w ww ww w. .f fr ro on nt ti ie er rs si in ne ec co ol lo og gy y. .o or rg g © © 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 feeding reduces the risk of starvation and may enhance reproductive performance (Newton 1998). Despite the impressive 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 overwinter 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 treatments 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 supplementary 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 layingThe acute need for energy during egg development and laying means that supplementary feeding is likely to affect REVIEWS REVIEWS REVIEWS I In n a a n nu ut ts sh he el ll l: :• Feeding birds is an enormously popular activity that can affect virtually every aspect of bird ecology, from daily survival 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 climate and predation GN Robb et al.Avian responses to supplementary feeding avian fecundity and resource allocation during reproduction. During the breeding season, time is constrained, and even small shifts toward...

Supplementary food given to birds can have contemporary effects by reducing the risk of starvation, increasing survival and altering movements and reproductive performance. There is, however, a widely held perception that birds benefit from extra food over winter, but that it is better that they ‘look after themselves’ during breeding. Here we describe a landscape-scale experiment showing for the first time that the effects of increasing food availability only during the winter can be carried over to the subsequent breeding season. Even though food supplementation stopped six weeks prior to breeding, birds living on sites provisioned over winter had advanced laying dates and increased fledging success compared with birds living on unprovisioned sites. Thus, supplemental feeding of wild birds during winter, in a manner mimicking householders provisioning in gardens and backyards, has the potential to alter bird population dynamics by altering future reproductive performance. With levels of bird feeding by the public continuing to increase, the impacts of this additional food supply on wild bird populations may be considerable.

Food availability influences multiple stages of the breeding cycle of birds, and supplementary feeding has helped in its understanding. Most supplementation studies have reported advancements of laying, whilst others, albeit less numerous, have also demonstrated fitness benefits such as larger clutches, shorter incubation periods, and greater hatching success. Relatively few studies, however, have investigated the effects of supplementary feeding for protracted periods across multiple stages of the breeding cycle. These effects are important to understand since long-term food supplementation of birds is recommended in urban habitats and is used as a tool to increase reproductive output in endangered species. Here, we compare the breeding phenology and productivity of blue tits Cyanistes caeruleus and great tits Parus major breeding in food-supplemented and non-supplemented blocks in a broadleaf woodland in central England over three seasons (2006-2008). Supplementation was provided continuously from several weeks pre-laying until hatching, and had multiple significant effects. Most notably, supplementation reduced brood size significantly in both species, by half a chick or more at hatching (after controlling for year and hatching date). Reduced brood sizes in supplemented pairs were driven by significantly smaller clutches in both species and, in blue tits, significantly lower hatching success. These are novel and concerning findings of food supplementation. As expected, supplementary feeding advanced laying and shortened incubation periods significantly in both species. We discuss the striking parallels between our findings and patterns in blue and great tit reproduction in urban habitats, and conclude that supplementary feeding may not always enhance the breeding productivity of birds.

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