476 www.frontiersinecology.org © The Ecological Society of America T he popularity of bird feeding has increased rapidly in the past few decades. Up to 43% of households in the US regularly feed birds (Martinson and Flashpoler 2003), while in the UK, this figure is close to 75% (Cowie and Hinsely 1988). Food availability is clearly one of the main factors limiting bird populations, and supplementary feed- ing reduces the risk of starvation and may enhance repro- ductive performance (Newton 1998). Despite the impres- sive scale of bird feeding, understanding of the ecological effects of these massive subsidies is minimal. While bird feeding can bring positive benefits, such as increased over- winter survival and enhanced breeding success, there are also a number of potential negative impacts. For example, aggregations of birds around feeders may attract predators or enhance the spread of diseases. Feeders may also act as ecological traps, by providing inaccurate cues regarding habitat quality based on potential food resources. Experimental studies investigating the potential impacts of supplementary feeding have dealt with a range of species, from small passerines (Figure 1) to birds of prey. Directed feeding experiments do not reflect the large-scale, diffuse nature of backyard feeding, but may provide some indication of the wider impacts. The form of supplementary feeding varies markedly among studies, replicates are few, and treat- ments often run consecutively, sometimes as single-site, before-and-after comparisons. As a result, meta-analysis of this disparate body of research is challenging. To focus advice and future research, we have drawn together and reviewed the many and varied responses of bird populations to supple- mentary feeding (Table 1). We begin with the effects on avian demography, starting with egg laying, and then progress through the avian life cycle to effects on adult survival. Finally, we consider the implications for adult behavior, species interactions, and indirect, community-level effects.  Egg laying The acute need for energy during egg development and laying means that supplementary feeding is likely to affect REVIEWS REVIEWS REVIEWS Food for thought: supplementary feeding as a driver of ecological change in avian populations Gillian N Robb 1 , Robbie A McDonald 2 , Dan E Chamberlain 3 , and Stuart Bearhop 1,4* Every year, millions of households provide huge quantities of supplementary food to wild birds. While alter- ation of the natural dynamics of food supply represents a major intervention in avian ecology, we have a remarkably limited understanding of the impacts of this widespread pastime. Here, we examine the many and varied responses of birds to supplementary feeding at backyard feeders – in large-scale management projects and in focused academic studies – and evaluate population responses to the bird-feeding phenomenon. Our review encompasses a wide range of species, from songbirds to raptors, and compares provisioning with a vari- ety of foods, at different times of year and in different locations. We consider positive impacts, such as aiding species conservation programs, and negative ones, such as increased risk of disease transmission. It seems highly likely that natural selection is being artificially perturbed, as feeding influences almost every aspect of bird ecology, including reproduction, behavior, demography, and distribution. As the effects of bird feeding cascade through ecosystems and interact with processes of environmental change, we suggest areas for future research and highlight the need for large-scale experiments, with a particular focus on the backyards of an increasingly urban and generous, but sometimes fickle, human population. Front Ecol Environ 2008; 6(9): 476– 484, doi:10.1890/060152 In a nutshell: • Feeding birds is an enormously popular activity that can affect virtually every aspect of bird ecology, from daily sur- vival to large-scale migration • Supplementary feeding has the potential to effect long-term changes in the population dynamics and distribution of some bird species • Although feeding birds generates mostly positive effects, some negative impacts, such as increased predation pressure and disease transmission, have also been observed • Research is required on the wider impacts of feeding and on the interactions between food supply and factors such as cli- mate and predation 1 School of Biological Sciences, Queen’s University Belfast, Belfast, UK; 2 Central Science Laboratory, Sand Hutton, York, UK; 3 British Trust for Ornithology, The Nunnery, Thetford, Norfolk, UK; 4 Centre for Ecology and Conservation, School of Bio- sciences, University of Exeter, Cornwall Campus, Penryn, UK * (s.bearhop@exeter.ac.uk)