This month I want to report on a bird conservation success story that is as close to home as Putah Creek. Yolo Audubon’s own Melanie Truan came up with the idea and made it happen, and has ensured its continuation ever since. Working at the UC Davis Museum of Wildlife and Fish Biology, and aiming to promote research, education, and conservation simultaneously, in 2000 Melanie established what she called the Putah Creek nest-box highway. A series of wooden boxes (now numbering over two hundred) were hung from tree branches along twenty miles of the creek, starting at Fishing Access #5 and ending downstream at Mace Boulevard. The idea was to supplement the tree cavities that certain small birds use as nest sites. Natural tree holes were scarce, and there was evidence that native hole-nesting birds were declining in numbers. The experiment seemed promising to Melanie because certain species like eastern bluebirds were benefiting from nest boxes in the east.
The nest-box program has been a huge success. Over 95% of boxes are used each year. More than ten thousand fledglings of seven species have been produced. The main users are tree swallow, western bluebird, and ash-throated flycatcher, plus smaller numbers of house wren, oak titmouse, and white-breasted nuthatch. At the UC Davis Riparian Reserve, where I have birded regularly for the last thirty years, the program has single-handedly brought bluebirds back as nesting birds, and greatly increased the numbers of ash-throated flycatchers and tree swallows.
The nest-box program has shown that the availability of nest cavities has a large effect on the abundance of certain species in this part of California, because demand for nest holes far exceeds supply. The large-scale removal of older trees for development and farming, plus our inclination to trim out dead and dying branches of trees near our homes and work-places, has meant the loss of nesting opportunities for many small birds. Furthermore, European starlings commonly displace smaller birds from the few remaining nest sites. (The nest-box program was careful to size entrance holes to exclude starlings.)
After sixteen years of operation, Melanie and her colleagues are examining the subtler effects of all those nest boxes on bird populations, such as the relation between the abundance of house wren and Bewick’s wren. One thing is certain, that the effort has been a boon to smaller cavity-nesting songbirds. We might consider introducing nest-boxes to other areas, since the cities of Yolo County have plenty of trees, but not many tree-holes.
— Michael Perrone, Conservation Chair