From the BTO

Although many of us provide food for garden birds, especially in winter, we are still in the early stages of understanding how this might affect wild bird populations. One possibility is that winter food enhances birds’ ability to invest in future reproduction. However, it is likely that the exact nutrition a bird receives from supplementary food will be important and a new study has shown exactly that.

In a three year study of woodland blue tit populations, researchers examined the consequences of providing different winter food supplements for egg production. Their results showed that providing fat alone resulted in reduced egg quality in early breeders. This suggests that females which utilised a fat-rich diet in winter were less able to acquire some of the important resources needed to form yolk during egg production. However, the addition of vitamin E to the fat mitigated these affects because, as an antioxidant, vitamin E provides protection after eating fatty foods.

As urban land cover expands, gardens are expected to play an increasingly important role in the conservation of biodiversity. The provision of food for garden birds has been thought likely to benefit this objective although there is limited evidence of its ecological impacts. More generally, food provision has also been applied as a conservation tool to manage endangered populations. This study is the first to suggest that there may be problems. Yet at the population level, these were mitigated by the provision of fat together with vitamin E. Therefore, care must be given to the nutritional composition of foods. Whether winter food for garden birds is considered to be beneficial or harmful may depend on whether effects are interpreted at the level of individuals or populations. If provisioning enables certain low-quality individuals to breed, when they might otherwise have died or survived only as non-breeders, this would clearly enhance their lifetime reproduction and may in fact boost the overall population size. Further work is needed to see how winter feeding may be used to benefit wild bird populations in the future.

These findings suggest that birds require a balanced diet, much like we do, to aid their reproduction, and it underlines the importance of considering the nutritional value of provisioned foods.

Read the abstract from the study here