Garden Birds. Mike Toms

severe winter. Despite this, Great Tits in a wood provisioned with sunflower seeds increased in number, while those in an unfed wood declined. The following year, in which there was a good crop of beechmast, saw populations in both woods increase.

      If the provision of food at garden feeding stations leads to an increase in the numbers of birds locally, then we might expect to see a relationship between feeder density and bird numbers across individual towns and cities. There is some evidence of this from the work carried out in Sheffield (Fuller et al., 2008; 2012), with the density of three of the seven urban-adapted species studied shown to be positively related to the density of bird feeders. House Sparrow showed the strongest pattern, with 57 per cent of the variation in House Sparrow densities explained by the density of bird feeders; the other two species were Blackbird and Starling. Interestingly, given the frequency with which Blue Tits and Great Tits are seen at garden feeding stations, it was surprising that they did not show such a relationship, yet Blackbird did.

      Some of the strongest evidence for community-level effects comes from work carried out in New Zealand, where most of the native bird species able to persist within urban areas are either predominantly insectivorous (e.g. Grey Warbler Geryone cigata), frugivorous (e.g. New Zealand Pigeon Hemiphaga novaeseelandiae) or nectarivorous (e.g. Tūī Prosthemadera novaeseelandiae). In contrast, many of the introduced species are either granviorous (e.g. House Sparrow and Spotted Dove) or adaptable enough to take bird table fare. Much of the food provided at garden feeding stations in New Zealand would appear to favour the introduced species, something confirmed by Galbraith et al. (2015) through a feeding experiment. The introduction of feeding altered the community structure, with significant increases seen in the abundance of House Sparrow and Spotted Dove, and to a lesser degree in Starling and Song Thrush. The introduction of feeding was also shown to have a negative effect on the abundance of the native Grey Warbler. Once the feeding experiment ended, virtually all of the observed changes in community structure disappeared. Observational work on these birds (Galbraith et al., 2017b) revealed that House Sparrow and Spotted Dove dominated the feeding stations.

      Supplementary feeding in the Sydney area, Australia, together with the establishment of both native and non-native fruiting shrubs in gardens, is thought to have been a significant factor in the major changes seen over the past 100 years in the diversity and abundance of parrots (Burgin & Saunders, 2007). The provision of food at garden feeding stations is thought to have aided colonisation and range expansion of the Collared Dove within the UK. Within North America, the northwards range expansions of House Finch, Tufted Titmouse Baeolophus bicolor and Northern Cardinal have also been attributed to the food available at garden feeding stations during the winter months.

      FIG 39. The presence of garden feeding stations is thought to have aided the successful colonisation of the UK by Collared Doves. (John Harding)

      INFLUENCE OF SUPPLEMENTARY FOOD ON NEST PREDATION

      We have already seen how the provision of supplementary food can alter the timing of breeding and community structure. A special case of the latter relates to the ways in which the provision of supplementary foods can influence relationships between predators and their prey. This is of particular relevance when discussing urban and suburban gardens because anthropogenic foods have become a defining characteristic of urbanised landscapes (Warren et al., 2006). The nature of interactions between breeding birds and their predators may be especially influenced by the provision of such supplementary foods because both songbirds and their generalist predators readily exploit these resources, something that can lead to them occupying the same urban sites. BTO work, in the form of a meta-analysis led by Dan Chamberlain, failed to reveal any strong underlying pattern to the effects of urbanisation – and the associated anthropogenic food sources – on the relationship between predators and levels of nest predation (Chamberlain et al., 2009a), but there have been some useful field experiments on the subject.

      FIG 40. There is little evidence that the presence of bird feeders and their use by Grey Squirrels leads to increased levels of nest predation in the local area, but such an effect has been seen for other predators. (John Harding)

      Supplementary food may reduce levels of nest predation by providing an alternative food supply for would-be nest predators; alternatively, it may increase levels of nest predation by attracting predators or elevating their population densities. Jennifer Malpass and her colleagues investigated how the presence of bird feeders affected predator abundance and nest survival of American Robin Turdus migratorius and Northern Cardinal across seven neighbourhoods in Ohio, US (Malpass et al., 2017). Malpass found that the relative abundance of both Brown-headed Cowbird and American Crow Corvus brachyrhynchos – known nest predators of the two study species – was positively associated with the number of bird feeders present within a neighbourhood. No similar relationship was found for the other nest predators studied, which included Grey Squirrel Sciurus carolinensis, Blue Jay Cyanocitta cristata and Common Grackle Quiscalus quiscula. While the relative abundance of American Crow and Brown-headed Cowbird was greater in areas with bird feeders, there was no consistent relationship between the numbers of bird feeders and predation of the songbird nests.

      DISEASE AND FOOD PROVISION

      The presence of supplementary food at garden feeding stations may contribute positively to bird populations by increasing the availability of food during those periods when natural food supplies are in short supply. However, as will be explored in Chapter 4, the provision of food at bird tables and in hanging feeders may also facilitate the spread of disease. The relationships between food provision and disease occurrence can be more complex than they first appear. It is not simply that garden feeding stations attract large numbers of birds, and through this lead to increased opportunities for disease transmission; in some instances the presence of the food can act to ameliorate the impacts of disease. This is something that has been studied in North America, in relation to the outbreak of the bacterial disease mycoplasmal conjunctivitis in North American House Finches (Fischer & Miller, 2015).

      Mycoplasmal conjunctivitis impairs vision, making it difficult for affected individuals to find and locate food. If such individuals have access to abundant and predictable food resources, this may afford them with the time and energy needed to recover. Interestingly, there is also some limited evidence that individuals infected at feeding stations may develop less severe symptoms than those infected elsewhere, something that may lead to a faster recovery. Fischer & Miller (2015) used information from the FHWAR (Fishing, Hunting and Wildlife-associated Recreation) census to examine how food provision influenced House Finch populations before and after the emergence of mycoplasmal conjunctivitis. The two researchers found that the availability of bird food appeared to have a positive effect on House Finch populations, with House Finch density prior to the disease outbreak strongly related to the density of US citizens providing food for birds. After the disease arrived, reducing House Finch numbers substantially, the relationship between feeder density and finch numbers remained.

      A comparison was then made between pre- and post-disease emergence populations, the researchers seeking to determine whether finch populations declined more strongly where feeder densities were high – which is what you might expect if high densities of feeders lead to high rates of disease transmission and, through this, to a correspondingly greater level of population decline. The results of the study revealed a positive relationship between changes in finch densities and changes in feeder densities between the two periods, indicating that higher densities of feeders actually resulted in lower rates of population decline. This adds weight to the work of Dhondt et al. (2007), suggesting that food provision might act as a ‘crutch’ for sick birds and improve their chances of survival and recovery. Other factors also need to be considered, not least that House Finches may spread mycoplasmal conjunctivitis to other species, including House Sparrow and American Goldfinch.

      FOOD PROVISION AND BEHAVIOUR

      As we have already discovered, food resources are central to many different aspects of a bird’s life, so it is little wonder then that the provision