It would be surprising if such a colossal input of food resources into the environment did not have impacts that ripple throughout ecosystems.
One important indirect effect could be increased competition for species that do not commonly use feeders themselves but co-exist alongside those that benefit from bird feeding. Since the 1960s, abundance of Blue Tits and Great Tits in Britain has increased by a respective 24% and 89%, and they reach higher densities in our woodlands than almost anywhere else in Europe (Cramp & Perrins 1993). Britain is a paradise for these two species, where we not only provide them with vast amounts of easy food but also with millions of nestboxes. What does this mean for the other species that share their habitat with these subsidised birds?
As woodland generalists, Blue Tits and Great Tits cross habitat boundaries with ease, moving between woodland, villages and suburbs to access garden bird feeders. A recent study by Shutt et al. (2021) found peanuts to be the most common food item in the diet of Blue Tits in Scotland, being present in the diet of birds as far as 1.4 km from the nearest garden. It is reasonable to question whether boosting the abundance of these generalists by year-round feeding has skewed the wider bird community, especially in the woodlands where many tits return to breed in spring.
Since the 1970s, there have been dramatic declines of some woodland specialists, notably Marsh Tits Poecile palustris and Willow Tits P. montanus, whose populations have collapsed by a respective 78% and 92% (Woodward et al. 2020). Research into these declines has not yet found definitive explanations, but much has been learnt about their ecology and the problems they face, which have been summarised in Lewis et al.(2009) and Broughton & Hinsley (2015).
From these studies, it is clear that habitat change alone cannot account for the declines of Marsh Tits and Willow Tits. Nationally, Marsh Tits should be benefiting from the increasing maturity of much of Britain’s existing woodland, although Willow Tits have likely lost out from this. Nevertheless, the expansion of new woodland and wetlands over recent decades should have created new habitats for Willow Tits (Broughton et al. 2021). Yet, in many areas where the habitat seems to have changed very little, or even improved for them, both species have still declined or gone extinct.
What could have changed across such a large scale to push Marsh Tits and Willow Tits into their current habitat refuges? Increased competition could be an important factor. As well as being far more numerous, Blue Tits and Great Tits are socially dominant over Marsh Tits and Willow Tits, so they tend to win any interactions where their niches overlap. That could mean dominating natural food resources and safe foraging areas, or taking over nest cavities and causing breeding failure.
Countering the dominance of Blue Tits and Great Tits, both Marsh Tits and Willow Tits have a survival strategy of establishing a large year-round territory that contains all the resources that a pair needs to support them throughout their life. This means that they naturally occur at much lower densities than Blue Tits or Great Tits. They underpin this strategy by hoarding natural food within their territories during the autumn and winter, which gives them an advantage during lean times. Non-hoarding Blue Tits and Great Tits have no natural buffer of stored food and are not tied to a fixed territory, and so they rely on what they can find as they forage each day (Perrins 1979).
Willow Tits have another tactical advantage of being able to excavate their own nest cavities in deadwood. This trait enables them to live and breed in young woodland and scrub before other tits that need pre-existing nest-holes, giving them a unique niche where they should be able to avoid much competition.
These strategies all work when there is a level playing field for the woodland bird community. If all of the tits are exposed to the same natural food availability, then harder winters should favour the strategic Marsh Tits and Willow Tits that stay put and hoard food, and milder winters favour the dominant and flexible Blue Tits and Great Tits; but what if bird feeding removes these natural checks and balances, by providing Blue Tits and Great Tits with a reliable food source so that they never have a lean year or a hard winter?
This is where questions arise as to whether supplementary bird feeding is having an indirect impact on specialists like Marsh Tits and Willow Tits through increased competition from dominant, generalist tits. This competition hypothesis is not new but has been explored more deeply in the recent paper by Shutt & Lees (2021). The hypothesis, linked to bird feeding, is centred on increased populations of Blue Tits and Great Tits in the woodlands and spilling over into more marginal habitats, where they are now common or abundant almost everywhere.
In theory, widespread bird feeding releases Blue Tits and Great Tits from the effects of food scarcity and bad winters, and Marsh Tits and Willow Tits then lose the advantage of staying in their territories and food-hoarding. Instead, the increasing numbers of Blue Tits and Great Tits in the woods can dominate the natural food resources. A neat study in Wytham Woods (Farine et al. 2015) showed that Marsh Tits, with their deep knowledge of their territory, are usually the first to discover a new food source, but Blue Tits and Great Tits soon exploit these discoveries and may ‘parasitise’ the Marsh Tits’ knowledge.
When the natural food dwindles in the woods, Blue Tits and Great Tits can simply leave to exploit bird feeders. That would leave Marsh Tits and Willow Tits stranded in their territories with depleted food. Even if there is access to a nearby feeder, Blue Tits and Great Tits still have a dominant advantage. As a basic ecological hypothesis, the competition effect is quite a strong one.
However, demonstrating that competition is happening is notoriously difficult. The interactions are difficult to observe and hard to unravel from local habitat quality, time lags and annual effects. Broad-scale analyses have not supported the hypothesis of generalist Blue Tits and Great Tits outcompeting Marsh Tits and Willow Tits (Siriwardena 2004, 2006; see also Lewis et al. 2009, Broughton & Hinsley 2015). That is not completely surprising, as broad analyses can smooth out regional effects of stable versus declining populations, or can miss key co-variables. Competition may not even be a main driver of population declines, but it may add additional pressure that makes a difference.
Is there any evidence at all of competition effects on Marsh Tits and Willow Tits? Detailed population studies do provide some suggestion, including a 20-year study of Marsh Tits in Cambridgeshire that monitors a once-thriving, but now much reduced, population (Broughton & Hinsley 2015). The main mechanism of this decline appears to be greater juvenile mortality in summer, leading to plummeting recruitment and a downward spiral as territories are left vacant. There is no sign of reduced breeding success or disastrous habitat changes in this population, so could competition be specific enough to eliminate the juveniles?
The answer may lie in the dominance hierarchy among the woodland tits. Juvenile Marsh Tits are near the very bottom of the social pecking order, dominated by adult Marsh Tits and all of the many Blue Tits and Great Tits that forage in the woods after breeding (Perrins 1979). The adult Marsh Tits are more experienced, but increased mortality of the subordinate juveniles suggests that competition has increased, and the young birds can no longer cope. More research is needed on this topic, but only through detailed population studies are these demographic changes becoming apparent.
For Willow Tits, studies show a clear impact of local competition. The increased numbers of Blue Tits and Great Tits spilling over into marginal habitats, such as young woodland, are desperately looking for nest sites in spring. Willow Tits can spend a week excavating a nest cavity, only for dominant Blue Tits or Great Tits to commandeer it. This is one of the main causes of nest loss, affecting almost one in five Willow Tit nests in northern England and 67% of nests in a Scottish population, which has since gone extinct (see Parry & Broughton 2018). Some Willow Tits lost up to four successive nest excavations to Blue Tits or Great Tits in a single season, and probably failed to breed entirely.
To make matters worse, the other major cause of Willow Tit breeding failure is nest predation by Great Spotted Woodpeckers. This species also benefits from bird feeding and has increased across all woody habitats by 387% overall since the 1960s. Consequently, Willow Tits have by far the highest nest failure rates of all the tits, ranging between 30% and 67% in northern England. Marsh Tits fare better, with failure rates of under 20% due to nesting in safer cavities in living trees and being better able to fend off Blue Tits.
What is an appropriate response to the negative effects of bird feeding when uncertainty remains over its true impact? The stakes are high, including potential extinction of Britain’s endemic subspecies of Willow Tit P. m. kleinschmidti, but this must be balanced against the immense enjoyment that people get from bird feeding, not to mention the importance of a multi-million-pound industry that has some commercial relationships with conservation charities.
Nevertheless, bird feeding in Britain has become a vast experiment with our wildlife, where we input huge food resources without fully understanding the consequences. Bird feeding has expanded beyond gardens, and there are now permanent, year-round feeding stations on many nature reserves. Increasingly, feeding stations are even appearing in public woodlands, stocked by local residents or photographers. With such interventions also comes responsibility, whether that is to avoid spreading diseases or not to contribute to the myriad pressures on declining species.
The precautionary principle is a central tenet of nature conservation strategies (www.iucn.org/files/2005-precautionary-principle-guidelines) and warns that delaying action until there is compelling evidence of harm will often mean it becomes too late to avert the threat. We suggest that there is now enough provisional evidence and theoretical basis to consider the precautionary principle with regard to bird feeding where it will largely benefit generalist, abundant Blue Tits and Great Tits at the likely expense of specialist, declining Marsh Tits and Willow Tits.
We are not proposing that bird feeding should end, as that would be unrealistic and unnecessary. However, we do suggest that, where possible, regular bird feeding should be cautiously reconsidered, reduced or discouraged around the remaining core habitats of Marsh Tits and Willow Tits, in light of their perilous population declines. The same may apply to providing nestboxes for Blue Tits and Great Tits in sensitive areas, unless they are for specific studies, as nestboxes increase the densities of the common tits and can help them to colonise habitats with few natural holes (Perrins 1979). With just 8% of our Willow Tits remaining, and only 22% of our Marsh Tits, assisting their main competitors or predators around the remaining populations seems counter-productive.
Although many people will be unwilling to reduce bird feeding, others can make informed choices on the necessity and impact around sensitive habitats. If a modest proportion of us stopped or reduced direct feeding, and instead, for example, donated the money to conservation organisations for restoring habitats, that could be a positive change. The farmland and carbon used for bird-food production and transportation could be spared and more species could benefit from habitat creation, while those of us with access to nature outdoors could continue enjoying wildlife. People with limited mobility or greater need could continue to enjoy bird feeding, while the overall impact would be reduced.
Even switching the foods used for bird feeding could be beneficial, by avoiding sunflowers, peanuts and fats that are favoured by the common tits and, instead, providing millet or cereals that are attractive to declining House Sparrows, Tree Sparrows Passer montanus and other farmland birds.
If reduced bird feeding resulted in a moderate fall in the numbers of Blue Tits, Great Tits or, indeed, Great Spotted Woodpeckers, this would not be a conservation disaster. These species would remain extremely common and would merely fall back to prior densities that are perhaps more typical of elsewhere in Europe. If we can avoid inflating the numbers of competitors and predators by rethinking our bird-feeding habits, that could give British Marsh Tits and Willow Tits a better chance of surviving the biodiversity crisis.
Broughton, R. K., & Hinsley, S. A. 2015. The ecology and conservation of the Marsh Tit in Britain. Brit. Birds108: 12–28.
—, Parry, W., & Maziarz, M. 2021. Wilding of a post-industrial site provides a habitat refuge for an endangered woodland songbird, the British Willow Tit Poecile montanus kleinschmidti. Bird Study 67: 269–278.
Cramp, S., & Perrins, C. M. (eds.) 1993. The Birds of the Western Palearctic. Vol. 3. OUP, Oxford.
Davies, Z. G., Fuller, R. A., Loram, A., Irvine, K. N., Sims, V., & Gaston, K. J. 2009. A national scale inventory of resource provision for biodiversity within domestic gardens. Biol. Conserv. 142: 761–771.
Farine, D. R., Aplin, L. M., Sheldon, B. C., & Hoppitt, W. 2015. Interspecific social networks promote information transmission in wild songbirds. Proc. Royal Soc. B 282: 20142804.
Lewis, A. J. G., Amar, A., Charman, E. C., & Stewart, F. R. P. 2009. The decline of the Willow Tit in Britain. Brit. Birds 102: 386–393.
Parry, W., & Broughton, R. K. 2018. Nesting behaviour and breeding success of Willow Tits Poecile montana in north-west England. Ringing & Migration 33: 75–85.
Perrins, C. 1979. British Tits. Collins, London.
Plummer, K. E., Risely, K., Toms, M. P., & Siriwardena, G. M. 2019. The composition of British bird communities is associated with long-term garden bird feeding. Nature Communications 10: 2088.
Shutt, J. D., & Lees, A. C. 2021. Killing with kindness: does widespread generalised provisioning of wildlife help or hinder biodiversity conservation efforts? Bio. Conserv. 261: 109295.
—, Trivedi, U. H., & Nicholls, J. A. 2021. Faecal metabarcoding reveals pervasive long-distance impacts of garden bird feeding. Proc. Royal Soc. B 288: 20210480.
Siriwardena, G. M. 2004. Possible roles of habitat, competition and avian nest predation in the decline of the Willow Tit Parus montanus in Britain. Bird Study 51: 193–202.
— 2006. Avian nest predation, competition and the decline of British Marsh Tits Parus palustris. Ibis 148: 255–265.
Toms, M. 2019. Garden Birds. William Collins, Glasgow.
Woodward, I. D., et al. 2020. BirdTrends 2020: trends in numbers, breeding success and survival for UK breeding birds. BTO Research Report 732. BTO, Thetford.
Richard K. Broughton, UK Centre for Ecology & Hydrology, Benson Lane, Wallingford OX10 8BB; e-mail[email protected]
Jack D. Shutt and Alexander C. Lees, Division of Biology and Conservation Ecology, Department of Natural Sciences, Manchester Metropolitan University, Manchester, M1 5GD; e-mail [email protected] and [email protected]
Richard Broughton is an ecologist at the UK Centre for Ecology & Hydrology in Wallingford, and a senior research associate at the University of Oxford’s Department of Zoology, with a special interest in woodland birds
Jack Shutt is a research associate at Manchester Metropolitan University interested in the responses of forest birds to human pressures around the world, from the Amazon to his back garden, and helping to find conservation solutions.
Alexander Lees is a Senior Lecturer in conservation biology at Manchester Metropolitan University and works primarily on avian responses to global change, especially in the Neotropics.