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

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