Over our heads

Canada warbler flight call (from here)

At the peak of migration along any major flyway, tens of thousands of warblers, sparrows, thrushes, vireos and other birds can pass overhead on a single night. As they fly, they emit very short, high-pitched bursts of sound. The calls intensify as they descend to roost in the hours before dawn, with birds on the ground responding to birds still in flight. Sometimes, birds even key in on spring peepers — maybe because after all that flying, the first thing they want is a drink of water!

To human ears, the night calls of migrating songbirds are hard to tell apart, and many are so high-pitched as to be virtually inaudible. But with the help of a microphone, a recording device, and a computer outfitted with special software, these night flight calls can be identified by species — and increasingly also by age, sex, and even geographic origin. Last night I attended a talk by Mike Lanzone, the coordinator of field research for the Carnegie Museum of Natural History’s Powdermill Avian Research Center, which has been gathering flight call data at two stations in western Pennsylvania for several years. Mike explained how they learned to take advantage of the birds’ tendency to reply to similar-sounding calls with calls of their own. As a now-routine part of their bird-banding process, all captured birds are placed for two minutes in large cotton tubes outfitted with microphones, where they can flutter about and call in response to recordings of other flight calls. At the same time, a feather sample is taken to analyze for DNA and stable isotope signatures, which are compared with data from museum specimens, in cooperation with the Cornell Lab of Ornithology. Using this data, they can figure out where each banded bird was hatched and raised.

The point of all this extra effort is to assemble a continent-wide map of night flight call dialects, as well as to hone their interpretive techniques. As the Powdermill website puts it,

Statistical analyses of intra- and inter-specific variation in flight call notes will help facilitate more robust methods for distinguishing species-specific flight notes using advanced software, and information on age, sex, and energetic condition of birds in relation to flight call rates in captivity will aid in the translation of the number of recorded calls overnight into an accurate estimate of the number of individuals of each flight-calling species that passes over a location in migration at night.

Presuming that Mike’s hypothesis is correct — that flight call dialects do exist and are distinct enough for computers to tell apart — in a few years, it should be possible for linked networks of listening stations across all major flyways to generate year-by-year summaries of population trends for a wide array of passerine species. They’ll be able to detect if a local palm warbler population in the Northwest Territories, say, has suddenly suffered a decline, and can notify folks at Northwest Territories Wildlife and Fisheries to look into it.

With all this data, though, Mike admitted that they still can’t answer the most basic of questions: Why do birds emit these calls in the first place? Their conversations are figuratively as well as literally over our heads.