Nutritional implications of feeding free-living birds in public urban areas

Abstract

Supplementary feeding can affect populations of birds. It reduces energy spent on foraging and reduces the risk of starvation, but it also increases the risk of disease transmission and predation. Supplementary feeding may reduce species richness if some species are better able to exploit supplementary food resources than others. Feeding may also artificially inflate the carrying capacity of the ecosystem, leading to bird nuisance in the form of droppings and noise. The aim of this study was to characterise and quantify the risk factors and consequences of feeding free-living birds in public areas in the western part of the city of Amsterdam. In seven study areas, the following data were collected: bird population size and species composition, feeding events, and the type and amount of supplementary food offered. Estimations were made of the nutritional energy provided and the number of birds that could be supported by the food offered. Members of the public who fed the birds were invited to complete a questionnaire on various aspects of feeding. Results showed that supplementary feeding attracts juvenile gulls and feral pigeons, which could in the long-term affect biodiversity. Bread was the main category of supplementary food being offered (estimated to be 67% of the total amount of food). The majority of respondents fed birds so as not to waste bread and meal leftovers. In six of the seven areas studied, an overabundance of nutritional energy was calculated. We conclude that the current type and extent of supplementary feeding in the city of Amsterdam is nutritionally unbalanced and affects species diversity at a local level. The overabundance is undesirable for reasons of both animal health, because it can lead to malnutrition, and public health, because surplus food attracts rats and may also have a negative effect on water quality.

Keywords: bread; ducks; feeding; gulls; pigeons; rats.

Figure 1

Species composition of (a) study areas containing a large body of water (b) study areas with no surface water. Numbers are means of all observations including feeding events [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 2

Types of food offered to the birds based on an estimate of weight. A total of 88 kg food was estimated to have been put out for all observation periods together

Figure 3

Correlation between the amount of food put out and the number of birds attracted to feed during the observation periods. The numbers of birds attracted to the food revealed that the numbers of birds attracted to a feeding event increased with increasing amount of food offered to around 50 and then levelled off, which fitted a logarithmic curve with best significance level (p = .004)

Figure 4

Feeding participation ratios for all species recorded (the mean proportions of birds present that were attracted to the food). Error bars indicate the 95% CI [Colour figure can be viewed at wileyonlinelibrary.com]