During the recent International Partners in Flight Conference in Snowbird, Utah, the emphasis was on protecting birds throughout their annual cycle. Yet, it is really difficult to set conservation priorities when there are uncertainties concerning the threats that birds face throughout the year. And in order to identify threats, we need to know exactly where populations of our northern breeding birds go during migration and winter.
Everyone at the meeting was talking about migratory connectivity. This refers to the way regional populations of breeding birds create linkages among geographic regions through their migratory behavior. Understanding connectivity is vital to the identification of factors that harm specific bird populations, and unfortunately there is a significant gap in scientific knowledge on this topic. The problem stems from the fact that it is extremely difficult to track such small animals as songbirds over the incredible distances they migrate.
GPS transmitters—which have been successfully used on raptors and shorebirds—are simply too heavy to place on most songbirds, which often weigh less than a few quarters. Radio transmitters are small enough, but these are limited by short signal ranges that would require a biologist to be within several miles of a migrating bird in order to detect it. Fortunately, recent advances in technology are helping us overcome these logistical challenges and are generating valuable knowledge about the movements of North American breeding birds.
Small tracking devices called light-level geolocators now weigh only 0.4 grams and have permitted some amazing advances in our knowledge of migratory connectivity. These geolocators are attached to a bird via a tiny backpack with two leg loops, and they record ambient light levels throughout the day. The light-level data they collect can later be used to estimate the bird’s location within about one hundred kilometers. Day length can give an indication of latitude, as days are longer in the north in summer, and longitude can be calculated from the timing of sunrise, as the sun rises earlier (relative to Greenwich Mean Time) in the east.
The latest advance has been the development of archival GPS geolocators. These weigh a bit more (~1g), but they are far more accurate – within a few meters! To fit this technology into such a small package, they can only record ten location points. However, you can program the geolocator to record these location points whenever you want—say, three fixes during spring migration, four during winter, and three during fall migration. A single bird with a geolocator backpack can depart its breeding grounds in the summer and return in spring with a wealth of information on migratory routes and wintering grounds.
The disadvantage of both types of geolocators is that they are only archival—which means you have to find and recapture your bird the following breeding season to retrieve the data. Due to this challenge, most geolocator studies have small sample sizes, but even so have produced amazing results. For instance, in a 2012 study by Franz Bairlein and colleagues, they discovered that Northern Wheatears in Alaska migrate 14,500 km across Asia to winter in eastern Africa—a unique and incredible journey that was previously undocumented.
In another study, Kira Delmore and colleagues discovered in 2012 that neighboring populations of Swainson’s Thrush in British Columbia exhibited dramatically different migration routes. Coastal birds traveled down the west coast to winter in western Mexico, whereas inland birds traveled overland across the Rockies and crossed the Gulf of Mexico to winter farther south in Central America. Clearly, the conservation of these two populations would require very different strategies.
I picked up brochures on both types of geolocators from the Lotek vendor booth at the conference. The challenge and opportunity now for KBO will be to determine how best to employ this technology to advance bird conservation.