In recent decades, migratory songbird populations declined throughout North America and other parts of the temperate zone. It is necessary to know whether these declines were the consequences of events that occurred on the birds’ breeding or wintering grounds for their protection. Tracking migratory birds involves observing them throughout their annual cycle, for instance, during which they may travel thousands of miles south from their breeding grounds located in the northern hemisphere. Migratory connectivity refers to the movement of individuals between summer and winter populations, which includes the stopover sites between the breeding and wintering grounds (Webster et al. 2002).
Intrinsic Markers: Unlocking Migration Clues
A range of new technologies has made it easier to study migratory connectivity. These technologies can be divided into two types: those that use markers intrinsic to the birds to indirectly track movements, and those that require attaching extrinsic markers to the birds to more directly track movements.
Intrinsic markers that indirectly track movement include stable isotopes and genetic markers.
Stable isotopes are naturally occurring forms of elements, and animals acquire them in their tissues either through the diet or water they drink. Because stable isotopes vary naturally and predictably in the environment due to a range of biological and biogeochemical processes, information about these naturally occurring patterns can therefore be used to identify the location of birds at the time they acquired these markers in their diet.
Extrinsic Markers: The Game Changer
In contrast to intrinsic markers, many extrinsic markers require recapturing the same individual at least twice. The first type of extrinsic marker to directly track movement was a uniquely numbered metal ring that could be attached primarily to the leg of a bird. Although ringing large numbers of birds allowed researchers to recapture individuals that returned to the same breeding or wintering sites annually, establishing migratory connectivity by recapturing a bird banded during the breeding season on its wintering grounds was like finding a needle in a haystack.
Modern Tracking Innovations
Light-level geolocators, for example, are used to estimate location based on seasonal and locational differences in the timing of day and night. Unlike geolocators, which need the recapture of birds, the tracking of birds by satellite just involves fixing GPS tags to the birds and retrieving information with ease. It is noted that due to historically placed battery limitations, GPS tags have been too large and heavy for most migratory songbirds, although this is changing as new technologies make the batteries smaller and lighter (Wikelski et al. 2007, Kays et al. 2015).
Ultimately, this and other new technologies will enable scientists to define songbirds’ migrations better and perhaps define a smaller current decline, as habitat loss continues on the breeding and winter grounds around the world.
