Thermal stress in artificial cavity-nesting Eastern Bluebirds: killing them with kindness?

Abstract

Artificial nest boxes are used to help cavity-nesting species with declining populations that compete for limited nest sites. Proper use of nest boxes has been shown to lower predation rates and ectoparasite loads. However, most nest boxes do not provide the insulation benefits of natural cavities and may be increasing thermal stress on cavity-nesting species. Recent studies have found evidence for the effects of extreme ambient temperatures on hatching and fledging success in passerines. Therefore, I hypothesized that clutches experiencing the highest mean temperatures would have a greater likelihood of hatching failure. I further expected that broods exposed to high mean temperatures during the nestling period would influence nestling growth and survival rates. I studied multiple-brooded Eastern Bluebirds breeding in nest boxes at ECU’s West Research Campus. I placed programmable thermochron iButtons in a consistent position within nest boxes to measure temperature at 10-minute intervals continuously during the incubation and nestling periods. I recorded data on hatching success, nestling size, and fledging success. I found that high mean temperatures during incubation significantly impacted the proportion of unhatched eggs in a nest. Yet, some late broods tolerated prolonged temperatures above 35º C where boxes remained in full sun because no natural shading was available. My results suggested that some nest boxes reached temperatures above a tolerable threshold for some bluebird embryos to survive. To mitigate the effects of heat stress, nest boxes should be designed with insulation properties or placed where they receive natural shading to provide a stable microclimate for cavity-nesting passerines.