The British Trust for Ornithology (BTO) estimates that the UK spends £200 million annually on garden bird feeding, a commitment that survived financial shocks. Since the late 1990s, the volume of wild bird food provided is thought to have doubled. Feed suppliers have developed diverse products that cater for varied dietary needs and the 18 species recorded feeding on supplementary food in gardens in 1987 has climbed to 130, according to BTO survey data. In North America, 50 million people feed birds over one million tons of seed annually. As much as 80% of Australians are involved in some kind of bird feeding.

When I came across this summary of three studies of the effects of fat-based supplementary foods on breeding success I became concerned about the possibility that my own bird feeding could be doing more harm than good. I undertook an informal review of what evidence I could easily find for the impacts of feeding. Given the scale of this human intervention, it seems impossible that populations would not be affected in some way. Trying to offer our avian visitors some kind of advantage is, after all, one of the primary motivations for feeding in the first place. Another is that we enjoy contact with wild birds and I was keen to explore how best to continue this while maximising the benefits to my guests.

Effects of feeding

Body condition and survival

The good

Perhaps the main reason for bird feeding is the hope that it improves their condition and helps them to survive, particularly through the winter. As expected, there is good evidence that provisioned birds can have higher feather growth rates, body mass, cholesterol levels (if fed appropriate foods), antioxidant levels, and reduced stress and better immune defence. An aviary-based study on sparrows demonstrates the complexity of the feeding issue, with urban birds having a consistently lower body mass than their rural counterparts under the same feeding regime. Several studies have suggested that beneficial effects disappear in the medium- to long-term following removal of feeders but that health does not crash, implying that the birds do not become dependent upon feeders for their primary nourishment.

There is also evidence that these physiological differences can indeed translate into enhanced survival. A review of bird productivity in urban environments reported that human-provided food can help blackbirds to survive over winter where natural foods are relatively scarce. In another study, black-capped chickadees with access to winter food had over-winter survival rates of 69%, compared to 37% for birds at unfed control sites. The latter was one of six cases reviewed in 1990 that all showed improved survival rates of fed birds. Some caution is required in interpreting these results due to the difficulties in distinguishing death from emigration in studies that do not not follow individuals (for example by radio tracking).

 The bad

Bringing birds into close proximity, where they deposit droppings on foodstuffs and exchange bacteria on feeders, clearly raises potential infection risks. This has been quantified, with 8.3% of birds at feeders showing sings of illness in one study, although this situation can be improved by good practices such as regular cleaning and providing a number of feeders to reduce bird densities.

In addition, high concentrations of birds attract wild and domestic predators. The Mammal Society estimates that UK cats kill up to 275 million animals annually, including 55 million birds. Although there is no evidence that this mortality impacts bird population levels, it seems somewhat unfair to attract them without taking reasonable steps to reduce the risks through some basic preventative measures such as thoughtful placement of feeders. Similarly, badly positioned feeding stations can increase the risk of window strikes.

Reproductive success (productivity) and distribution

It has been known for some time that food supplementation just before or during the breeding period can lead to earlier egg-laying. In one review, 58% of 59 studies found that feeding led to significantly earlier laying dates. There are two potential explanations — that females’ ability to produce eggs is food-constrained, or that they use food availability as a a cue for the inset of breeding. However, most feeding by humans occurs in winter, outside the breeding season, due to the conventional wisdom that in spring and summer birds should “look after themselves”.

For winter feeding to impact breeding biology, there would need to be a “carry-over effect” — where past events influence current condition (or zoological “fitness”). These effects are often observed in migratory birds but are less well known in resident species. A controlled experiment with blue tits in woodlands in Northern Ireland found that birds fed over winter laid eggs 2.5 days earlier and fledged one additional chick, on average, than their unfed counterparts in control groups. This outcome is consistent with the evidence for improved body condition as a result of supplementary feeding.

Great spotted woodpeckers were observed to lay eggs 4-5 days earlier and were twice as productive when fed before (but not during) the breeding season. During the study year the spring was particularly warm, with caterpillar numbers reaching their maximum very early. Although this meant that even fed birds missed “peak caterpillar”, their early breeding gave them an advantage in this case. In the review referred to above, an increase in at least one variable of clutch size, hatching success, chick growth rate or fledging success due to supplementary was found in 44% to 64% of the 59 studies, with no significant effects on one of the measures in 36% to 55% of cases and negative impacts reported in only three instances.

However, in other cases feeding effects could be too good — early laying may cause young to be in the nest before peak wild food abundance, causing decreased survival of both adults and nestlings. In addition, there is concern that subsidising a high density resident population to breed earlier and, potentially, produce more offspring could lead to increased competition for returning migrants.

A study in New Zealand demonstrated how feeding can assist invasive species to edge out natives — in gardens with feeders there were fewer species and house sparrows (native to Europe and Asia) and spotted doves (native to Asia) were 2.5 and 3.5 times more common, respectively, than in non-fed gardens.

In order to test a simple prediction that if feeding is harmful, the species fed the most should be doing the worst (with all the caveats that go with this logical conjecture — this was not a controlled experimental approach), the Cornell Lab of Ornithology analysed 30 years of data and found that the US species most commonly associated with feeders tended to be doing just as well as, or better than, species that are rarer visitors. For example, red-bellied woodpeckers visit feeders regularly and are thriving, and northern cardinals, another feeder regular, has a growing population and an expanding range. However, pinyon jays visit feeders more sporadically and are showing declines. The feeder species that showed declines seem to be faced with non-feeder–re­lated pressures, such as habitat loss. This study did not consider the possibility that fed species were being assisted to out-compete non-fed birds.

According to evidence from the BTO, the goldfinch was in long-term decline but with the introduction of sunflower hearts and nyjer seed to bird feeders, the population has been steadily increasing. Farmland birds, such as reed buntings and yellowhammers, which have been hit by the change to autumn sowing, removing winter stubbles and a valuable source of seeds, also move into gardens in the winter and early spring to take advantage of garden bird feeders.

Two UK studies examined the specific effects of feeding fat-based foods to tits. One found that both blue and great tits that were fed beef suet and ground peanuts during the breeding season had earlier laying dates but lower brood sizes, by half a chick on average, than their unfed counterparts. Another established that blue tits supplemented with fat alone produced eggs that were lower in nutrients and yolk mass. This effect was not observed in birds fed fat fortified with Vitamin E, underlining the importance of the quality of supplemental food and a “balanced diet”.

As with most research on this topic, both studies were performed in natural woodland, where birds have access to abundant, nutritious wild food. Birds in gardens are logistically harder to study and less is known about effects of supplementary feeding in poorer habitat. In addition, the mechanism for the poorer productivity observed in the first study is unknown. It is possible that, rather than the feeding having been “bad” for the birds, it allowed poorer quality individuals to survive to breed, thus reducing the average productivity rate.

In his seminal Population Limitation in Birds, Ian Newton reviewed experimental manipulations of food supply (again, in natural habitats) and found that in 15 cases subsequent breeding density increased by a factor ranging from 1.2 to 2.4. In a further 11 cases, there was no clear increase in bird numbers. Again, there several possible mechanisms for observed population increases — increasing survival and/or immigration, reducing emigration, and affecting territorial behaviour so that male territories are smaller and more densely arranged — and this issue is poorly understood.

Winter feeding seems to affect breeding numbers most when natural food supply is poor. A study in Sheffield concluded that there were more birds where feeding households were more numerous but there was no apparent effect upon the number of species. However, in an individual garden, bird feeding usually increases both the range of species and number of individuals present. These studies were correlational, again highlighting the difficulties of manipulative experimentation in urban areas.

Song and territorial behaviour

This is another area in which the evidence is mixed and complex. Several species have been observed to devote more time to signing and territorial defence when food supplemented, suggesting that the additional food allows them to meet their requirements more quickly, allowing them to invest energy in other activities. However, other research has recorded that feeding can attract competitors, causing males to spend more time chasing and attacking their rivals, but not in the singing or display activities needed to attract a mate. Other species sang less frequently and later when fed. Our understanding is further complicated by the fact that many species form and consolidate pairs during the winter and, therefore, any changes to male singing and territorial behaviour due to feeding during this period could potentially have impacts on breeding performance.

Evolution

Bird species are thought to have changed their behaviour, and even their morphology, in response to the widespread availability of supplementary food sources. This relationship is not straightforward and other factors are influential — climate change, for instance, is already driving new dynamics in bird populations.

Blackcaps that winter in the UK are not only adapting their feeding habits to exploit food (especially fats and sunflower hearts) supplied by humans, but are becoming physically different to their counterparts that overwinter in Spain, with longer and narrower beaks associated with a more generalist diet. Similarly, British great tits with slightly longer beaks are thought to be gaining an advantage in accessing garden feeders.

Bird feeding tips

It is surprising how poorly understood the effects of supplementary bird feeding are, particularly in the urban areas where most human feeders live. However, there are some general conclusions that can inform how we provide food to wild birds.

The quality of the food offered is very important. Some estimates suggest that human-provided bird food account for around 25% of regular feeder visitors’ diet. Others argue that the proportion is greater, although most agree that total dependence is not an issue. However, it stands to reason that even if as little as a quarter of a bird’s food intake is nutritionally poor, this will have serious consequences for the individual. Using the best available commercial seed products (not cheap supermarket offerings), suet and certified toxin-free peanuts, for example, and avoiding bread, processed foods and cooked meats should maximise the chances of providing a safe, balanced diet.

Another major point to consider is shifting the balance from artificial feeding to making a garden more naturally bird-friendly, for example through planting native seeding grasses and fruit/berry bearing trees and shrubs, mulching and leaving leaves to attract insects and small mammals, introducing butterfly food plants (resulting in more caterpillars for birds as well as adult butterflies to enjoy), providing “bug hotel” type structures and creating water features. The evidence shows that natural food is always better than artificial supplements, and improved habitat reduces dependence upon individual people to replenish, clean and monitor food supplies.

Other advice that emerges from current knowledge includes:

• Provide water as well as food. This can range from a pet dish, a vessel large enough for bathing or even a wildlife pond
• Remove netting from fat balls and other foods before offering them to birds (also see note below) and avoid badly designed feeders, both of which can potentially trap birds
• Keep feeders, water dishes, baths and the area around feeding stations clean in order to avoid disease transmission and attracting rats and other vectors of pathogens
• Site them near cover and well off the ground to reduce risk from predators, including cats. Take measures to discourage cats from visiting the feeding area
• Keep them away from windows and use stickers to reveal large areas of glass to birds in their vicinity
• August and September, when juveniles are most numerous, is a beneficial time to feed

Note: the video at the end of this post depicts food offered in green netting at my own feeding station. Some manufacturers have withdrawn such products from sale due to incidences of birds trapping their feet and beaks. Although this has not occurred on my table the RSPB reports a number of such incidents annually and I now remove the bags as a precaution.

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Some video footage of my own bird table in late summer 2017, when most of the visitors were juvenile blue tits and great tits. Please note the potential issue with green netting, as described above. I now remove the balls from the bags before putting them out.