How many birds outlast migration is a key question for biologists
Published: November 1, 2005
|Hurricane Katrina's recent destruction along the Gulf Coast called to mind a scene I came upon nearly 20 years ago on the beach of Mississippi's Horn Island. I saw feathers washing up along the shore and occasionally found a carcass or a wing. The remains included a Rose-breasted Grosbeak and an unidentifiable thrush. Most of the feathers in the wrack line were the flight feathers of warblers, vireos, and other small birds that had been caught in a storm during spring migration over the northern Gulf of Mexico, probably only a few miles from land, and perished. |
I had witnessed a graphic demonstration of natural selection. Birds that I didn't see had made it to shore safely and survived their ordeal. But the evidence of mortality made me wonder about how many birds outlast the perils of migration, and how the total compares with survival during the nesting and wintering seasons.
The proportion of birds that cross the Gulf successfully is not known, and with the exception of a few species, the share that survives migration in general is pretty much a guess. We also lack data for most species on differences in survivorship between breeding, migration, and wintering seasons.
Determining survivorship is perhaps the most challenging aspect of studying migration or, for that matter, population biology. Ornithologists who study migrant birds use banding, color-marking, and radar, and they analyze observation records from stopover locations. Banding involves capturing birds in mist nets and placing uniquely numbered bands on their legs. Approximately six percent of the 1.1 million birds banded annually in the United States are recovered each year - a small return rate, but valuable nonetheless. During banding, some birds are color-marked to make them easy to identify at a distance. (I wrote about color marking shorebirds in our June 2004 issue.) Color markers include plastic cylinders or collars for geese, nasal markers for ducks, and non-toxic dyes or leg bands for many other species. To track birds, banders attach radio or satellite transmitters using harnesses or other means.
Banding and color-marking allow researchers to identify individual birds and track them over several years at nesting, migration, and wintering sites, at least among species that return to the same areas year after year. However, once a marked or banded bird departs its nesting forest, for example, it disappears until the following spring. Annual survivorship statistics, based on numbers of birds that return to a site, are high for some species and low for others.
In virtually all species, birds less than one year old die more often than adults. The chart on page 60 shows how variable survivorship is for several birds. A lot of credit for what we know about survivorship must go to the Institute for Bird Populations based at Point Reyes Station, California. In 1989, it developed the Monitoring Avian Productivity and Survivorship (MAPS) program to track bird populations; today the program receives data from more than 500 banding stations across North America.
Survivorship rates are high for large birds such as Sandhill Crane, a bird that, not surprisingly, can live for many years. Its annual survivorship has been reported to be between 85 and 90 percent, and its maximum age ranges from 19 to 21 years. Adult songbird survival rates vary quite a bit, from about 43 percent for Eastern Bluebird to 67 percent for Black-throated Green Warbler. Smaller birds also live short lives, infrequently reaching 5 years of age. It's fascinating to realize that birds that live the longest also migrate the most - 42 times in the case of a 21-year-old Sandhill Crane!
Average annual survival rates are valuable, but they don't tell us how bird survival changes between a single year's wintering, migration, and breeding seasons. One of the few birds that we have such information for is the Black-throated Blue Warbler.
For 14 years, Scott Sillett of the Smithsonian Migratory Bird Center and Richard Holmes of Dartmouth University studied banded, color-marked individuals. They found that survivorship during migration differed from survivorship during the winter and nesting seasons. Virtually all adults survived the nesting season in New Hampshire and the winter in Jamaica, but only about 79 percent survived migratory periods.
Sillett and Holmes, who reported their findings in the Journal of Animal Ecology in 2002, determined that mortality rates were at least 15 times greater during migration than during nesting and wintering seasons, and more than 85 percent of all mortality for the species occurred during migration.
In the long run, why should ornithologists study survivorship? Sillett and Holmes note that, "The importance of the migratory period has frequently been ignored when developing conservation strategies."
Because so many birds die during migration, ornithologists now recognize the importance of stopover sites with high-quality habitat as key to maintaining populations. Indeed, the lead paper in the May 2005 issue of the Cooper Ornithological Society's journal Condor says that migration biologists intend to coordinate their research on stopover sites in hopes of having an impact on conservation actions.
Studying survivorship is one piece in the larger puzzle of understanding birds and how we can help them.