Finding Out What Sparrows Do in Summer

May 05, 2017

By Bruce Lyon, Professor of Ecology and Evolutionary Biology 

           My students and I have been studying Golden-crowned Sparrows in the UCSC Arboretum for the past fourteen years. We have learned a great deal about the social lives of these migratory songbirds and, thanks to a recent grant from the National Geographic Society, we are now adding an exciting new dimension to the project.  Although the breeding range of the species is known generally—Alaska and British Columbia—we have no idea where the birds from our winter population spend their summers. With our grant we purchased 35 GPS tags that can be programmed to take up to 80 very accurate GPS readings (<10 m) on dates and times of our choosing.

            A bit of background information will help put the new aspect into perspective. Right from the start of the study, a critically important aspect has been that every sparrow is given its own unique color band combination—like an avian social security number. This allows us to recognize, census, and observe known individuals in the field without having to catch them to read the unique numbers stamped into their metal bands. Color banding for individual recognition, first adopted in the 1930’s, revolutionized the study of birds because biologists could shift focus to studying patterns of variation among individual birds in various features, including behavior. It was Margaret Morse Nice who pioneered the use of individual tagging in her now famous study of song sparrows—her study showed the powerful information that can come from following the lives of known individual birds.

            In our own study, color banding has revealed several important findings. We now know that the same individual birds return back to the Arboretum each year, typically to the exact same part of the Arboretum they occupied the previous winter. The returnees also tend to hang out with the same birds as in previous years, given that their buddies also survived. About half of our birds survive across seasons, which is a typical rate for small migratory songbirds. We have also found that survival is independent of age, another typical bird pattern. Put another way, a two-year old bird and a five-year old bird have the same chances of returning across years. And, to answer a question we are often asked: the oldest individual from the 617 color-banded birds we have followed was at least nine years old when it disappeared this past December.

            Another discovery from our color-banding study is that the sparrows live in fairly distinct social communities, and each of these communities tends to use different parts of the Arboretum. The tight social bonds of the birds in these communities are maintained across years, and it appears that the birds form close associations for their entire lives (best friends forever!).  Tight affiliations like these in the animal world often promote close cooperation, and we have been interested in finding out what is special about the individuals that cooperate. One obvious answer is that they are relatives, but our genetic study showed the birds that form close associations are not related. Another possibility is that these friendships are first formed on the breeding grounds and that summer buddies migrate together and then hang out in winter together. Data from the GPS tags will allow us to test this idea—by programming the tags to take several readings in June, when the sparrows are known to be nesting, we will determine whether the winter buddies are also summer buddies. In addition, by carefully choosing the dates when the tags will obtain readings, we will also be able to determine the exact migration routes of the birds, whether some birds migrate together, and when the birds first arrive back in the Arboretum the following autumn.

 Golden-crowned Sparrow           The tags weigh one gram, just over three percent of their body weight, and are attached with a backpack harness system with slightly stretch loops that go around the legs. With a good harness system, the tag itself lays concealed under the bird’s back feathers, and the only evidence that the bird is tagged is a little white-colored antenna sticking out above the tail, as can be seen in the photo at right. The tags collect GPS information by getting readings from satellites but do not themselves transmit any information to satellites. That means we can get only the data on the tags by recapturing birds next fall and recovering the tags. Based on our long-term survival rates, we can expect to get half of the 35 tags we deployed back. Here’s hoping that the birds have a great summer vacation!