Food for thought: supplementary feeding as a driver of ecological change in avian populations

Abstract

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...

An appetite for connection: why we need to understand the effect and value of feeding wild birds

“…Although little is known about the negative effects of feeding birds on their ecosystems, the fundamental influence of food supply on the lives of animals has long been appreciated, largely through many carefully conducted supplementary feeding experiments (see Martin 1987;Robb et al 2008). These studies have explored, for example, the influence of the timing, quantity and quality of food provisioning on hatching and laying dates, clutchsize, and survival of chicks and fledglings (Boutin 1990;Newton 1998).…”

“…Thus, supplemented birds typically breed earlier and more often. However, evidence for direct benefits to fitness, such as producing larger clutches and enhancing hatching survival is far less obvious (Robb et al 2008). In an important recent experiment conducted in the UK, Great Tits (Parus major) and Blue Tits (Cyanistes caeruleus) were provided with supplementary foods over three consecutive breeding seasons (Harrison et al 2010).…”

“…From these important emotional origins has developed a massive industry, with poorly understood -but likely profound -influences over large areas (Robb et al 2008). In the UK, for example, the survival of several species appears to be sustained at least in part by the provision of seed (Sterry and Toms 2008).…”

Supplementary feeding restructures urban bird communities

“…For species capable of exploiting these anthropogenic food sources there may be profound effects on almost every aspect of their ecology (14,15). Direct benefits for feeder-visiting birds may include reduced time foraging or improved body condition, which in turn may increase reproductive success or survival and lead to population level changes (16)(17)(18). A greater availability of food may artificially inflate the carrying capacity of the urban environment, resulting in higher densities of species capable of exploiting anthropogenic food resources (15,19).…”

“…However, the reality is far more complex (17). Biodiversity may be reduced where a subset of species become dominant at feeding locales, either through competitive advantage or numerical dominance.…”