2004: Bluebirds & Disturbance

Bert Harris (Biology)The University of the South:  Jennifer Phillips (Biology) Lewis & Clark College

Human disturbance of wildlife caused by outdoor recreation or everyday activities is inherently on the rise as the human population increases. The effects of these activities must be assessed in order to devise management practices that give wildlife suitable buffer space to maintain stable populations. In the past, the vast majority of disturbance studies on wild birds have focused on criteria such as flush distance and distance flown, rarely linking disturbance directly to the fitness of the birds population. We related human disturbance to the nesting success of eastern bluebirds (Sialia sialis) described by nestling condition (mass versus wing length), nestling growth (wing length versus day), parental feeding rate, and parental fecal sac removal rate.
We observed each nesting box for 90 minute time periods four times at a distance of 50 m. During that time, every two minutes all disturbances within a 50 m radius of the box were recorded. We divided disturbances into six categories: foot traffic, non-motor, gas-powered vehicles, movable machinery, predators, and electric golf carts. For each of the six categories, we calculated number of disturbances off track, number on track, minimum distance, mean distance from box, mode distance, temporal variance, and spatial variance. On track disturbances are disturbances that are in the normal area for that type of disturbance, e.g. golfers on greens, cars on roads. In addition, we continuously recorded certain parental and competitor behaviors, specifically predator and competitor chasing, feeding of chicks, fecal sac removal, and competitors entering the box. Each of the disturbance sub-categories and behaviors were compared to chick residual condition, residual growth, residual feeding rate, and fecal sac removal rate using a forward stepwise linear multiple regression with SPSS v11.5.
We hypothesized that bluebird nests with high levels of disturbance would have a higher nesting success than those with intermediate amounts of disturbance. This is based on the assumption that the bluebirds would become habituated to the disturbances in the high disturbance areas because disturbances were usually present, in contrast to the nests with intermediate amounts of disturbance, which would not become habituated. Nests with little or no disturbance, under this hypothesis, would have the highest nesting success, since even if the bluebirds were habituated to high levels of disturbance, disturbance would still put stress on bluebirds.
Disturbance was negatively related to nestling fitness in two cases: spatial variation in foot traffic around the box negatively influenced growth (R2=0.554, F2,21=13.054, p<0.001), and temporal variation in golf cart presence around the box was negatively related to condition (R2=0.166, F1,22=4.377, p=0.048). Also, feeding rate was negatively impacted by the number of cars observed on track (R2=0.775, F3,20=10.024, p<0.001). Unexpectedly, maximum sound level was related positively to growth (R2=0.388, F1,22=13.951, p=0.001) and feeding rate (R2=0.401, F2,21=7.039, p=0.005). These unintuitive results could have been caused by wind noise being registered as a higher sound level, leading to artificial inflation of the maximum sound level of a site. Interestingly, we found condition to be negatively influenced by feeding rate (R2=0.405, F2,21=7.159, p=0.004). This result, again unexpected, can possibly be explained in a number of ways, namely the fact that feeding quantity might not relate directly to condition if the quality of the food varies. Further, a tradeoff between growth and condition found by our colleagues might offer an explanation. If the chicks use their food to grow faster instead of build up more mass, then feeding rate would not affect condition as strongly. Each of our dependent variables except fecal sac relate negatively to at least one form of disturbance. Disturbance also related in unexpected yet explainable ways to our dependent variables. Further study is needed with respect to food quality, habitat openness, and off track disturbances.

For additional documentation Bert Harris and Jennifer Phillip provided a PowerPoint Presentation entitled "Bluebirds & Disturbance" provided here in PDF form.