A Time and Place for PVC Bluebird Houses
By Gary Gaard Summary About one fourth of the 150 bluebird houses I monitor are PVC. PVC houses are the safest houses currently available. The well being of adult bluebird and brood is more important for me than the numbers of bluebirds produced.
Introduction Bluebird monitoring for me began in the mid 1990’s. The mode of operation at that time was “build a house, put it up, and the bluebirds will come”. In general, bluebird monitors believed you could design a perfect birdhouse, place it anywhere and the bluebirds would flock to your door. I built houses and put them up, but didn’t fledge many bluebirds. The number of failed nests was really disturbing. Approximately 50% of eggs laid failed to develop into fledged baby bluebirds. Early on I decided there was not a one size fits all bluebird house and there would probably never be the sought after perfect house. Today my philosophy is entirely different. When placing a bluebird house I consider in order of importance: 1) Habitat, including site competition from other cavity nesters. 2). Safety, of both adult bluebirds and their nest 3). Monitoring, meaning repair and cleaning of houses 4) Fledge of bluebirds and other cavity nesters 5). Preference of bluebirds to style of house used. The PVC houses I now have in the field are at sites where bluebirds are not safe in a wooden house. If a House Sparrow tries to nest or kills an adult bluebird in a wooden house, I swap the wooden house for a PVC house. The PVC houses are at sites preferred by bluebirds as bluebirds have already nested there in wooden houses. Safety and the Black Fly In 2000 and 2001 I published articles in Wisconsin Bluebird, results of field experiments with black flies. The recommendation was to eliminate ventilation holes thus preventing Wisconsin Bluebird
black flies from taking blood meals from young and adult bluebirds in the nest box. Since then I have lost, in unvented houses, several broods to the black fly. My current theory is this. The black fly has to locate the source of its blood meal. The fly can detect increasing concentration of Carbon Dioxide, and moves to the source, or higher
Jack Bartholmai
concentration of Carbon Dioxide. The higher concentration of Carbon Dioxide is in the house, and if air moves through the house the fly is able to find its meal and the feeding frenzy begins. If there are two holes in the house, the fly will enter. The entrance hole is always present, and the second hole can be as small as an old nail hole or a sixteenth inch crack where two boards don’t mesh. The K-box is highly susceptible to black fly entry. It takes a very good carpenter to fit the slanted side to sloping roof without a gap. Joints will separate to form cracks as the house gets older. If wood houses are near a black fly river, all open joints, cracks in wood, and knot holes should be caulked. Pine is not a good choice of construction materials because it warps when exposed to weather. PVC houses have only the entrance hole, no seams or knots. Air does not move through the house, therefore the PVC house has excellent black fly protection. More than 95% of black fly 10
attacks are prevented – this is excellent biological control!!
Safety and the House Sparrow Schedule 40 PVC houses with no head room above the entrance are infrequently used as nesting sites by House Sparrows (WI Bluebird 22-4, P10 and WI Bluebird, 24-1, p5). Many of my houses are in areas where House Sparrow population is high. I prefer to prevent House Sparrow nesting rather than having to throw their nests or offspring in the bushes. Not all people with bluebird houses have the time or inclination to toss out sparrow nests or trap adult sparrows, and PVC houses relieve the monitor of this unpleasant duty. If House Sparrows evict a bluebird from a wooden house I replace the wooden house with a PVC house. These houses are in sites the bluebird has predetermined to be a preferred nest site. Most PVCs are now “double nesters” for bluebirds. Most of my PVC houses are at isolated sites so there are no nearby wooden houses for bluebirds to move to. So any data from these houses are skewed positive – conversely data from Aldo Leopold Audubon in Stevens Point (WI Bluebird, 24-1, p5) is skewed negative for these reasons; they placed PVC houses in 1). bluebird predetermined non-preferred bluebird nest sites, 2). the middle of the first round of bluebird nesting season, an open invitation to Tree Swallows seeking a nest site, 3). sites where wooden houses were nearby.
A simple model A 38 year career in research taught me that often the best way to analyze a situation or problem is to work from a model. Establish an ideal situation, and remove all variables. Then, mentally or on paper, you begin asking “What if?” National Audubon says nesting bluebirds require 2-3 acres per breeding pair. Assume four acres of excellent bluebird habitat. Let’s make the four acres our model and begin to ask “What if”? Assume no predators, competition, infertility, equal weather, etc. What if you placed two wooden houses? You should fledge two nests/ house or 16 bluebirds. Summer 2009