People putting out food on bird tables and artificial feeders is a common occurrence in the UK. It gives us a chance to engage with the natural world and see garden birds up close. The food however is not just being eaten by our beloved garden birds, it is also affecting their behaviour.
In the past feeding birds used to involve throwing out some left over bread or taking it to the pond to feed the ducks, but now there is a huge amount of bird feed available, from seed to invertebrates, which you can feed to your garden birds. Around half of householders put out supplementary food for garden birds, which is around 50-60 thousand tonnes of food every year (BTO, 2011). The presence of this abundant food source is likely to affect several aspects of bird behaviour, including reproduction and distribution (Robb et al, 2008). For example the rise of the red kites around urban areas is a well-known phenomenon and a study by Orros and Fellowes (2015), on red kites in Reading, suggested that the food residents provided in their gardens, specifically for these birds of prey, was a key reason for their presence in these areas during the daytime.
Although the immediate thought is that an abundance of food may surely mean increased survival rates, some species can be affected in different ways. A study of great tits and blue tits showed that the presence of supplementary food influenced their breeding behaviour, reducing brood size and clutch size (Harrison et al, 2010). Some negative influences of providing bird food on feeding stations include the threat of disease transmission, dependency issues and the problem of the intermittency of the food resource.
The presence of artificial food has also been linked to a change in the migration of blackcaps. Blackcaps from areas of central Europe have historically migrated to the Mediterranean, to areas of southern Spain, during the winter however many now fly to Britain for the colder months of the year. A study by Plummer et al (2015), suggested the reliable garden food supply provided by many British households, as well as the weather conditions (the milder winters) has helped to cause this change in migration pattern. It has even been suggested that there are physical differences noted in those birds wintering in Britain compared to Spain, as they have beaks which are longer and thinner (BTO, no date). The fact that food provided in the garden by humans has potentially influenced migration on an extensive scale is rather remarkable. It is an important reminder however that our actions have consequences for the natural world, whether positive or negative and whether done intentionally or not.
BTO. (2011) Garden feeders support more birds than ever [www document]. http://www.bto.org/news-events/press-releases/garden-feeders-support-more-birds-ever
BTO. (no date) Garden bird feeding and a changing climate are driving evolutionary change in Blackcaps [www document]. http://www.bto.org/science/latest-research/garden-bird-feeding-and-changing-climate-are-driving-evolutionary-change-bla
Harrison, T., Smith, J., Martin, G., Chamberlain, D., Bearhop, S., Robb, G. and Reynolds, J. (2010) ‘Does food supplementation really enhance productivity of breeding birds?’,Oecologica, 164, 311-320.
Orros, M. and Fellowes, M. (2015) ‘Widespread supplementary feeding in domestic gardens explains the return of reintroduced red kites Milvus milvus to an urban area’, Ibis, 157, 230-238.
Plummer, K., Siriwardena, G., Conway, G., Risley, K. and Toms, M. (2015) ‘Is supplementary feeding in gardens a driver of evolutionary change in a migratory bird species?’, Global Change Biology, DOI: 10.1111/gcb.13070.
Robb, G., McDonald, R., Chamberlain, D. and Bearhop, S. (2008) ‘Food for thought: Supplementary feeding as a driver of ecological change in avian populations’, Frontiers in Ecology and the Environment, 6, 476-484.