Dan E. Chamberlain

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

Every year, millions of households provide huge quantities of supplementary food to wild birds. While alteration 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 variety 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...

Differences in the climatic debts of birds and butterflies at a continental scale

Climate changes have profound effects on the distribution of numerous plant and animal species(1-3). However, whether and how different taxonomic groups are able to track climate changes at large spatial scales is still unclear. Here, we measure and compare the climatic debt accumulated by bird and butterfly communities at a European scale over two decades (1990-2008). We quantified the yearly change in community composition in response to climate change for 9,490 bird and 2,130 butterfly communities distributed across Europe(4). We show that changes in community composition are rapid but different between birds and butterflies and equivalent to a 37 and 114 km northward shift in bird and butterfly communities, respectively. We further found that, during the same period, the northward shift in temperature in Europe was even faster, so that the climatic debts of birds and butterflies correspond to a 212 and 135 km lag behind climate. Our results indicate both that birds and butterflies do not keep up with temperature increase and the accumulation of different climatic debts for these groups at national and continental scales.

A comparison of bird populations on organic and conventional farm systems in southern Britain

Abstract Field boundaries and fields on 22 pairs of organic and conventional farms in England and Wales were surveyed over three breeding seasons (April–July) and two autumn (September–November) and winter (December–February) periods in order to ascertain whether organic and conventional farms differed in the size and diversity of their associated bird populations. Species diversity was significantly higher on organic farms in the 1994 breeding season, but in no other year or season. Of 18 species, eight showed a significantly higher density on organic field boundaries in at least one season/year, with a greater number of significant results being detected in the autumn. There were very few significant differences in bird density in fields outside the breeding season. The density of breeding skylarks Alauda arvensis, the principal field nesting species, was significantly greater on organic farms in one breeding season. Hedges tended to be higher and wider, field boundaries tended to have more trees and field sizes tended to be smaller on organic farms. Canonical correspondence analysis indicated that components of habitat structure were important in explaining differences in bird density between farm types for certain species.

Winter feeding of birds increases productivity in the subsequent breeding season.

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.

The House Sparrow Passer domesticus in urban areas: reviewing a possible link between post-decline distribution and human socioeconomic status

The House Sparrow Passer domesticus is traditionally associated with human habitation. However, the species has undergone dramatic declines in many urban areas in north-western Europe. There are many theories as to why this decline has occurred, but the lack of data on House Sparrow numbers prior to their decline has hampered efforts to investigate these theories in detail. This review summarises the demographic changes in urban House Sparrow populations since the 1970s, and considers evidence that the current distribution of House Sparrows may reflect changes in urban habitats caused by socioeconomic change. Evidence is mounting that, within urban landscapes, House Sparrows appear to be more prevalent in areas with a relatively low human socioeconomic status. Here, we present evidence to suggest that House Sparrows may have disappeared predominantly from more affluent areas, and that these areas are more likely to have undergone changes to habitat structure. We also show how these changes in habitat could influence House Sparrow populations via impacts upon nesting success, foraging and predation risk.

Does food supplementation really enhance productivity of breeding birds

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.

Conserving the Birds of Uganda’s Banana-Coffee Arc: Land Sparing and Land Sharing Compared

Reconciling the aims of feeding an ever more demanding human population and conserving biodiversity is a difficult challenge. Here, we explore potential solutions by assessing whether land sparing (farming for high yield, potentially enabling the protection of non-farmland habitat), land sharing (lower yielding farming with more biodiversity within farmland) or a mixed strategy would result in better bird conservation outcomes for a specified level of agricultural production. We surveyed forest and farmland study areas in southern Uganda, measuring the population density of 256 bird species and agricultural yield: food energy and gross income. Parametric non-linear functions relating density to yield were fitted. Species were identified as “winners” (total population size always at least as great with agriculture present as without it) or “losers” (total population sometimes or always reduced with agriculture present) for a range of targets for total agricultural production. For each target we determined whether each species would be predicted to have a higher total population with land sparing, land sharing or with any intermediate level of sparing at an intermediate yield. We found that most species were expected to have their highest total populations with land sparing, particularly loser species and species with small global range sizes. Hence, more species would benefit from high-yield farming if used as part of a strategy to reduce forest loss than from low-yield farming and land sharing, as has been found in Ghana and India in a previous study. We caution against advocacy for high-yield farming alone as a means to deliver land sparing if it is done without strong protection for natural habitats, other ecosystem services and social welfare. Instead, we suggest that conservationists explore how conservation and agricultural policies can be better integrated to deliver land sparing by, for example, combining land-use planning and agronomic support for small farmers.

Winter food provisioning reduces future breeding performance in a wild bird

Supplementation of food to wild birds occurs on an enormous scale worldwide, and is often cited as an exemplar of beneficial human-wildlife interaction. Recently it has been speculated that winter feeding could have negative consequences for future reproduction, for example by enabling low quality individuals to recruit into breeding populations. However, evidence that winter feeding has deleterious impacts on reproductive success is lacking. Here, in a landscape-scale study of blue tits (Cyanistes caeruleus) across multiple years, we show that winter food supplementation reduced breeding performance the following spring. Compared to unfed populations, winter-fed birds produced offspring that weighed less, were smaller, and had lower survival. This impairment was observed in parents that had received fat only, or in combination with vitamin E, suggesting some generality in the mechanism by which supplementary feeding affected reproduction. Our results highlight the potential for deleterious population-level consequences of winter food supplementation on wild birds.

Fat provisioning in winter impairs egg production during the following spring: a landscape-scale study of blue tits

1. Provisioning of garden birds is a growing phenomenon, particularly during winter, but there is little empirical evidence of its true ecological impacts. One possibility is that winter provisioning could enhance subsequent breeding performance, but this seems likely to depend on the types of nutrients provided. For example, whereas effects of macronutrients such as fat are unlikely to be carried over to influence breeding in small passerines, micronutrients such as dietary vitamin E (an antioxidant) may be stored or have lasting health benefits. 2. Here, we examine the carry-over effects of winter food supplements on egg production in wild populations of blue tits (Cyanistes caeruleus). Over three consecutive years, birds were provisioned with fat, fat plus vitamin E or remained unfed (controls). 3. The provision of fat in winter resulted in smaller relative yolk mass in larger eggs and reduced yolk carotenoid concentrations in early breeders. However, these effects were not seen in birds provisioned with fat plus vitamin E. Lay date, clutch size, egg mass and yolk vitamin E concentrations were not significantly affected by winter provisioning treatment. 4. Our results indicate that winter provisioning can have important downstream consequences, in particular affecting investment in egg production several weeks or months later. 5. Provisioning is widely applied to support garden bird populations and for the conservation management of endangered species. However, our results challenge the assumption that such practices are always beneficial at the population level and emphasize how the ecological impacts can depend on the specific nutritional profile of provisioned foods.

Bird use of cultivated fallow ‘Lapwing plots’ within English agri‐environment schemes

Capsule Northern Lapwings Vanellus vanellus occurred on about 40% of fallow plot options within agri‐environment schemes; this could be increased by improved management and better placement of plots in the landscape. Aims To determine the use by Lapwings (and other species) of fallow plot options from the UK Countryside Stewardship and Higher Level Stewardship (HLS) agri‐environment options. Methods The number of Lapwings and other bird species using a plot and any evidence of breeding by Lapwings was recorded. Adjacent crop and boundary features, and vegetation height and bare ground within the plot, were recorded. Results Approximately 40% of the 212 plots surveyed were used by Lapwings. Breeding was suspected on 25% of plots and was proven on 11%. Lapwing presence was lower where woodland was close to the plot and breeding evidence was more likely on plots with more bare ground. Sky Larks Alauda arvensis, Woodpigeons Columba palumbus and Yellowhammers Emberiza citrinella were also frequently recorded. Conclusions Fallow plot options are one of the most expensive per‐hectare agri‐environment options under HLS. Better ‘value for money’ could be achieved by ensuring that a greater proportion of plots are (1) managed to promote a short broken sward, with plenty of bare ground; and (2) placed in open landscapes away from woods and vertical features.