Geolocators are very tiny—one device and the harness used to hold it just above a bird’s rump weigh less than one gram. All a geolocator does is to record light levels and the time of day, and scientists must retrap a bird and remove the geolocator to retrieve any data, so the technology is useful only for species with high site fidelity, likely to be caught again the following year. Day-length varies with latitude and the time of solar noon varies with longitude. Data from a geolocator can reveal a bird’s entire migratory path and where it spent the winter accurately to about 125 miles for latitude. They provide much better resolution for longitude. Satellite tracking devices, used on loons, osprey, and other large birds, are significantly more precise but much too heavy for songbirds. Scientists don’t put any device weighing more than 4 percent of a bird’s body weight on it. Geolocators are tiny enough to weigh more like 2 percent of the weight of medium-sized songbirds such as thrushes and martins, and for now provide the best information we have for working out migration and wintering locations for Neotropical songbirds.
Bridget Stutchbury’s seminal research led to her 2007 book, The Silence of the Songbirds, subtitled "How we are losing the world’s songbirds and what we can do to save them," a wonderful book I devoured as soon as it came out. She also is co-author of a fantastic textbook, Behavioral Ecology of Tropical Birds, published in 2001. So her Private Lives of Birds is based on a vast body of research put together by her and her students through her career, using state-of-the-art technology and good old fashioned field work.
It’s a fun read, because she elegantly explains how she and other researchers figure out each piece of information even as she’s keeping her focus on exactly what the title says, the private lives of birds. For example, scientists banding nesting birds have long known that most songbirds are monogamous. So they were mystified when DNA analysis in more and more species established that a single brood of nestlings raised by supposedly monogamous birds all were likely to have the same mother but one or more of the chicks may have been fathered by one or more males other than the one raising them. Stutchbury explains the advantages to birds of this “extra-pair paternity,” and explains what male birds do to try to keep their own mates faithful even as they try to mate with other females.
Her writing is fun and accessible as well as authoritative. Here and there she or her editors made minor errors: one does not see large numbers of crocodiles on a Texas island. And sometimes I squirmed thinking about the individual birds that were experimented upon for her to make many of her discoveries. It sounds like she didn't "sacrifice" any birds, but trapping one of a mated pair and keeping it off territory for hours or days to see whether it could get the territory back was probably pretty unpleasant for both that bird and the one trying to take over the territory. Fortunately, she has a track record of using the valuable information from these experiments to promote bird conservation that ultimately can benefit those individual birds as well as larger populations.
The book is chock full of valuable facts and insights, and is well worth reading for anyone who wants to understand more about the private lives of birds.