Effects of Supplemental Feeding on Stress Physiology and Nesting Success of Eastern Bluebirds, Sialia Sialis

Abstract

A common anthropogenic influence on wildlife is the use of supplemental bird feeders. Dependent on abundance and natural food availability, this supplemental food source could influence individual survival and productivity. In this study, supplemental food availability was experimentally manipulated in a wild population of Eastern Bluebirds, Sialia sialis. This was done to examine the influence of the common pastime of bird feeding on physiology and reproductive success of bluebirds, especially when supplemental feeding is inconsistent. Adult and nestling bluebirds were assigned to one of three groups. In the first group, birds received mealworms (Tenebrionidae larvae) throughout the breeding attempt. In the second group, birds received mealworms from nest completion until nestlings hatched. Birds in the third group received no supplementation but were disturbed at the same frequency as birds in the other two groups. Nestling growth and reproductive success were calculated to examine differences resulting from my experimental manipulation. I also collected blood samples from adults and nestlings to quantify differences in bacterial killing ability, circulating corticosterone levels, and heterophil to lymphocyte ratios between the experimental groups. Finally to determine if differences in habitat quality contributed to the effect of food supplementation on bluebird physiology and nest success, data on invertebrate abundance were collected on a subset of territories. I found bacterial killing ability, baseline corticosterone and heterophil to lymphocyte ratios of adults and nestlings were not significantly different across the experimental groups. Nestling mass, tarsus, and wing chord length were unaffected by experimental treatment. Invertebrate abundance and richness were similar between years as well as across nest box trails. Invertebrate abundance and richness were not correlated with any of the nesting success metrics and did not statistically influence nesting success. Experimental manipulation of supplemental feeding did not appear to influence physiology or nesting success of Eastern Bluebirds. With regard to immune function, it is possible female condition prior to egg laying has a greater influence on nestlings than changes in food availability post-laying and later. Supplemental food availability may only have significant effects on physiology metrics and nest success in years with low environmental food availability.