The biological effects of what is commonly called 'backyard feeding' were studied in Australian Magpies (Gymnorhina tibicen) during the non-breeding season in 1999 in the Greater Brisbane and the Lockyer Valley regions, southeast Qld. Furthermore, a widespread survey on backyard bird feeding was also conducted in 2003 in the same areas. The survey was designed to collect the information necessary to extrapolate my findings about the physiological effects of feeding birds, over a large, wild population and to gain new sociological information related to the activity.
A captive feeding experiment was conducted in November 1999, after the free-ranging experiment, to determine if selected foods (pet mince, cheese, dog sausage) had the potential to alter the birds' blood chemistry (plasma cholesterol (PC), uric acid (UA), non-esterified fatty acids ( NEFA)) that had been examined in free-ranging magpies. Six birds were captured from the wild and held for seven days. It was demonstrated here that all of the processed foods, when provided ad libitum, influence at least two of three blood parameters (plasma cholesterol and non-esterified fatty acids). Hence, the use of these foods in the free-ranging experiment was validated. The observed increase in mean PC level may be due to lipids (i.e. saturated fat and cholesterol) in the foods provided, and decreased FA level could be explained by ready access to high quality food preventing the breakdown of stored fat.
The free-ranging experiment was commenced at the end of March 1999 and continued for 20 weeks including four weeks of non-feeding period. Six magpie populations (three in suburban, three in rural areas) were selected and in total 70 birds were individually tagged for identification. The birds were provided daily with a moderate amount of processed foods (20-40g per bird). The same food types as the captive experiment were used. To monitor the effects of the food provision, blood chemistry, body weights (BW), and the daily feather growth dates (Ptilochronology) were used as indices. Only Dog sausage was found to have significant effects on magpies' biology with PC and BW levels showing a great deal of sensitivity to the food type. No evidence of changes in nutritional status was found from Ptilochronology. There was no observable evidence to suggest dependency on the artificial food source.
Survey results showed that backyard feeding is, in both urban and rural environments, an activity that is common (one in two households have the experience), prolonged (most current-feeders have fed more than a year and few stopped afterwards) and intense (most households feed more than a few days a week). Among the total 43 species reported, Australian magpies were one of the most commonly fed species in all survey units. Besides bird seeds, common food types used by feeders comprised of processed foods such as household scraps, bread, pet foods and cheese, justifying the selection of the food types in the feeding experiments. Most feeders (roughly 90%) regarded the effects of bird feeding either positive or none. They are, however, subjective opinions and lack a concrete basis, highlighting the need for research-based information.
What the physiological challenges I reported here mean to the birds' life-style and/or fitness is crucial but out of direct scope of this project. However, considering the results of feeding experiments and the widespread popularity of this activity revealed by the survey, it is possible that food provision has had some degree of influence on the ecology of magpies and other wild bird populations across the nation. Despite that, the growing popularity of wildlife feeding inside and outside Australia and the many benefits to the practitioners means that wildlife feeding in some form will likely to continue. There is obviously a need for larger scaled studies in various species before a reasonably comprehensive assessment of the biological effects of wildlife feeding can be made, which will be an important step toward making wildlife feeding activities truly sustainable. However, this study does unequivocally demonstrate that the physiology of wild magpies can be affected by what is commonly called "backyard feeding." Thus, any consequences, positive or negative, that may follow from a change in diet already have the precursors in place.