Electronic twitchers spot the night birds

时间:2019-03-01 11:06:04166网络整理admin

By BERNICE WUETHRICH in WASHINGTON DC Tracking songbirds that migrate at night can be a frustrating job. They are hidden by darkness, and although they call to each other to keep in touch their night calls are often different from their daytime songs. Even when the birds’ night-time calls are known, they are often hard to pick out from the background of noise made by insects, wind and water. This makes it almost impossible to tell by ear whether there are thrushes and warblers flying by or cuckoos and sparrows. Bill Evans, a researcher at the Laboratory of Ornithology at Cornell University in Ithaca, New York, has been trying to solve this problem. He has spent the past decade eavesdropping on songbirds that migrate at night, listening in as they fly from their northern breeding grounds to the tropics where they spend the winter. With the aid of sensitive microphones and specially designed computer software, he has distinguished the characteristic night calls of more than sixty species. Armed with this information, he and his colleagues plan a census that will automatically record the movements of most of America’s migrating songbirds, and show the extent to which these species are declining. Evans began by taping the calls of birds migrating over Florida, Texas and Alabama. To match sound with species, he compared the night calls with known flight paths of particular species, and in many cases with known daytime calls. Eventually, he was able to assign notes as short as a twentieth of a second to individual species. ‘During the course of one spring night in Florida, you can hear a call every second all night long,’ says Evans. He records these calls and feeds them into a computer programmed to extract bird calls from background noise. The computer generates a spectrograph for each call (see Graphs), showing its nuances of frequency and duration. By comparing these ‘call signatures’ with those made by various species, Evans can identify which ones are passing overhead. He can also count how many birds of each species have flown by. As a first step in the census, Evans set up an array of seven listening posts on rooftops in New York state, mainly in the countryside. Each station can pick up sounds from birds as high as 1000 metres over an area of 7 square kilometres. Three seasons of monitoring have revealed that songbirds migrate in broad fronts up to several hundred kilometres wide. Each front is made up of many species, but at a particular place and time – depending on the weather – the density of each species is remarkably consistent. The weather is important, says Evans, because on nights with low cloud, when the stars are blocked, the birds pack together and call more frequently in order to stay in touch. Evans suggests that because the patterns are so consistent, monitoring nocturnal flight calls offers a good way to check on how populations change over time. For example, in each of the past three years, Evans estimates that about 100 000 Swainson’s thrushes passed over his line of monitoring stations, which stretches about halfway across New York state. Ornithologists predict that in the next quarter of a century, the number of these thrushes will fall by between 25 and 50 per cent because of deforestation in its wintering grounds in Central America. The sound census should register any change in the size of the population much sooner than any other census technique. These other census methods rely on volunteers counting birds on land. The breeding grounds in the boreal forests and wintering grounds in the tropics are generally inaccessible, so census takers estimate the numbers en route, counting the number of night-migrating species at rest during the day. Sam Drogy, an ornithologist with the National Biological Survey in Washington DC says this can be misleading because the roosting birds may be the ‘losers’ that are unable to keep up with the flock. But Evans’s approach has its own drawbacks, says Drogy. For one thing, identifying birds from their spectrographs is time-consuming. Researchers at the Bioacoustics Research Program at Cornell have recognised this and are developing software that will count the number of birds of each species automatically, eliminating the need to check every spectrograph manually. Evans’s technique can only count birds that fly at 1000 metres or less; some travel at five times that altitude, and so are missed. The technique also assumes that a constant proportion of each species calls as they fly. ‘This is unknown, and may be unknowable,’ says Drogy. Evans admits that he does not know what proportion of birds call as they fly, or if some call too quietly to be picked up on the microphones. To fill this gap in understanding, Evans plans to team up with the electricity company Niagara Mohawk Power, which supplies power throughout New York state. The company wants to know where it can safely put up new power lines, and will track the next migration through the state using radar. Although radar cannot distinguish between species, it can give a picture of the mass of birds, says Evans. By comparing the radar data with the number of calling birds, he hopes to estimate how many are callers. Evans’s first spectrographs of nocturnal flight calls have just appeared in The Wilson Bulletin, an American ornithological journal. From the cacophony of a Florida night, the computer picked out a migrating Bicknell’s thrush, a bird that is now in decline. Previously, the species had flown through Florida incognito,