Effects of bird-feeding activities on the health of wild birds

Abstract

Among the most popular reasons that people feed wild birds is that they want to help birds. The extent to which supplemental food helps birds, however, is not well established. From spring 2011 to spring 2014, we examined how feeding of wild birds influences the health of individual birds at forested sites in central Illinois, USA. Specifically, we compared three forested sites where we provided supplemental food with three forested sites for which no supplemental food was available and monitored changes in the individual health of birds. In addition, we determined whether any changes in bird health had occurred after feeders had been removed from sites 10 months before. Generally, the individual health of birds improved with supplemental feeding, including increased antioxidant levels, reduced stress (heterophil-to-lymphocyte ratio) and more rapid feather growth. In some species, we also found improved body condition index scores and innate immune defense. The difference among sites was not present 10 months after feeders were removed, suggesting that the impact on health was indeed related to supplemental feeding. Potential negative effects of supplemental feeding were also found, including an increase in infectious disease prevalence among individual birds at forested sites where supplemental food was offered. Birds with clear signs of pathology showed deficits in most of the physiological metrics in which birds at feeder sites typically showed improved health condition. At the peak of prevalence of infectious disease, 8.3% of all birds at feeders exhibited symptoms of conjunctivitis, pox, dermal disease or cloacal disease. We found both positive and negative impacts of wild bird feeding, and that, in general, birds that had access to supplemental food were in better physiological condition. Moreover, the negative effects we found may be mitigated by hobbyists engaging in safer bird-feeding practices.

Keywords: Anthropogenic food; avian physiology; disease; songbirds.

Figure 1:

Physiological differences in 11 bird species at sites with feeders and sites without feeders, sampled in Central Illinois from 2011 to 2014. Shown here are heterophil-to-lymphocyte ratio means (a), fat score means (b) and total antioxidant capacity (copper-reducing equivalent) means (c). The differences were not year specific or species specific; therefore, all years and species are combined in this figure. Differences are statistically significant in all comparisons shown here.

Figure 2:

Body condition index means (a) and microbial killing ability index means (b) from 11 bird species at sites with feeders and sites without feeders in central Illinois from 2011 to 2014. *Significant difference (P < 0.001).

Figure 3:

Difference in mean growth bar length for six species at sites with feeders and sites without feeders in central Illinois from 2011, when feathers were grown before feeders were available at any sites, from 2012 and 2013, when feathers were grown with some birds having access to feeders and other birds at sites without feeders, and from 2014, when feathers were grown at sites with feeders removed 10 months before and sites with no bird-feeding history. The differences were not species specific; therefore, data from all six species are combined in this figure. *Significant difference (P < 0.001).

Figure 4:

Comparison of heterophil-to-lymphocyte ratio (a), body condition index (b), total plasma protein (c), total antioxidant capacity (d) and microbial killing ability index (e) for 11 species of feeder-using birds showing symptoms of disease (open circles) and birds without symptoms of disease (filled circles). Differences are statistically significant in all comparisons shown here.