Ecological impacts of supplementary feeding on urban bird communities in New Zealand

Abstract

Millions of people worldwide engage in the practice of feeding birds. Although there are many purported impacts associated with bird feeding – including improved winter survival, increased reproductive success, increased aggression, and disease outbreaks – there has been little consideration of both the human and ecological consequences of the practice from the scientific community. The majority of bird feeding takes place in urban areas, yet in situ experimental studies of the effects are extremely rare. The practice of feeding is particularly poorly studied in New Zealand. In this thesis I investigate bird feeding and its impacts in New Zealand, to determine what risks bird feeding poses to urban bird communities in this context. The study consisted of two main components. First, a mail survey of feeding practices in six cities nationwide was carried out, to quantify the current feeding practices and motivations for feeding in New Zealand and identify potential risks from typical feeding practices. Results confirmed that bird feeding is a common activity, with 46.6% of households feeding birds. Bread was most commonly provided, with an estimated 5.1 million loaves/annum fed to birds across the six surveyed cities. Two key risks identified were: 1) that introduced birds are likely to be the main consumers of supplementary food sources in New Zealand; and 2) that poor hygiene practices may contribute to transmission of disease at feeders. Second, a 2-year in situ experiment was conducted to examine the impacts of typical feeding practices on local bird community structure and disease transmission; volunteer households in urban Auckland provided food for 18 months. The grain-based feeding regime caused a significant shift toward communities dominated by a few introduced bird species at feeding compared to non-feeding properties. House sparrow (Passer domesticus) and spotted dove (Streptopelia chinensis) particularly benefitted. There was a detrimental effect on one native insectivorous species, the grey warbler (Gerygone igata), which did not utilise the feeders. Disease risks were identified in feeder-visiting bird species, with all pathogens and parasite groups detected in at least one of the three focal species screened (house sparrow, silvereye Zosterops lateralis, and Eurasian blackbird Turdus merula). Feeding stations tested positive for Salmonella enterica Typhimurium on ~7% of occasions, confirming that structures used in feeding are a potential transmission pathway. Feeding had varying effects on disease dynamics, including no change on infectious pathogen parameters, and both positive and negative effects on parasitic infection parameters. The overall findings of this research confirm that bird feeding is a popular activity in New Zealand, one that generates positive feelings for those participating. Moreover, this research confirms that the practice of bird feeding has discernible consequences for urban bird communities. Further investigation into the effects of feeding is certainly warranted, and all conservation practitioners should take into account bird feeding as an important driver of avian ecology in urban ecosystems.